7.4: Modes of Exchange
The idea of exchange was first explained by Marcel Mauss in terms of two types of exchange: non-capitalist gift exchanges (which have to do with social relations and building, which require a gift for exchange), and impersonal commodity exchanges. Impersonal commodity exchanges are more common in Capitalist societies which don’t link those who are exchanging with one another, aside through the use of cash. These aspects are also characteristic of egalitarian societies. Later, Marshall Sahlins used the work of Karl Polanyi to develop the idea of three modes of exchange, which could be identified throughout more specific cultures than just Capitalist and non-capitalist. These are reciprocity, redistribution, and market exchange. Although these modes of exchanges are drastically different, aspects of more than one mode may be present in any one society.
Reciprocity
The oldest mode of exchange is Reciprocity which is used in egalitarian societies, like that of the Ju/’hoansi. There are three different types of reciprocity: generalized, balanced, and negative reciprocity.
- Generalized reciprocity is an exchange where return isn't expected right away and the value of this return isn’t specified. This is based on the assumptions that all exchange balances out, like that between family members. This is largely based on trust as social situations can become hostile as an individual could use an exchange to build a debt in their favor.
- Balanced reciprocity (or also known as Symmetrical reciprocity) is when exchange is made with the expectations that those who give an amount will get the same in return. This, unlike generalized reciprocity, has a specified time limit as to when the return should be made. The Ju/’hoansi, who use reciprocity in their societies in all forms, use balanced reciprocity. They distinguish between what they barter, which requires immediate balanced exchange (this is similar to our shopping experiences, where it's expected that money will be immediately exchanged for goods). Within the Ju/'hoansi, this also includes hxaro, which establishes that this exchange entitles obligations between the two in the future.
- Negative reciprocity is when a party tries to exchange without having to give up any value, which is the opposite of balanced exchange. "This can range from haggling prices to outright seizure." [12]
Reciprocity, the most ancient mode of exchange, is the exchange of goods and services of equal value. Generalized reciprocity can be defined as when the individuals involved just assume that the exchange will balance out. Nothing is expected immediately and a value of return is not established before the exchange is made. This type of reciprocity occurs often between parents and children. Balanced reciprocity, the opposite of generalized, is when a specific value of return and under an established time limit is expected. This exchange can be found between those in relationships. For example, when boyfriends and girlfriends exchange gifts of equal value and expect the same in return at Christmas. Negative reciprocity is the exchange of goods or services when at least one party attempts receive something for nothing in return without suffering consequences. This type of reciprocity can involve haggling or in some cases seizure.
Redistribution
Redistribution is a mode of exchange that involves some sort of centralized social organization. Members of a group contribute items such as food, money, clothing, etc. to the central organization, and the organization then redistributes the items to the members of the group. An example of this on a humanitarian level could be Goodwill, a food bank, etc.
Redistribution can occur on a small scale or a very large scale. A small scale example of redistribution is a class party. Each person is assigned something to bring – chips, salsa, pop, brownies, napkins, utensils, etc. On the day of the party, everyone brings in their items to share with each other. The Salvation Army is a good example of a rather medium-sized scale of redistribution. The Salvation Army collects money, clothing, household goods, cars, and even airline miles to redistribute to those in need. A large scale example of redistribution is the Internal Revenue Service. The IRS collects taxes from citizens and redistributes the money throughout our governmental system, to education, post offices, road construction, and the like.
One widespread local example of redistribution is church potlucks. For special events, several churches hold potlucks, (potlucks with noteworthy Native American origins). Every family contributes to the meal. All the contributions are placed in a central area, and when everyone has arrived, the congregation can eat a meal together from the assortment of dishes.
Another example of this is under Big Man/Big Woman political groupings in the South Pacific where the leader, chosen by favor, is in charge of general affairs, and collects a certain sum (i.e. a pig) from their group and redistributes it. This gives the Big Man/Big Woman the label of generous, although they personally don’t give any more than anyone else. This is also an example of the tradition of potlatch, where group members all give goods to one, who evenly distributes these goods among the community. The main point of this is to redistribute wealth.
Redistribution requires some form of centralized social organization. Those who own the central position of the organization receive economic contributions from all members of the group. With the contributions they receive from all members of the group they redistribute those goods to all the members of the groups in fair amounts to meet the needs of every member of the group. A potlatch is a good example of redistribution. When people go an event and are provided with food they then take that food and redistribute it to all members of their family or some kind of group they belong to. An example of this is the indigenous Americans of the northwest coast of North America. This is a very common mode of exchange among tribes and groups in all part of the world. It is a fair and normally well organized mode of exchange and valued by the members of most tribes and groups.
Potlatching
Potlatches are ceremonies held by First Nations peoples on the Pacific Northwest coast of the United States and Canadian province of British Columbia such as the Haida, Tlingit, Salish and Kwakiutl (Kwakwaka'kawakw). The potlatch takes the form of a ceremonial feast traditionally featuring seal meat or salmon. In it, hierarchical relations between groups were observed and reinforced through the exchange of gifts and other ceremonies. The potlatch is an example of a gift economy, whereby the host demonstrates their wealth and prominence through giving away their possessions and thus prompt participants to reciprocate when they hold their own potlatch. Although this sort exchange is widely practiced across the planet (consider, for example, the Western practice of buying one's friends rounds of drinks), the Northwest Coast potlatch is of a massive scale, and also served to redistribute goods from coastal to inland ecological zones.
The native peoples of the Northwest Coast of North America institutionalized this ceremonial redistribution of food and gifts. The southern Kwakiutl people were the most elaborative on this custom until 1904 when the potlatch was outlawed, however the ceremony did continue to be practiced in many societies. In 2004, the Tlingit clan members re-enacted the ritual in Sitka, Alaska, for the 100th Anniversary Commemoration of "The Last Potlatch". The clan members dressed in traditional Tlingit attire and practiced Tlingit traditions for the two day long celebration.
Market Exchange
Market Exchange is used in Capitalist societies and relies on private trade of goods and services. Value is assigned based on a standard symbol, typically money. Although trade and money were developed independently, they are used together to create market exchange. This is the dominant mode of exchange in Western Societies.
Market Exchange was invented by the capitalist society that uses an economic system in which wealth are privately owned rather than commonly, publicly, or state-owned. It is where currency exchange takes place. It is where banks and other official institutions facilitate the buying and selling of foreign currencies. Trade, money, and market institutions developed independently and were not invented to work together. Capitalism is unique because the three (trade, money, and market) ended up working together. This was first done in the societies of early modern Europe. It is said that different modes of exchange often co-exist within a single society meaning that each society has their own way of operating and exchanging good in their day-to-day lives.
For example, in the United States we use the market mode or exchange, but you can still find redistribution and reciprocity. Reciprocity if you recall is the most ancient mode of exchange, was the exchange of goods and services of equal value and redistribution requires some form of centralized social organization. In families in the U.S., most parents have an income and then redistribute that income to their children and loved ones. Parents using their income to buy their children food and clothing without expecting return is an example of reciprocity. Some people believe that you cannot properly understand the exchange process without first fully understanding the production process.[19]
Currency
Currency is a system of money that is used as a medium of exchange such as banknotes, coins, or digital currency. Systems of currency, such as the Euro, U.S. dollar, bitcoin, etc., are often produced and monitored by governments. Governments often give value to banknotes, referred to by some as fiat currency. The values of the independent currencies are often influenced by the economy in a global market driven by trade and foreign exchange markets. Usually monetary values remain within the boundaries of its intended nations and states.
Alternative Currency
Some forms of currency are given value by the global market and have no tie to a state, namely commodities such as gold, silver, and oil as well as stocks, bonds, derivatives, and cryptocurrencies. These currencies can be traded on the global markets, some being traded at every hour of the day. Physical commodities are given value by their practical uses, appearance, and even superstitions, regardless, they are transacted with and accepted as payment.
Bitcoin is a form of cryptocurrency and it is given value via the inherent principle of scarcity programmed at its core. Bitcoin is limited by computing power of its competitive network and the collaborative confirmation by the millions of nodes in its network of every single transaction. Each in every bitcoin transaction is in a sense, public information.
A more familiar type of alternative currency is currency that can only be used within a certain brand or company such a "Starbucks Stars". This is an example of currency brought about by a nation recently formed in the Pacific Northwest. Not all forms of currency are ancient and currency that begins from a company can be recognized worldwide.


20.6: Plane Electromagnetic Waves
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LEARNING OBJECTIVES
By the end of this section, you will be able to:
- Describe how Maxwell’s equations predict the relative directions of the electric fields and magnetic fields, and the direction of propagation of plane electromagnetic waves
- Explain how Maxwell’s equations predict that the speed of propagation of electromagnetic waves in free space is exactly the speed of light
- Calculate the relative magnitude of the electric and magnetic fields in an electromagnetic plane wave
- Describe how electromagnetic waves are produced and detected
Mechanical waves travel through a medium such as a string, water, or air. Perhaps the most significant prediction of Maxwell’s equations is the existence of combined electric and magnetic (or electromagnetic) fields that propagate through space as electromagnetic waves. Because Maxwell’s equations hold in free space, the predicted electromagnetic waves, unlike mechanical waves, do not require a medium for their propagation.
A general treatment of the physics of electromagnetic waves is beyond the scope of this textbook. We can, however, investigate the special case of an electromagnetic wave that propagates through free space along the x -axis of a given coordinate system.
Electromagnetic Waves in One Direction
An electromagnetic wave consists of an electric field, defined as usual in terms of the force per charge on a stationary charge, and a magnetic field, defined in terms of the force per charge on a moving charge. The electromagnetic field is assumed to be a function of only the x -coordinate and time. The y -component of the electric field is then written as \(E_y (x,t)\), the z -component of the magnetic field as \(B_z (x,t)\), etc. Because we are assuming free space, there are no free charges or currents, so we can set \(Q_{in} = 0\) and \(I = 0\) in Maxwell’s equations.
The transverse nature of electromagnetic waves
We examine first what Gauss’s law for electric fields implies about the relative directions of the electric field and the propagation direction in an electromagnetic wave. Assume the Gaussian surface to be the surface of a rectangular box whose cross-section is a square of side l and whose third side has length \(\Delta x\), as shown in Figure \(\PageIndex{1}\). Because the electric field is a function only of x and t , the y -component of the electric field is the same on both the top (labeled Side 2) and bottom (labeled Side 1) of the box, so that these two contributions to the flux cancel. The corresponding argument also holds for the net flux from the z -component of the electric field through Sides 3 and 4. Any net flux through the surface therefore comes entirely from the x -component of the electric field. Because the electric field has no y - or z -dependence, \(E_x(x,t)\) is constant over the face of the box with area A and has a possibly different value \(E_x (x + \Delta x, t)\) that is constant over the opposite face of the box.
Applying Gauss’s law gives
\[\text{Net flux} = - E_x (x,t) A + E_x (x + \Delta x, t) A = \dfrac{Q_{in}}{\epsilon_0} \label{16.13} \]
where \(A = l \times l\) is the area of the front and back faces of the rectangular surface. But the charge enclosed is \(Q_{in} = 0\), so this component’s net flux is also zero, and Equation \ref{16.13} implies \(E_x (x,t) = E_x (x + \Delta x, t)\) for any \(\Delta x\). Therefore, if there is an x -component of the electric field, it cannot vary with x . A uniform field of that kind would merely be superposed artificially on the traveling wave, for example, by having a pair of parallel-charged plates. Such a component \(E_x(x,t)\) would not be part of an electromagnetic wave propagating along the x -axis; so \(E_x(x,t) = 0\) for this wave. Therefore, the only nonzero components of the electric field are \(E_y(x,t)\) and \(E_z(x,t)\) perpendicular to the direction of propagation of the wave.
A similar argument holds by substituting E for B and using Gauss’s law for magnetism instead of Gauss’s law for electric fields. This shows that the B field is also perpendicular to the direction of propagation of the wave. The electromagnetic wave is therefore a transverse wave, with its oscillating electric and magnetic fields perpendicular to its direction of propagation.
The speed of propagation of electromagnetic waves
We can next apply Maxwell’s equations to the description given in connection with Figure 16.2.3 in the previous section to obtain an equation for the E field from the changing B field, and for the B field from a changing E field. We then combine the two equations to show how the changing E and B fields propagate through space at a speed precisely equal to the speed of light.
First, we apply Faraday’s law over Side 3 of the Gaussian surface, using the path shown in Figure \(\PageIndex{2}\). Because \(E_x(x,t) = 0\), we have
\[\oint \vec{E} \cdot d\vec{s} = - E_y(x,t)l + E_y(x + \Delta x,t)l. \nonumber \]
Assuming \(\Delta x\) is small and approximating \(E_y (x + \Delta x,t)\) by
\[E_y (x + \Delta x,t) = E_y (x,t) + \dfrac{\partial E_y(x,t)}{\partial x} \Delta x, \nonumber \]
we obtain
\[\oint \vec{E} \cdot d\vec{s} = \dfrac{\partial E_y (x,t)}{\partial x} (l\Delta x). \nonumber \]
Because \(\Delta x\) is small, the magnetic flux through the face can be approximated by its value in the center of the area traversed, namely \(B_z\left(x + \dfrac{\Delta x}{2}, t\right)\). The flux of the B field through Face 3 is then the B field times the area,
\[\oint_S \vec{B} \cdot \vec{n} dA = B_z \left(x + \dfrac{\Delta x}{2}, t\right) (l \Delta x). \label{16.14} \]
From Faraday’s law ,
\[\oint \vec{E} \cdot d\vec{s} = -\dfrac{d}{dt} \int_S \vec{B} \cdot \vec{n} dA.\label{16.15} \]
Therefore, from Equations \ref{16.13} and \ref{16.14},
\[\dfrac{\partial E_y (x,t)}{\partial x} (l \Delta x) = - \dfrac{\partial}{\partial t} \left[ B_z \left( x + \dfrac{\Delta x}{2}, t\right) \right] (l\Delta x). \nonumber \]
Canceling \(l \Delta x\) and taking the limit as \(\Delta x = 0\), we are left with
\[\dfrac{\partial E_y (x,t)}{\partial x} = - \dfrac{\partial B_z(x,t)}{\partial t}. \label{16.16} \]
We could have applied Faraday’s law instead to the top surface (numbered 2) in Figure \(\PageIndex{2}\), to obtain the resulting equation
\[\dfrac{\partial B_z(x,t)}{\partial t} = - \dfrac{\partial E_y (x,t)}{\partial x}. \label{16.17} \]
This is the equation describing the spatially dependent E field produced by the time-dependent B field.
Next we apply the Ampère-Maxwell law (with \(I = 0\)) over the same two faces (Surface 3 and then Surface 2) of the rectangular box of Figure \(\PageIndex{2}\). Applying Equation 16.2.16 ,
\[\oint \vec{B} \cdot d\vec{s} = \mu_0 \epsilon_0 (d/dt) \int_S \vec{E} \cdot n \, da \nonumber \]
to Surface 3, and then to Surface 2, yields the two equations
\[\dfrac{\partial E_y (x,t)}{\partial x} = - \epsilon_0 \mu_0 \dfrac{\partial E_z (x,t)}{\partial t}, \label{16.18} \]
and
\[\dfrac{\partial B_z(x,t)}{\partial x} = - \epsilon_0 \mu_0 \dfrac{\partial E_y (x,t)}{\partial t}. \label{16.19} \]
These equations describe the spatially dependent B field produced by the time-dependent E field.
We next combine the equations showing the changing B field producing an E field with the equation showing the changing E field producing a B field. Taking the derivative of Equation \ref{16.16} with respect to x and using Equation \ref{16.26} gives
\[\dfrac{\partial^2E_y}{\partial x^2} = \dfrac{\partial}{\partial x}\left(\dfrac{\partial E_y}{\partial x}\right) = - \dfrac{\partial}{\partial x}\left(\dfrac{\partial B_z}{\partial t}\right) = - \dfrac{\partial}{\partial t}\left(\dfrac{\partial B_z}{\partial x}\right) = \dfrac{\partial}{\partial t}\left(\epsilon_0 \mu_0 \dfrac{\partial E_y}{\partial t}\right) \nonumber \]
or
\[\dfrac{\partial^2E_y}{\partial x^2} = \epsilon_0 \mu_0 \dfrac{\partial^2 E_y}{\partial t^2} \nonumber \]
This is the form taken by the general wave equation for our plane wave. Because the equations describe a wave traveling at some as-yet-unspecified speed c , we can assume the field components are each functions of x – ct for the wave traveling in the + x -direction, that is,
\[E_y (x,t) = f(\xi) \, where \, \xi = x - ct. \label{16.21} \]
It is left as a mathematical exercise to show, using the chain rule for differentiation, that Equations \ref{16.17} and \ref{16.18} imply
\[1 = \epsilon_0 \mu_0 c^2. \nonumber \]
The speed of the electromagnetic wave in free space is therefore given in terms of the permeability and the permittivity of free space by
\[c = \dfrac{1}{\sqrt{\epsilon_0\mu_0}}. \label{16.22} \]
We could just as easily have assumed an electromagnetic wave with field components \(E_z (x,t)\) and \(B_y (x,t)\). The same type of analysis with Equation \ref{16.25} and \ref{16.24} would also show that the speed of an electromagnetic wave is \(c = 1/\sqrt{\epsilon_0\mu_0}\).
The physics of traveling electromagnetic fields was worked out by Maxwell in 1873. He showed in a more general way than our derivation that electromagnetic waves always travel in free space with a speed given by Equation \ref{16.18}. If we evaluate the speed \(c = \dfrac{1}{\sqrt{\epsilon_0\mu_0}}\), we find that
\[c = \dfrac{1}{\sqrt{\left(8.85 \times 10^{-12} \dfrac{C^2}{N \cdot m^2}\right)\left(4\pi \times 10^{-7} \dfrac{T \cdot m}{A}\right)}} = 3.00 \times 10^8 m/s, \nonumber \]
which is the speed of light . Imagine the excitement that Maxwell must have felt when he discovered this equation! He had found a fundamental connection between two seemingly unrelated phenomena: electromagnetic fields and light.
Exercise \(\PageIndex{1}\)
The wave equation was obtained by (1) finding the E field produced by the changing B field, (2) finding the B field produced by the changing E field, and combining the two results. Which of Maxwell’s equations was the basis of step (1) and which of step (2)?
- Answer (step 1)
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Faraday’s law
- Answer (step 2)
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the Ampère-Maxwell law
How the E and B Fields Are Related
So far, we have seen that the rates of change of different components of the E and B fields are related, that the electromagnetic wave is transverse, and that the wave propagates at speed c . We next show what Maxwell’s equations imply about the ratio of the E and B field magnitudes and the relative directions of the E and B fields.
We now consider solutions to Equation \ref{16.16} in the form of plane waves for the electric field:
\[E_y(x,t) = E_0 \, \cos \, (kx - \omega t). \label{16.23} \]
We have arbitrarily taken the wave to be traveling in the +x -direction and chosen its phase so that the maximum field strength occurs at the origin at time \(t = 0\). We are justified in considering only sines and cosines in this way, and generalizing the results, because Fourier’s theorem implies we can express any wave, including even square step functions, as a superposition of sines and cosines.
At any one specific point in space, the E field oscillates sinusoidally at angular frequency \(\omega\) between \(+E_0\) and \(-E_0\) and similarly, the B field oscillates between \(+B_0\) and \(-B_0\). The amplitude of the wave is the maximum value of \(E_y(x,t)\). The period of oscillation T is the time required for a complete oscillation. The frequency f is the number of complete oscillations per unit of time, and is related to the angular frequency \(\omega\) by \(\omega = 2\pi f\). The wavelength \(\lambda\) is the distance covered by one complete cycle of the wave, and the wavenumber k is the number of wavelengths that fit into a distance of \(2\pi\) in the units being used. These quantities are related in the same way as for a mechanical wave:
\[\omega = 2\pi f, \, \, f = \dfrac{1}{T}, \, \, k = \dfrac{2\pi}{\lambda}, \, \, and \, \, c = f\lambda = \omega/k. \nonumber \]
Given that the solution of \(E_y\) has the form shown in Equation \ref{16.20}, we need to determine the \(B\) field that accompanies it. From Equation \ref{16.24}, the magnetic field component \(B_z\) must obey
\[\dfrac{\partial B_z}{\partial t} = - \dfrac{\partial E_y}{\partial x} \nonumber \]
\[\dfrac{\partial B_z}{\partial t} = - \dfrac{\partial}{\partial x} E_0 \, \cos \, (kx - \omega t) = kE_0 \, sin\, (kx - \omega t). \label{16.24} \]
Because the solution for the B -field pattern of the wave propagates in the + x -direction at the same speed c as the E- field pattern, it must be a function of \(k(x - ct) = kx - \omega t\). Thus, we conclude from Equation \ref{16.21} that \(B_z\) is
\[B_z(x,t) = \dfrac{k}{\omega} E_0 \, \cos \, (kx - \omega t) = \dfrac{1}{c}E_0 \, \cos \, (kx - \omega t). \nonumber \]
These results may be written as
\[E_y(x,t) = E_0 \, \cos \, (kx - \omega t) \nonumber \]
\[B_z(x,t) = B_0 \, \cos \, (kx - \omega t) \label{16.25} \]
\[\dfrac{E_y}{B_z} = \dfrac{E_0}{B_0} = c. \label{16.26} \]
Therefore, the peaks of the E and B fields coincide, as do the troughs of the wave, and at each point, the E and B fields are in the same ratio equal to the speed of light c . The plane wave has the form shown in Figure \(\PageIndex{3}\).
Example \(\PageIndex{1}\): Calculating B-Field Strength in an Electromagnetic Wave
What is the maximum strength of the B field in an electromagnetic wave that has a maximum E -field strength of 1000 V/m?
Strategy
To find the B -field strength, we rearrange Equation \ref{16.23} to solve for \(B\), yielding
\[B = \dfrac{E}{c}. \nonumber \]
Solution We are given E , and c is the speed of light. Entering these into the expression for B yields
\[B = \dfrac{1000 \, V/m}{3.00 \times 10^8 \, m/s} = 3.33 \times 10^{-6} T. \nonumber \]
Significance
The B -field strength is less than a tenth of Earth’s admittedly weak magnetic field. This means that a relatively strong electric field of 1000 V/m is accompanied by a relatively weak magnetic field.
Changing electric fields create relatively weak magnetic fields. The combined electric and magnetic fields can be detected in electromagnetic waves, however, by taking advantage of the phenomenon of resonance, as Hertz did. A system with the same natural frequency as the electromagnetic wave can be made to oscillate. All radio and TV receivers use this principle to pick up and then amplify weak electromagnetic waves, while rejecting all others not at their resonant frequency.
Exercise \(\PageIndex{2}\)
What conclusions did our analysis of Maxwell’s equations lead to about these properties of a plane electromagnetic wave:
- the relative directions of wave propagation, of the E field, and of B field,
- the speed of travel of the wave and how the speed depends on frequency, and
- the relative magnitudes of the E and B fields.
- Answer a
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The directions of wave propagation, of the E field, and of B field are all mutually perpendicular.
- Answer b
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The speed of the electromagnetic wave is the speed of light \(c = 1/\sqrt{\epsilon_0\mu_0}\) independent of frequency.
- Answer c
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The ratio of electric and magnetic field amplitudes is \(E/B = c\).
Production and Detection of Electromagnetic Waves
A steady electric current produces a magnetic field that is constant in time and which does not propagate as a wave. Accelerating charges, however, produce electromagnetic waves. An electric charge oscillating up and down, or an alternating current or flow of charge in a conductor, emit radiation at the frequencies of their oscillations. The electromagnetic field of a dipole antenna is shown in Figure \(\PageIndex{4}\). The positive and negative charges on the two conductors are made to reverse at the desired frequency by the output of a transmitter as the power source. The continually changing current accelerates charge in the antenna, and this results in an oscillating electric field a distance away from the antenna. The changing electric fields produce changing magnetic fields that in turn produce changing electric fields, which thereby propagate as electromagnetic waves. The frequency of this radiation is the same as the frequency of the ac source that is accelerating the electrons in the antenna. The two conducting elements of the dipole antenna are commonly straight wires. The total length of the two wires is typically about one-half of the desired wavelength (hence, the alternative name half-wave antenna ), because this allows standing waves to be set up and enhances the effectiveness of the radiation.
The electric field lines in one plane are shown. The magnetic field is perpendicular to this plane. This radiation field has cylindrical symmetry around the axis of the dipole. Field lines near the dipole are not shown. The pattern is not at all uniform in all directions. The strongest signal is in directions perpendicular to the axis of the antenna, which would be horizontal if the antenna is mounted vertically. There is zero intensity along the axis of the antenna. The fields detected far from the antenna are from the changing electric and magnetic fields inducing each other and traveling as electromagnetic waves. Far from the antenna, the wave fronts, or surfaces of equal phase for the electromagnetic wave, are almost spherical. Even farther from the antenna, the radiation propagates like electromagnetic plane waves.
The electromagnetic waves carry energy away from their source, similar to a sound wave carrying energy away from a standing wave on a guitar string. An antenna for receiving electromagnetic signals works in reverse. Incoming electromagnetic waves induce oscillating currents in the antenna, each at its own frequency. The radio receiver includes a tuner circuit, whose resonant frequency can be adjusted. The tuner responds strongly to tshe desired frequency but not others, allowing the user to tune to the desired broadcast. Electrical components amplify the signal formed by the moving electrons. The signal is then converted into an audio and/or video format.
Note
Use this simulation to broadcast radio waves. Wiggle the transmitter electron manually or have it oscillate automatically. Display the field as a curve or vectors. The strip chart shows the electron positions at the transmitter and at the receiver.


1.6: Documentation for IV Initiation
Documentation Cues:
Accurate and complete documentation regarding IV initiation should include the following:
- Date/Time of procedure
- Manufacturer’s brand name of device
- Gauge and length of device
- Location of the accessed vein
- Use of local anesthetic
- Number of attempts for a successful IV start
- Description of the insertion site, such as “cephalic vein on dorsal surface of right lower arm, 2.5 cm (1 inch) above the wrist”
- Condition of extremity and IV site
- Type of dressing with which the cannula was secured
- Patient’s tolerance of the procedure and patient education provided
- If saline lock was established or if fluid was infused after IV initiation. If fluid was infused, the method of infusion (gravity or infusion pump), type and rate of infusion should be included.
- Patient’s status and integrity and patency of the system according to agency policy
Sample Documentation:
12/7/20XX 0845
Obtained peripheral IV access as ordered by provider. A suitable vein was identified in the patient’s right hand, and the site was cleansed with chlorhexidine per protocol. A 20-gauge 1-inch Protect IV was placed in the patient’s right hand. The vein was cannulated with one attempt, and no complications were noted during cannulation. The catheter was freely threaded into the vein after blood return was noted, and the site was flushed with normal saline and clamped. The site was dressed with a sterile tegaderm dressing and extension set tubing was secured. Following application of the dressing, the site was saline locked. The patient tolerated the procedure well with no signs of redness, swelling, or other complications.
Janika Smith, RN


21.3: Postpartum Hemorrhage
By the end of this section, you will be able to:
- Differentiate between immediate (early) postpartum hemorrhage and secondary (late) postpartum hemorrhage
- Analyze the risk factors and causes of postpartum hemorrhage
- Explain the management of a postpartum hemorrhage
- Develop a nursing diagnosis that would be appropriate for a person experiencing a postpartum hemorrhage
Postpartum hemorrhage (PPH) is one of the most common complications of birth, occurring after 1 percent to 5 percent of births. PPH is defined by the American College of Obstetricians and Gynecologists (ACOG; 2017) as 1,000 mL of blood loss within 24 hours of birth for both vaginal and cesarean births. However, if 500 mL of blood is lost after a vaginal birth, this indicates a need for intervention.
Detection of Postpartum Hemorrhage
Labor and postpartum nurses must be aware of risk factors, prevention, and treatment of PPH. Risk factors include large for gestational age (LGA) infant, multiple gestation, grand multiparity, prolonged labor, chorioamnionitis , prolonged or precipitous labor, and placenta previa or placental abruption. Facilities have instituted a PPH risk-assessment tool to be used in recognizing those at risk for PPH (Faysal et al., 2023). This tool allows the nurse to prepare and monitor for a potential PPH. Some providers practice active management of the third stage of labor, meaning the health-care provider may apply umbilical cord traction to deliver the placenta quickly, order oxytocin (Pitocin) to be administered after the birth of the placenta, and begin uterine massage after delivery of the placenta (ACOG, 2017). Estimating blood loss is difficult because blood is mixed with amniotic fluid. The nurse plays a role in early detection of PPH by performing quantitative blood loss (QBL) , the process of weighing and measuring the amount of blood lost during the birth and immediately postpartum. Many birthing facilities have a rapid response or hemorrhage team the nurse can activate during PPH cases. A multidisciplinary team is vital in treating PPH and preventing maternal mortality.
The Association of Women’s Health, Obstetric and Neonatal Nurses ( AWHONN ) provides guidelines for quantifying blood loss during childbirth. Accurate measurement of blood loss is crucial for identifying postpartum hemorrhage and ensuring timely interventions. This video describes how to accurately quantify blood loss during childbirth.
Morbidity and Mortality Specific to Postpartum Hemorrhage in the United States
Approximately 11 percent of maternal deaths in the United States are caused by PPH; unfortunately, 54 percent to 93 percent of those deaths may have been preventable (ACOG, 2019). ACOG notes that some preventable cases are due to health-care providers underestimating the amount of blood loss and delaying treatment of PPH. This led to ACOG (2019) recommending that quantitative blood loss be assessed at all births.
Global Results of Postpartum Hemorrhage
Postpartum hemorrhage causes 25 percent of all maternal deaths globally and is the leading cause of maternal mortality in low-income countries (Yeshitila et al., 2021). In low-income countries such as Ethiopia, delay in receiving care for obstetric complications is a major risk factor for maternal mortality. Non-pneumatic anti-shock garment s ( NASG s) are used in low-resource areas to prevent shock and death from postpartum hemorrhage. NASGs are neoprene and Velcro devices applied to the legs, similar to a sequential compression device (Figure 21.3). The NASG is then applied to the pelvis and abdomen to compress those areas and increase blood flow to the heart, lungs, and brain. This device can prevent hypovolemic shock while transporting the postpartum person to a higher level of care. In Ethiopia, the NASG was introduced in 2011 and has reduced maternal deaths from postpartum hemorrhage by 79 percent (Yeshitila et al., 2021). In low-income areas, this device has made an overwhelming difference for birthing women.
Early (Primary) Postpartum Hemorrhage
Postpartum hemorrhage that occurs after the birth of the placenta up to 24 hours postpartum is considered early PPH or primary PPH . It is more common than secondary, or late, PPH. Common causes of PPH are the four T’s—tone, trauma, tissue, and thrombin—with uterine atony being the most common cause of early PPH (Salati et al., 2019). Because of the increased blood volume of the pregnant person, classic signs of hypovolemia may not occur until a significant hemorrhage has occurred. See Chapter 19 Complications of Labor and Birth for further discussion of early (primary) postpartum hemorrhage.
Late (Secondary) Postpartum Hemorrhage
Postpartum hemorrhage that occurs after the first 24 hours up to 12 weeks postpartum is considered late PPH or secondary PPH . The most common cause of late PPH is subinvolution , which is the inability of the uterus to return to its original size. At the 6-week follow-up, subinvolution is diagnosed when the patient’s uterus is larger than expected and the lochia has not progressed to lochia alba. Retained placenta or membranes can be another cause of late PPH and can also lead to infection and bleeding. Just as coagulation disorders can cause early PPH, these disorders can also cause late PPH. The nurse will give the patient anticipatory guidance and signs of when to call their health-care provider if late PPH were to occur.
Recognizing Postpartum Hemorrhage
Nurses caring for postpartum patients need to monitor for and recognize the signs and symptoms of postpartum hemorrhage before the patient is compromised. Two important nursing actions are monitoring blood loss and recognizing the signs and symptoms of hypovolemia. Both of these actions provide nurses with cues to diagnose fluid volume deficit .
Quantitative Blood Loss
Measuring quantitative blood loss (QBL) has been recommended for all births by ACOG (2019) because estimated blood loss is inaccurate. Research has shown that blood loss is underestimated with larger blood loss and overestimated with smaller blood loss (ACOG, 2019). To perform QBL, birthing facilities must provide scales for weighing sponges and drapes that allow quantification of blood loss. Research has also found that facilities using QBL have reduced rates of maternal morbidity, decreased time until escalation of care, and increased team awareness of the need for emergency intervention (AWHONN, 2021).
Determining Quantitative Blood Loss
The Association of Women’s Health, Obstetric and Neonatal Nurses (AWHONN) recommends the following education and supplies be utilized for determining QBL:
- using standardized items to calculate QBL
- ensuring all labor and postpartum rooms and operative suites have access to scales and PPH carts
- attaching a laminated list of the dry weight of sponges and pads to all scales for accurate measurement
- ensuring electronic charting automatically deducts dry weights from wet weights
- providing laminated staging algorithms in each room or on the PPH cart
- practicing simulation drills for PPH with all members of the interprofessional team
- debriefing after each PPH to identify areas of needed education
(AWHONN, 2021)
Signs and Symptoms of Hypovolemia
The blood loss from postpartum hemorrhage results in hypovolemia . Early symptoms and signs of hypovolemia include feeling weak, dizzy, and anxious; decreased blood pressure; and increased pulse and respiratory rates. Severe hypovolemia causes tachycardia , hypotension, decreased oxygen saturation, thirst, restlessness, low body temperature, and decreased urinary output (Figure 21.4). The postpartum person becomes pale and diaphoretic, confused, and loses consciousness as the hypovolemia becomes critical. The pregnant person has 50 percent more blood volume than the nonpregnant person (Agrawal et al., 2018). As a result, the postpartum person may not exhibit signs of hypovolemia until they have lost a significant amount of blood. Therefore, the nurse monitors blood loss closely for all postpartum patients, especially for those persons at higher risk for PPH.
Causes of Postpartum Hemorrhage
As mentioned earlier, the causes of postpartum hemorrhage can be summarized using the four T’s of tone, trauma, tissue, and thrombin. See Chapter 19 Complications of Labor and Birth for further discussion of the four Ts .
Tone
Tone refers to the tone of the uterus. To stop bleeding from the placental site, the uterine muscles contract to strangulate the spiral arteries that are feeding blood to the placenta. If the uterine muscles are not contracted, the fundus is soft and boggy, and the spiral arteries are free to pump blood and cause rapid blood loss. Risk factors associated with PPH secondary to uterine atony include a large number of previous births, chorioamnionitis , prolonged labor with oxytocin (Pitocin) use, general anesthesia, macrosomic babies, multiple gestations, and polyhydramnios. A full bladder can also impede the uterus from contracting (Milton, 2024).
Trauma
Trauma refers to lacerations in the reproductive tract, uterine rupture, hematomas, and uterine inversion. All these circumstances can cause rapid blood loss. Hematomas might not be identified immediately, but the postpartum person will report increased pain, and vital sign s will show a decrease in blood pressure and an increase in heart rate. Risk factors for PPH secondary to trauma include precipitous birth, delivery of an LGA infant, poor nutritional status, use of oxytocin during labor, uterine tachysystole, and the use of vacuum extractor or obstetric forceps. Risk factors leading to uterine inversion are excessive traction on the umbilical cord and fundal placenta implantation (Milton, 2024).
Tissue
Tissue refers to retained placental tissue.  This tissue interrupts the uterus from contracting and leads to PPH. Risk factors for retained placenta are early preterm birth (less than 32 weeks’ gestation), extra placental lobes, placenta accreta, and a history of uterine surgery. Retained tissue can also lead to postpartum infection (Shields et al., 2023).
Thrombin
Thrombin refers to coagulation disorders. Disorders in the activation or amount of thrombin include von Willebrand disease and thrombocytopenia . Other bleeding disorders include lack of required features of the clotting cascade (ACOG, 2017). Disseminated intravascular coagulation (DIC) can also lead to PPH because of the inability of the blood to clot. See Chapter 19 Complications of Labor and Birth for further discussion of DIC. Risk factors for coagulation issues are intrauterine fetal demise, placental abruption, sepsis , DIC, and genetic coagulation disease (ACOG, 2017).
Management of Postpartum Hemorrhage
Management of PPH differs for early and late PPH. Late PPH is managed in the office or perhaps in the emergency department and many times is treated with antibiotics or methylergonovine (Methergine). If retained placental products are suspected, dilation and curettage may be performed. Management of early PPH occurs in the birthing room and will depend on the cause of the hemorrhage. Because uterine atony is the most common cause of early PPH, interventions to induce uterine contractions are quickly performed.
Fundal Massage
Fundal massage is the technique of massaging the uterus to stimulate contractions to decrease postpartum bleeding. The nurse places one hand on the fundus and the other hand guards the uterus. The hand on the fundus massages until the uterus becomes firm. When the uterus contracts, it stops the blood from being pumped by the spiral arteries (Bilgin & Komurcu, 2020). This is almost always the first intervention for postpartum bleeding. Figure 21.5 demonstrates fundal massage .
Oxytocin
Oxytocin (Pitocin) is the first-line medication to treat PPH. The usual dose is 10 to 40 units in 500 to 1,000 mL of intravenous (IV) fluid (ACOG, 2017). During PPH, oxytocin is infused as a bolus. It can also be administered intramuscularly (IM) with the usual dose of 10 units when IV access is not available. The goal is to stimulate contractions to treat uterine atony. The nurse explains to the postpartum person that strong uterine contractions will occur due to oxytocin administration but that the medication will treat or prevent PPH. See Chapter 19 Complications of Labor and Birth for further information regarding oxytocin.
Misoprostol
Misoprostol (Cytotec) is a prostaglandin used for cervical ripening or labor induction. It is also used to treat PPH because it induces uterine contractions. The usual dose for PPH is 600 to 1,000 mcg orally, rectally, or sublingually (ACOG, 2017). Even though it is a prostaglandin, misoprostol does not cause reactive airway constriction or an asthma flare. It can be given to persons with hypertension. Therefore, it is a safe prostaglandin to be administered to most postpartum persons. Misoprostol can, however, cause nausea, vomiting, diarrhea, chills, some rise in blood pressure, and fever. These symptoms are not severe and resolve over several hours.
Methylergonovine
Methylergonovine ( Methergine ) is also a uterotonic. It produces sustained uterine contractions to help treat PPH. It is the only uterotonic that causes sustained versus rhythmic contractions. The usual dose is 0.2 mg IM for early PPH (ACOG, 2017). It should not be given IV. It should also not be given to persons with hypertension or preeclampsia. Methylergonovine can be used for late PPH and subinvolution, but it is administered orally for several days. The side effects of methylergonovine include nausea, vomiting, and hypertension.
Carboprost
Carboprost (Hemabate) is a strong prostaglandin that induces uterine contractions. The usual dose is 250 mcg IM (ACOG, 2017). It can also be administered into the muscle of the uterus by the health-care provider in a severe case of PPH. Carboprost has serious side effects, and the nurse should inform the postpartum person that the following side effects may occur: nausea, vomiting, fever, diarrhea, chills, and bronchospasm. The nurse may medicate the person with an antipyretic and antidiarrheal, as prescribed, to possibly prevent these side effects. Carboprost should not be given to a person with asthma due to its bronchospasm side effect.
Tranexamic Acid
Tranexamic acid , or TXA (Cyklokapron), is an antifibrinolytic that can reduce mortality from PPH by reducing complications from the clotting cascade (ACOG, 2017). TXA blocks the breakdown of clots; prevents bleeding complications, such as DIC; and has very few side effects. The usual dose is 1 g in 100 mL of IV fluid administered over 10 minutes. It should be given within 3 hours of birth and is prescribed in conjunction with a uterotonic.
Intrauterine Tamponade
When PPH continues despite uterotonic administration and bimanual massage, compression of the uterus through bimanual compression or intrauterine tamponade may be the next intervention; the uterine tamponade system is inserted by the health-care provider (ACOG, 2017). Two systems of intrauterine tamponade are available in the United States, the Bakri Balloon and the Jada System. If neither is available, some health-care providers will pack the uterus with gauze. The Bakri Balloon is designed to be used as a tamponade during a PPH (Figure 21.6). The balloon is placed in the uterus and inflated by the health-care provider. Acting as a tamponade, the balloon compresses the inside of the uterus to stop the bleeding.
The Jada System is a device that uses an intrauterine vacuum to contract the uterus around the device to stop the bleeding. It is also placed by a health-care provider. The system is then connected to a vacuum device or wall suction (Figure 21.7).
Hysterectomy
Hysterectomy is the last procedure if all other interventions to control a PPH have failed.  Because the uterus is removed, a hysterectomy results in permanent sterilization (Kallianidis et al., 2021). The risk of surgical complications is increased, including infection, further blood loss, bladder and ureteral injuries, and damage to the bowel. The postpartum person will need a great amount of emotional support from the nurse after an unexpected hysterectomy. The nurse will provide postsurgical care and education, including encouragement of ambulation , teaching about signs of infection of the incision, pain control, and help with breast-feeding to avoid pain from the surgical site.
Nursing Assessment and Diagnosis
Nursing assessment for PPH begins by determining risk factors prior to birth. Several tools have been developed to measure a postpartum person’s risk for PPH (Lagrew et al., 2022). Assessment for PPH continues until discharge. The nurse will watch for bladder distention that could cause uterine atony and assist the patient in emptying the bladder in those cases. The nurse encourages the patient to void frequently (every 2 hours), performs QBL throughout the postpartum hospitalization, monitors vital sign s for symptoms of hypovolemia , performs fundal assessment of the tone of the uterus and massages the fundus when needed, and assesses the perineum for signs of hematoma (Elsevier, 2024). When caring for a patient who had a cesarean birth , the nurse assesses the abdomen and incision for bleeding. Nursing diagnoses made according to these assessments include fluid volume deficit , impaired tissue perfusion , and risk for excessive bleeding.
Fluid Volume Deficit
Postpartum hemorrhage causes the postpartum person to have a deficiency in fluid volume. The nurse will assess for signs of dehydration (increased thirst, weakness, tachycardia , decreased urine output). The nurse will administer a rapid bolus of IV fluids or a blood transfusion to replace the lost fluid volume as prescribed by the health-care provider. When a blood transfusion is ordered, the nurse will also monitor for signs of a transfusion reaction and fluid overload. Urine output is monitored and should be at least 30 mL/hr. If the person does not have an indwelling urinary catheter, the nurse may insert one to monitor output more closely, when ordered.
Impaired Tissue Perfusion
The nurse will monitor capillary refill and oxygen saturation to assess for decreased tissue perfusion. During a PPH, the vessels of the extremities contract to shunt blood to the lungs, heart, and brain. Therefore, peripheral perfusion is decreased. A warm blanket is wrapped around the person to keep the body and extremities warm. The nurse positions the person with elevated legs and keeps the person on bed rest. Supplemental oxygen can be administered and titrated based on the oxygen saturation, as prescribed.
Risk for Excessive Bleeding
The nurse assesses for signs of excessive bleeding from the uterus, incision, and potential sites of hematomas. The nurse also assesses for cues that bleeding continues to be excessive. These cues include worsening vital signs, change in neurologic status, shortness of breath, pallor, and soaking a peripad in less than 1 hour. Lab results (CBC and clotting studies) are monitored often. Fundal assessment and massage are performed frequently to ensure the uterus continues to stay contracted (firm).
Labor and Delivery: Part 3
See Labor and Delivery: Part 2 for a review of the patient data.
After 20 hours of labor, Brianne had a vaginal delivery of a male infant with mild shoulder dystocia (shoulders delivered within 30 seconds). Infant birth weight is 3,995 g. Placenta delivered spontaneously and intact by inspection. A second-degree vaginal laceration was repaired. Quantitative blood loss is 450 mL. The postpartum admitting nurse received the past medical history during the SBAR (situation, background, assessment, and recommendation tool) from the labor and birth unit.
| Past medical history |
36 years old, G1 P1
Allergy to penicillin A negative, antibody screen negative Rubella nonimmune Negative tests for STIs at initial prenatal visit and 36 weeks’ gestation GBS positive and treated during labor Gestational diabetes diagnosed at 26 weeks Husband, Trey, present for labor and birth, very supportive |
|
| Nursing notes | Postpartum Admission Assessment | |
| Postpartum Data | Data Obtained at Admission to the Floor | |
| Maternal BP | 128/84 | |
| Maternal TPR | 99.6°F, 88, 18 | |
| Pain/location | 2/perineal | |
| Breasts | Soft/nipples intact | |
| Location of fundus | Midline/2 finger breadths below umbilicus | |
| Consistency of fundus | Firm | |
| Color of lochia | Rubra | |
| Amount of lochia | Moderate | |
| Abnormal odor to lochia | Absent | |
| Labial edema | Present | |
| Laceration | Intact | |
| Perineum | Soft, no bruising | |
| Flowchart |
Vital Signs
Admission to L&B Unit BP 128/74; Temp 98.2 (F), Pulse 84, Resp 18 10 hours after admission 120/64; 98.4°F, 76, 20 14 hours after admission 124/68; 99.8°F, 80, 20 1 hour after birth 118/68; 99.4°F, 84, 18 |
|
| Provider’s orders |
Transfer to mother/baby unit
IV 1,000 mL LR with 20 units oxytocin (Pitocin) at 125 mL/hr May discontinue IV fluids when patient is tolerating regular diet and voiding without difficulty Regular diet Continue pad count to monitor cumulative QBL (quantitative blood loss) Ice pack to perineum for 24 hours Ibuprofen 600 mg po every 6 hours prn pain May get out of bed and shower with assistance when stable and has recovered from epidural RhoGAM workup if Rh negative Administer rubella vaccine before discharge if nonimmune |
What are the priority nursing actions at this time? Select three that apply.
- Administer ibuprofen 600 mg po now.
- Discuss the purpose of Rho(D) immune globulin.
- Insert a Foley catheter to empty the bladder.
- Educate the patient on signs and symptoms of preeclampsia.
- Determine the patient’s QBL.
- Educate the patient on the importance of ice to the perineum for the first 24 hours.
- Massage the patient’s uterus.
- Assist the patient to the bathroom to void.


1.14.30: Heat, Work, and Energy
Thermodynamics asserts that the energy of a closed system increases if
- heat \(\mathrm{q}\) flows from the surroundings into a system and
Separation of the heat term from the work term is extremely important in the context of the Second Law of Thermodynamics. Heat flows spontaneously from high to low temperatures, the word ‘spontaneous’ being absolutely crucial in the context of the Second Law.
There are many ways in which the surroundings can do work on a system. At this stage we note the distinction which is drawn between the three variables \(\mathrm{U}\), \(\mathrm{q}\) and \(\mathrm{w}\). [The point is emphasized by use of upper and lower case letters.] The variables \(\mathrm{q}\) and \(\mathrm{w}\) describe pathways which can result in a change in thermodynamic energy. We make the point by rewriting equation (a) to show the change in thermodynamic energy on going from state I to state II. Thus,
\[\Delta \mathrm{U}=\mathrm{U}(\mathrm{II})-\mathrm{U}(\mathrm{I})=\mathrm{q}+\mathrm{w} \nonumber \]
If for example \(\Delta \mathrm{U} = 100 \mathrm{~J}\), this can be a consequence of many pathways between state I and state II: e.g.
- \(\mathrm{q} = 50 \mathrm{~J}, \mathrm{~w} = 50 \mathrm{~J}\),
- \(\mathrm{q} = 0 \mathrm{~J}, \mathrm{~w} = 100 \mathrm{~J}\) and
- \(\mathrm{q} = 150 \mathrm{~J}, \mathrm{~w} = −50 \mathrm{~J}\).
Hence \(\mathrm{U}\) is a function of state (or, state variable) although \(\mathrm{q}\) and \(\mathrm{w}\) are not state variables. This is a triumph of the First Law of Thermodynamics. The task faced by chemists is to identify and describe quantitatively the actual pathway accompanying, for example, a given chemical reaction. Equation (a) signals the energy difference \(\Delta \mathrm{U}\) between two states which might involve a comparison of the energies at the start and finish of a chemical reaction in a closed system. In developing our argument there is merit in considering the change in energy of the original system following a small change along the overall reaction pathway.
We consider a closed reaction vessel containing ethyl ethanoate (aq; \(0.1 \mathrm{~mol}\)) and NaOH(aq; excess) . Spontaneous chemical reaction leads to hydrolysis of the ester to form EtOH(aq). The change in thermodynamic energy \(\Delta \mathrm{U}\) equals \(\mathrm{U}(\mathrm{II}) − \mathrm{~U}(\mathrm{I})\). We subdivide the total chemical reaction into small steps where the change in composition, (i.e. \(\mathrm{d}\xi\)) is accompanied by a change in thermodynamic energy \(\mathrm{dU}\).
\[\Delta \mathrm{U}=\int_{\text {state } \mathrm{I}}^{\text {state II }} \mathrm{dU} \nonumber \]
If the volume of the system changes by the differential amount \(\mathrm{dV}\) such that the pressure within the closed system equals the confining pressure \(\mathrm{p}\) [2],
\[\mathrm{w}=-\mathrm{p} \, \mathrm{dV} \nonumber \]
Then [3],
\[\mathrm{dU}=\mathrm{q}-\mathrm{p} \, \mathrm{dV} \nonumber \]
We write equation (e) in the following form;
\[\mathrm{q}=\mathrm{dU}+\mathrm{p} \, \mathrm{dV} \nonumber \]
The right hand side of equation (f) contains the differential changes in two extensive state variables, \(\mathrm{U}\) and \(\mathrm{V}\). Consequently heat \(\mathrm{q}\) is precisely defined by the changes in thermodynamic energy and volume at pressure \(\mathrm{p}\).
Footnotes
[1] The ‘equivalence ‘ of heat and work was first demonstrated in many experiments carried out in the 19th Century by James Joule, the son of a brewer (Salford, England). Joule showed that by doing work on a thermally isolated system the temperature of the latter increases. In other words, doing work on a system is equivalent to passing heat into the system.
The SI unit of energy is the joule, symbol \(\mathrm{J}\); \(\mathrm{J} \equiv \mathrm{kg} \mathrm{m}^{2} \mathrm{~s}^{-2}\) Sometimes one reads that thermodynamics is not concerned with ‘time’. However the concept of energy and the unit of energy involves ’time’. Of course the origins of these concepts are classical mechanics and accompanying discussion of potential and kinetic energies.
[2] \(\mathrm{p} \, \mathrm{V}=\left[\mathrm{N} \mathrm{m}{ }^{-2}\right] \,\left[\mathrm{m}^{3}\right]=[\mathrm{N} \mathrm{m}]=[\mathrm{J}]\)
[3] The fundamental link between heat and work was established by Joule. Interestingly the link between heat and work was apparent previously to A. Haller who suggested that human bodies are heated by the friction between solid particles in the blood passing through the capillaries in the lungs; see comments by
- P. Epstein, Textbook of Thermodynamics, Wiley, New York, 1937.
- M. A. Paul, Principles of Chemical Thermodynamics, McGraw-Hill, New York, 1957, p52.


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22.2A: Enteric Nervous System
The enteric nervous system (ENS) is a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system.
- Describe the structure and function of the enteric nervous system (ENS)
Key Points
- The enteric nervous system (ENS), which is embedded in the lining of the gastrointestinal system, can operate independently of the brain and the spinal cord.
- The ENS consists of two plexuses, the submucosal and the myenteric. The myenteric plexus increases the tone of the gut and the velocity and intensity of contractions. The submucosal plexus is involved with local conditions and controls local secretion, absorption, and muscle movements.
- While described as a second brain, the enteric nervous system normally communicates with the central nervous system (CNS) through the parasympathetic (via the vagus nerve ) and sympathetic (via the prevertebral ganglia) nervous systems, but can still function when the vagus nerve is severed.
- The ENS includes efferent neurons, afferent neurons, and interneurons, all of which make the ENS capable of carrying reflexes and acting as an integrating center in the absence of CNS input.
- The ENS contains support cells, which are similar to the astroglia of the brain, and a diffusion barrier around the capillaries surrounding the ganglia, which is similar to the blood –brain barrier of cerebral blood vessels.
Key Terms
- enteric nervous system : A subdivision of the peripheral nervous system that directly controls the gastrointestinal system.
EXAMPLES
The second brain of the enteric nervous system is the reason we get butterflies in our stomach or need to use the restroom more frequently when we are nervous and/or under stress.
The gastrointestinal (GI) system has its own nervous system, the enteric nervous system (ENS). Neurogastroenterology is the study of the enteric nervous system, a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system. The ENS is capable of autonomous functions such as the coordination of reflexes.
Although it receives considerable innervation from the autonomic nervous system, it can and does operate independently of the brain and the spinal cord. The ENS consists of some 100 million neurons, one-thousandth of the number of neurons in the brain, and about one-tenth the number of neurons in the spinal cord. The enteric nervous system is embedded in the lining of the gastrointestinal system.
Ganglia of the ENS
The neurons of the ENS are collected into two types of ganglia:
- The myenteric (Auerbach’s) plexus, located between the inner and outer layers of the muscularis externa
- The submucosal (Meissner’s) plexus, located in the submucosa
The Myenteric Plexus
The myenteric plexus is mainly organized as a longitudinal chains of neurons. When stimulated, this plexus increases the tone of the gut as well as the velocity and intensity of its contractions. This plexus is concerned with motility throughout the whole gut. Inhibition of the myenteric system helps to relax the sphincters —the muscular rings that control the flow of digested food or food waste.
The Submucosal Plexus
The submucosal plexus is more involved with local conditions and controls local secretion and absorption, as well as local muscle movements. The mucosa and epithelial tissue associated with the submucosal plexus have sensory nerve endings that feed signals to both layers of the enteric plexus. These tissues also send information back to the sympathetic pre-vertebral ganglia, the spinal cord, and the brain stem.
Neural control of the gut : An illustration of neural control of the gut wall by the autonomic nervous system and the enteric nervous system.
Function and Structure of the ENS
The enteric nervous system has been described as a second brain. There are several reasons for this. For instance, the enteric nervous system can operate autonomously. It normally communicates with the central nervous system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems. However, vertebrate studies show that when the vagus nerve is severed, the enteric nervous system continues to function.
In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as an integrating center in the absence of CNS input. For instance, the sensory neurons report mechanical and chemical conditions, while the motor neurons control peristalsis and the churning of intestinal contents through the intestinal muscles. Other neurons control the secretion of enzymes.
The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in the CNS, such as acetylcholine, dopamine, and serotonin. More than 90% of the body’s serotonin is in the gut, as well as about 50% of the body’s dopamine, which is currently being studied to further our understanding of its utility in the brain.
The enteric nervous system has the capacity to alter its response depending on factors such as bulk and nutrient composition. In addition, the ENS contains support cells that are similar to the astroglia of the brain, as well as a diffusion barrier around the capillaries that surround the ganglia, which is similar to the blood–brain barrier of the cerebral blood vessels.
Regulation of ENS Function
The parasympathetic nervous system is able to stimulate the enteric nerves in order to increase enteric function. The parasympathetic enteric neurons function in defecation and provide a rich nerve supply to the sigmoid colon, the rectum, and the anus.
Conversely, stimulation of the enteric nerves by the sympathetic nervous system will inhibit enteric function and capabilities. Neurotransmitter secretion and direct inhibition of the enteric plexuses cause this stall in function. If the gut tract is irritated or distended, afferent nerves will send signals to the medulla of the brain for further processing.


4.2: Market
What you’ll learn to do: explain what money is and what makes it useful
When we think of money, the paper bills in our wallet or the coins that are in our pockets come to mind. But money is much more than that, and how we define money determines where and how we use it to obtain the goods and services that businesses offer the consumer. In this section we’ll look at what money is, why it’s useful, and what may be the future of money.
Learning Objectives
- Discuss the advantages of using money versus barter
- Discuss alternatives to traditional currency used today
What Is Money?
Money is really anything that people use to pay for goods and services and to pay people for their work. Historically, money has taken different forms in different cultures—everything from salt, stones, and beads to gold, silver, and copper coins and, more recently, virtual currency has been used. Regardless of the form it takes, money needs to be widely accepted by both buyers and sellers in order to be useful.
Barter and the Double Coincidence of Wants
To understand the usefulness of money, we must consider what the world would be like without money. How would people exchange goods and services? Economies without money typically engage in the barter system. Barter —literally trading one good or service for another—is highly inefficient for trying to coordinate the trades in a modern advanced economy. In an economy without money, an exchange between two people would involve a double coincidence of wants, a situation in which two people each want some good or service that the other person can provide. For example, if a hairstylist wants a pair of shoes, she must find a shoemaker who has a pair of shoes in the correct size and who is willing to exchange the shoes for a certain number of hairdos. Such a trade is likely to be difficult to arrange. Think about the complexity of such trades in a modern economy, with its extensive division of labor that involves thousands upon thousands of different jobs and goods.
Another problem with the barter system is that it doesn’t allow people to easily enter into future contracts for the purchase of many goods and services. For example, if the goods are perishable, it may be difficult to exchange them for other goods in the future. Imagine a farmer wanting to buy a tractor in six months using a fresh crop of strawberries. Also, while the barter system might work all right in small economies, it will keep those economies from growing. The time that individuals might otherwise spend producing goods and services and enjoying leisure time is spent bartering.
Functions of Money
Money solves the problems created by the barter system. First, money serves as a medium of exchange , which means that money acts as an intermediary between the buyer and the seller. Instead of exchanging hairdos for shoes, the hairstylist now exchanges hairdos for money. This money is then used to buy shoes. To serve as a medium of exchange, money must be very widely accepted as a method of payment in the markets for goods, labor, and financial capital.
In addition, money needs to have the following properties:
- It must be divisible —that is, easily divided into usable quantities or fractions. A $5 bill, for example, is equal to five $1 bills. If something costs $3, you don’t have to tear up a $5 bill; you can pay with three $1 bills.
- It must be portable —easy to carry; it can’t be too heavy or bulky.
- It must be durable . It can’t fall apart or wear out after a few uses.
- It must be difficult to counterfeit . It won’t have much value if people can make their own.
Second, money must serve as a store of value . Consider the barter between the hairstylist and shoemaker again. The shoemaker risks having his shoes go out of style, especially if he keeps them in a warehouse for future use—their value will decrease with each season. Shoes are not a good store of value. Holding money is a much easier way of storing value. You know that you don’t need to spend it immediately, because it will still hold its value the next day or the next year. This function of money doesn’t require that money is a perfect store of value. In an economy with inflation, money loses some buying power each year, but it remains money.
Third, money serves as a unit of account , which means that it’s the ruler by which other values are measured. For example, a hairstylist may charge $30 to style someone’s hair. That $30 can buy two shirts (but probably not a pair of shoes). Money acts as a common denominator, an accounting method that simplifies thinking about trade-offs.
So money serves all of these functions—it’s a medium of exchange, store of value, and unit of account.
Uninscribed electrum coin from Lydia, 6th century BCE.
Commodity versus Fiat Money
Commodity money consists of objects that have value in themselves as well as value in their use as money. Gold, for example, has been used throughout the ages as money, although today it is not used as money but rather is valued for its other attributes. Gold is a good conductor of electricity and is used in the electronics and aerospace industry. Gold is also used in the manufacturing of energy efficient reflective glass for skyscrapers and is used in the medical industry as well. Of course, gold also has value because of its beauty and malleability in the creation of jewelry.
As commodity money, gold has historically served its purpose as a medium of exchange, a store of value, and as a unit of account. Commodity-backed currencies are dollar bills or other currencies with values backed up by gold or another commodity held at a bank. During much of its history, the money supply in the United States was backed by gold and silver. Interestingly, antique dollars dated as late as 1957 have “Silver Certificate” printed above the portrait of George Washington, as shown below. This meant that the holder could take the bill to the appropriate bank and exchange it for a dollar’s worth of silver.
A Silver Certificate and a Modern U.S. Bill. Until 1958, silver certificates were commodity-backed money—backed by silver, as indicated by the words “Silver Certificate” printed on the bill. Today, U.S. bills are backed by the Federal Reserve, but as fiat money.
As economies grew and became more global in nature, the use of commodity monies became more cumbersome. Countries moved toward the use of fiat money. Fiat money is legal tender whose value is backed by the government that issued it. The United States’ paper money—like the dollar bill, for instance—carries this statement: “This note is legal tender for all debts, public and private.” In other words, by government decree, if you owe a debt, then legally speaking, you can pay that debt with the U.S. currency, even though it’s not backed by a commodity. The only backing of our money is widespread faith and trust that the currency has value—and nothing more.
The following video discusses some additional characteristics of money:
Alternatives to Traditional Currency
Whoever said that “cash is king” hasn’t been paying attention to how consumers choose to pay for their purchases, particularly in developed economies. Take a look at the following chart:
Is a cashless future in the cards? A 2013 MasterCard report found that, in 2011, cashless payments made up 66 percent of global spending. Consumers in Belgium, France, and Canada used cash the least in 2011, as cashless payments made up 93 percent, 92 percent, and 90 percent of payments respectively. In the United States, 80 percent of payments were cashless in 2011.
Consumers expect to make fewer cash payments in the future, while also cutting back on credit- and debit-card use in favor of other forms of payments. In an Accenture survey, 66 percent of North Americans said they used cash daily or weekly in 2014 while only 54 percent expected to do so by 2020.
Money is an abstraction built on trust. As such, alternatives to the most tangible form of money—currency or cash—and its replacement with cashless payments have become possible. In this new emerging landscape, no transaction requires money in the form of notes and coins, and value can be exchanged through the transfer of information between transacting parties. There have been multiple waves of such alternatives.
Established alternatives to cash include checks, credit cards, debit cards, and prepaid debit cards. More recently, innovative options have sprung up that not only threaten to imperil the ubiquity of cash but also upend the traditional payment ecosystem. These include smartphone-enabled credit-card acquirers, such as Square, and Automated Clearing House or ACH acquirers, such as PayPal and Dwolla. And then there are even more ambitious alternatives to cash that have been proposed, such as Bitcoin, a Web-based cryptocurrency.
Unlike traditional money, such alternatives do not derive their value from government fiat—i.e., the government has not established their legitimacy or value. Each of these alternatives has an evolved network within which it is uniformly accepted as a means of payment; the more established alternatives, of course, have the widest networks. [1]
We will examine this trend further by taking a look at Bitcoin and Mobile Commerce.
Bitcoin
Today, there are more than seven hundred digital currencies in existence. Entering the marketplace is undertaken by so many due to the low cost of entry and the profit opportunity. In 2014, the European Banking Authority defined virtual currency as “a digital representation of value that is neither issued by a central bank nor a public authority, nor necessarily attached to a fiat currency, but is accepted by natural or legal persons as a means of payment and can be transferred, stored, or traded electronically.” The first and most widely known instance of such digital, virtual, or cryptocurrency is Bitcoin.
Bitcoin Background
Bitcoin, a peer-to-peer digital currency or cryptocurrency, operates without the involvement of traditional financial institutions, and it provides a direct digital alternative to physical currencies. Bitcoin transactions take place online directly between the buyer and seller, with each transaction having a unique encryption. Transactions are recorded on a decentralized public ledger available for network users to verify valid transactions. Special users on the network (“miners”) oversee this verification process. After verifying a block of transactions, miners are paid with twenty-five newly generated Bitcoins and the transactions are processed and approved; this is how the total number of Bitcoins grows. The number in circulation as of January 2015 was approximately 13.7 million, with the maximum set at 21 million. As of April 2015, their total value was $3 to $4 billion.
Governments worldwide generally do not yet see it and other digital currencies as a destabilizing “threat,” and some scholars have argued that it may best be seen as a speculative investment. Bitcoin has certainly had its ups and downs: As of April 1, 2015, its value stood at $242 per Bitcoin, after a January 14 low of $177 and a March 11 high of $296. The currency has also had a long run of troubles with hackers and fraud, most spectacularly in 2014 when the exchange Mt. Gox declared bankruptcy after Bitcoins worth $460 million at the time were apparently stolen. Bitcoin’s decentralized model and degree of anonymity have also raised concerns over its use in illegal money transfers, fueling potential illicit commerce across the “dark Web” and on sites such as Silk Road.
The organization Bitcoin.org, meanwhile, touts the currency’s potential for opening up a “whole new platform for innovation”:
Advantages and Disadvantages
A 2015 Congressional Research Service (CRS) report, “Bitcoin: Questions, Answers and Analysis of Legal Issues, explores the following technical, functional, and legal issues associated with Bitcoin—and, by extension, all virtual currencies.
Bitcoin advantages:
- Lower transaction costs: Because Bitcoin operates without a third-party intermediary, merchants are able to avoid the fees traditionally charged by payment systems such as credit cards.
- The possibility of increased privacy: Bitcoin provides a heightened degree of privacy for purchases and transactions, though by the system’s nature, a complete list of all transactions is forever recorded to each user’s encrypted identity.
- Protection from inflation: Since Bitcoin’s circulation is not linked to currency or government regulation, it is not subject to standard inflation. However, it more than makes up for this in volatility.
Bitcoin disadvantages:
- Severe price volatility: The value of a Bitcoin is determined by supply and demand and, as a result, can fluctuate rapidly. The value was as high as $1,100 in December 2013, then hit a low of $177 in January 2015. This extreme fluctuation is more characteristic of a commodity than a currency.
- Not legal tender: Debtors are not required to accept it, and without any formal backing other than the computer program to which it is linked, Bitcoin can be seen as an “unattractive vehicle” for holding and accumulating wealth.
- Uncertain security against theft and fraud: While the counterfeiting of Bitcoins is allegedly impossible, the system has at times found itself vulnerable to large security breaches and cyberattacks. Most recently, Bitstamp, a large European Bitcoin exchange, lost 19,000 Bitcoins (valued at about $5 million) in a digital security breach. This follows the massive problems with Mt. Gox in 2014 and the collapse of other exchanges in 2011.
- Vulnerability of Bitcoin “wallets”: Purchased or mined Bitcoins are stored in a digital wallet on the user’s computer or mobile device, and digital keys can be lost, damaged, or stolen. Paper or offline storage is an option, but it’s not always practiced.
Federal banking regulators have yet to issue guidance or regulations governing how banks are to deal with Bitcoins. In a February 2014 statement, Federal Reserve chair Janet Yellen said: “Bitcoin is a payment innovation that’s taking place outside the banking industry. . . . There’s no intersection at all, in any way, between Bitcoin and banks that the Federal Reserve has the ability to supervise and regulate.” Some state financial authorities have taken steps to devise regulations, with New York’s Department of Financial Services (NYDFS) in the lead.
The responsibility to oversee digital currency falls upon Congress. As of now, Congressional actions remain in the exploratory phase. The tax code lacks clarity on how such currency should be treated: Is it digital currency, property, barter, or foreign currency? Early concerns have focused more on tackling consumer-protection issues than tax ambiguities, and as a result, the Consumer Financial Protection Bureau has become involved regarding questions related to Bitcoin.
The following video explains further some of the gray areas in which this virtual currency is operating.
Whether it’s Bitcoin or another cryptocurrency, the fact that more than seven hundred of these unregulated digital currencies have emerged in just the last two years is just one more indication that consumers may be breaking off their longstanding love affair with traditional cash.
Mobile Commerce and Mobile Payment Systems
The term mobile commerce was first coined in 1997 by Kevin Duffey at the launch of the Global Mobile Commerce Forum to mean “the delivery of electronic commerce capabilities directly into the consumer’s hand, anywhere, via wireless technology.” Many think of mobile commerce as “a retail outlet in your customer’s pocket.”
Mobile commerce is worth US$230 billion annually, with Asia representing almost half of the market, and it’s expected to reach US$700 billion in 2017. According to BI Intelligence, in January 2013, 29 percent of mobile users have now made a purchase with their phones. Walmart estimated that 40 percent of all visits to their Internet shopping site in December 2012 was from a mobile device. Bank of America projected that $67.1 billion in purchases would be made from mobile devices by European and U.S. shoppers in 2015.
Mobile Payment
Mobile payment , also referred to as mobile money, mobile money transfer, and mobile wallet, generally refers to payment services operated under financial regulation and performed from or via a mobile device. Instead of paying with cash, check, or credit cards, a consumer can use a mobile phone to pay for a wide range of services and digital or hard goods. Although the concept of using non-coin-based currency systems has a long history, only recently has the technology to support such systems become widely available.
Mobile payment is being adopted all over the world in different ways. In 2008, the combined market for all types of mobile payments was projected to reach more than $600 billion globally by 2013, which would be double the figure as of February, 2011. The mobile payment market for goods and services, excluding contactless Near Field Communication or NFC transactions and money transfers, exceeded $300 billion globally in 2013. Investment on mobile money services is expected to grow by 22.2 percent during the next two years across the globe. It will result in revenue share of mobile money reaching up to 9 percent by 2018. Asia and Africa will observe significant growth for mobile money, with technological innovation and focus on interoperability emerging as prominent trends by 2018.
In developing countries, mobile payment solutions have been deployed as a means of extending financial services to communities known as the “unbanked” or “underbanked,” which are estimated to represent as much as 50 percent of the world’s adult population, according to Financial Access’s 2009 Report “Half the World is Unbanked.”
Forms of Mobile Payment
Apple Pay is a mobile payment service that lets certain Apple mobile devices make payments at retail and online checkout. It digitizes and replaces the credit or debit magnetic stripe card transaction at credit card terminals. The service lets Apple devices wirelessly communicate with point of sale systems using a near field communication (NFC) antenna, a “dedicated chip that stores encrypted payment information” (known as the Secure Element), and Apple’s Touch ID and Passbook.
The service keeps customer payment information private from the retailer, and creates a “dynamic security code [ . . .] generated for each transaction.” Apple added that they would not track usage, which would stay between the customers, the vendors, and the banks. Users can also remotely halt the service on a lost phone via the Find My iPhone service.
Google Wallet is a mobile payment system developed by Google that allows its users to store debit cards, credit cards, loyalty cards, and gift cards among other things, as well as redeeming sales promotions on their mobile phone. Google Wallet can use near field communication (NFC) to “make secure payments fast and convenient by simply tapping the phone on any PayPass-enabled terminal at checkout.”
Where this new technology will lead the world economy and what its impact on the existing monetary system will be remain to be seen, but we are certain it will continue to evolve rapidly!
- Chakravorti, B., & Mazzotta, B. (2013, September). The Cost of Cash in the United States. Retrieved September 8, 2016, from fletcher.tufts.edu/CostofCash...icrosites/Cost of Cash/CostofCashStudyFinal.pdf ↵