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The electromotive force (e.m.f.), E, of a cell is the p.d. between its terminals when it is
not connected to a load (i.e. the cell is on no load).
The electrical force or pressure that causes the electrons to move in a particular
direction is called as the electromotive force i.e. E.M.F.
The unit of emf is volts.


Electric potential: If charge B is to be brought closer to charge A, then work has to be

done against this force of repulsion.
This work done against the force of repulsion to bring a charge closer to the other one is
called as electric potential, and it is measured in volts.
Potential difference between any two points is defined as the difference between the
electric potentials at those points.


Current is defined as the movement of electrons inside a conducting material.

It is denoted as I and measured in ampere (A).


Resistance of a material is defined as the opposition to flow of current.

It is measured in ohms ().
Mathematical expression for resistance: R=(pxl)/a
Where p = resistivity of a material and it is constant.
l = length of the conductor and
a = cross sectional area.
Resistivity (p): The resistivity is also called as specific resistance and it is denoted as p

How Temperature Affects Resistance

The value of a resistor changes with changing temperature, but this is not as we might
expect, mainly due to a change in the dimensions of the component as it expands or
contracts. It is due mainly to a change in the resistivity of the material caused by the
changing activity of the atoms of that make up the resistor.
Materials which are classed as conductors tend to increase their resistivity with an
increase in temperature. Insulators however are liable to decrease their resistivity with
an increase in temperature. It should be said however, that materials used for practical
insulators (glass plastic etc) only exhibit a marked drop in their resistivity at very high
encounter in use.
small negatively charged particles will be repelled by a negative electric charge and
attracted by a positive electric charge. Thus if we apply an electric potential across a
conductor (positive at one end, negative at the other) electrons will "migrate" from atom
to atom towards the positive terminal.
tightly to their particular atom that even an electric field will not dislodge them. We say
therefore that the current flowing in the material is due to the movement of "free
and the higher the temperature the more violently the atoms vibrate.
heat it sufficiently the atoms vibrate violently enough to actually shake some of their
captive electrons free, creating free electrons to become carriers of current. Thus at
high temperatures the resistance of an insulator can fall, sometimes dramatically.
When we have a material in which the resistance increases with temperature we say the
material has a positive temperature coefficient.
When resistance falls with an increase in temperature we say that the material has a
negative temperature coefficient.
Therefore in general, we can say that conductors have a positive temperature coefficient
Whilst (at high temperatures) insulators have a negative temperature coefficient.
Materials chosen for the construction of resistors therefore are most likely to be carefully
selected conductors that also have a very low positive temperature coefficient.
Therefore in use resistors will have only very slight increases in resistance as
temperature rises.