Macroeconomics/Aggregate Demand

After discussing the AE model, we will discuss aggregate demand and aggregate supply model (AD-AS model), in which AD curve is based on the concept of AE.
To build the AD-AS model, we need to build and examine the AD curve and AS curve(there are two types of AS curves, 
namely short run aggregate supply (SRAS) curve and long run aggregate supply (LRAS) curve). 
In this chapter, we will examine the AD curve. In the next chapter about aggregate supply, we will build and examine the SRAS and LRAS curves.
We should recall how AD is built using the relationship between AE and formula_1, in the chapter about AE. The following section (one section with its subsections) is a brief review about that, 
and it can be safely skipped if you are familiar with it .
Brief review: components of Aggregate Demand.
Aggregate Demand or Aggregate Expenditure (abbreviated to AD and AE respectively) is essentially the demand curve for goods and services for a "whole" economy. There are determinable "parts", known as components of aggregate demand, which the next few pages will explain in more detail.
The formula for finding formula_2 is as follows:
formula_3
Consumption (C).
This is the expenditure by consumer on goods and services, including both durable and non-durable goods but not including "saving". While values vary widely between economies, 'C' usually accounts for 40-60% of GDP. Because of this, high or low consumer confidence, and consequently high or low consumer spending can change the course in an economy.
Investment (I).
Strictly known as Gross Domestic Fixed Capital Formation, this is the money spent by firms (not the government or consumers) on capital investment i.e. new machinery and factories. There is some argument as to whether "human capital" (training) should be included. Investment also includes stocks and goods and services not yet completed; for example, car firms will likely have many machines part-built or stored at any one time. Since they cannot be included in consumption since they have not been purchased, stores must be included in I.
Investment makes up approximately 10% of GDP in most economies, and governments are often keen to increase this value, as investment helps to increase the possible output of economy (strictly LRAS or the PPF).
Government purchases (G).
Strictly known as General Government Final Consumption, it is the expenditure by the government, "not" including transfer payments (e.g. social security, welfare benefits since they are simply moving money around the economy). Governments usually spend on public and merit goods. These are microeconomic terms. A public good is one that is provided to everybody and cannot be exclusive by its nature (think street lighting), whilst a merit good is one that is under-supplied and under-demanded as consumers do not appreciate all the benefits, because some of those benefits do not directly apply to the buyer (think environmentally friendly vehicles).
Changes in government spending as a method of controlling the economy is known as fiscal policy. It will be discussed later.
Net exports (NX).
Exports are a positive figure on Aggregate Demand as overseas customers purchase goods and services produced here. Imports, on the other hand, are not produced here, though they are consumed here. For imports, "money" is leaving the economy, while for exports, "money" is entering the economy.
Movements of AD curve.
Movements along AD curve vs. shifting AD curve.
In this section, we will consider of AD curve.
There are two ways of moving the AD curve, namely movements the AD curve and shifting the AD curve. 
The movements the AD curve is caused by formula_4, caused 
by a change in component of formula_2 (i.e. not caused by formula_6 or formula_7). We have discussed the movement along the AD curve when we explain why AD curve is downward sloping (how formula_4 (only) affects formula_2) .
Illustration of formula_10 movement AD curve:
On the other hand, the of AD curve is caused by changing in some component of AD (i.e. component of formula_2: formula_12 
or formula_7) 
Under the parallel (we may assume this for simplicity) shift of AD curve, for each formula_1, we have the same formula_15 (for formula_16 shift) 
or formula_17 (for formula_18 shift) in formula_2. 
Illustration of formula_16 of AD curve, from formula_21 to formula_22:
Shifting the AD Curve.
Indeed, we can use some policies to shift AD curve, e.g. monetary policy 
and fiscal policy .
We will discuss these two policies in later chapters.
We will introduce several important factors that shift AD curve in the following proposition, by affecting the component(s) of AD (formula_12 or formula_7) 
&gt;(&lt;)\uparrow\text{ in }Y_{\text{foreign}}\Rightarrow AD\swarrow(\nearrow)&lt;/math&gt; 
&gt;(&lt;)\uparrow\text{ in }Y_{\text{foreign}}\Rightarrow X\downarrow\&amp;\;M\uparrow\Rightarrow AD\swarrow(\nearrow) &lt;/math&gt;