Equilibrium in the Aggregate Demand-Aggregate Supply (AD-AS) Model - AP Macroeconomics Study Guide

Core Concepts & Learning Goals

This section introduces the concept of macroeconomic equilibrium, where the total quantity of goods and services demanded by households, firms, the government, and foreign buyers equals the total quantity supplied by producers. Understanding this topic is crucial for diagnosing the overall health of an economy—whether it is operating at its potential, in a recession, or overheating.

After studying this topic, you will be able to use the Aggregate Demand-Aggregate Supply (AD-AS) model to graphically and verbally explain the economy's short-run and long-run equilibrium price level and real output. You will also be able to identify and describe recessionary and inflationary gaps.

Key Concepts Breakdown

The AD-AS model combines three key curves to illustrate the state of the economy: the Aggregate Demand (AD) curve, the Short-Run Aggregate Supply (SRAS) curve, and the Long-Run Aggregate Supply (LRAS) curve. The interaction of these curves determines the equilibrium price level and real GDP.

1. Short-Run Equilibrium

In the short run, the economy reaches equilibrium at the price level and output level where the quantity of real GDP demanded equals the quantity of real GDP supplied.

• Short-Run Equilibrium is defined as the point where the aggregate demand (AD) curve intersects the short-run aggregate supply (SRAS) curve.

• This intersection determines the current equilibrium price level, which we can label (PL_E), and the current equilibrium level of real output (real GDP), which we can label (Y_E).

• This short-run equilibrium represents the economy's current performance, which may or may not be sustainable in the long run.

2. Long-Run Equilibrium

Long-run equilibrium represents an ideal state for the economy where it is operating at its maximum sustainable capacity.

• Long-Run Equilibrium occurs when the short-run equilibrium point also lies on the long-run aggregate supply (LRAS) curve.

• This means all three curves (AD, SRAS, and LRAS) intersect at a single point.

• At this point, the short-run equilibrium output ((Y_E)) is equal to the full-employment level of output ((Y_F)). This is the level of real GDP an economy produces when unemployment is at its natural rate. It is also referred to as potential output.

3. Output Gaps

When the short-run equilibrium does not coincide with the long-run equilibrium, the economy experiences an output gap. An output gap is the difference between the actual short-run output ((Y_E)) and the economy's potential output ((Y_F)).

• Recessionary Gap (Negative Output Gap): This occurs when the short-run equilibrium output is less than the full-employment level of output ((Y_E < Y_F)).

  • The AD and SRAS curves intersect to the left of the vertical LRAS curve.

  • This indicates the economy is underperforming, with high unemployment and idle resources.

• Inflationary Gap (Positive Output Gap): This occurs when the short-run equilibrium output is greater than the full-employment level of output ((Y_E > Y_F)).

  • The AD and SRAS curves intersect to the right of the vertical LRAS curve.

  • This indicates the economy is "overheating." To produce beyond its sustainable capacity, firms are using resources (like labor) so intensively that it drives up wages and prices, leading to inflationary pressure.

The table below compares the three possible states of the economy in the AD-AS model.

Graphical Analysis (Text-Only)

To visualize these concepts, we use the AD-AS graph. This graph is essential for macroeconomic analysis.

• Axes Declaration:

  • Vertical Axis: Price Level (PL)

  • Horizontal Axis: Real GDP (Y)

• Curve Specifications:

  • Aggregate Demand (AD): A downward-sloping curve, showing the inverse relationship between the price level and the quantity of real GDP demanded.

  • Short-Run Aggregate Supply (SRAS): An upward-sloping curve, showing the positive relationship between the price level and the quantity of real GDP supplied in the short run.

  • Long-Run Aggregate Supply (LRAS): A vertical line at the full-employment level of output ((Y_F)). This shows that in the long run, output is determined by the economy's resources and technology, not the price level.

Scenario 1: Long-Run Equilibrium

1. The AD curve slopes downward.

2. The SRAS curve slopes upward.

3. The vertical LRAS curve is positioned at the full-employment output level, (Y_F).

4. All three curves (AD, SRAS, LRAS) intersect at a single point, E.

5. This intersection determines the equilibrium price level, (PL_E), and the equilibrium real GDP, (Y_E).

6. In this state, (Y_E = Y_F). The economy is stable.

Scenario 2: Recessionary Gap

1. The AD and SRAS curves intersect at a point (E_1), to the left of the LRAS curve.

2. This intersection determines the short-run equilibrium price level, (PL_1), and real GDP, (Y_1).

3. The vertical LRAS curve is located at (Y_F), to the right of (Y_1).

4. The condition is (Y_1 < Y_F). The horizontal distance between (Y_1) and (Y_F) represents the size of the recessionary gap.

Scenario 3: Inflationary Gap

1. The AD and SRAS curves intersect at a point (E_1), to the right of the LRAS curve.

2. This intersection determines the short-run equilibrium price level, (PL_1), and real GDP, (Y_1).

3. The vertical LRAS curve is located at (Y_F), to the left of (Y_1).

4. The condition is (Y_1 > Y_F). The horizontal distance between (Y_F) and (Y_1) represents the size of the inflationary gap.

Step-by-Step Example

Imagine an economist provides you with the following information about the economy of Macroland: The AD and SRAS curves currently intersect at an output level of $900 billion and a price level of 110. The country's full-employment level of output is estimated to be $1,000 billion. How would you diagnose the state of Macroland's economy?

• Step 1: Identify the Short-Run Equilibrium.

  The short-run equilibrium is where AD intersects SRAS. We are told this occurs at a real GDP of $900 billion. Therefore, (Y_E = $900) billion. The current price level is (PL_E = 110).

• Step 2: Identify the Full-Employment Output.

  We are given that the full-employment level of output is $1,000 billion. Therefore, (Y_F = $1,000) billion. On a graph, the vertical LRAS curve would be placed at this value on the horizontal axis.

• Step 3: Compare Short-Run Output to Full-Employment Output.

  We compare the two values: (Y_E = $900) billion and (Y_F = $1,000) billion. It is clear that (Y_E < Y_F).

• Step 4: Conclude the State of the Economy.

  Because the current short-run equilibrium output is less than the economy's full-employment output, Macroland is experiencing a recessionary gap. This means the economy is operating below its potential, and unemployment is likely higher than its natural rate.

AP Exam Tips & Common Pitfalls

• [FRQ Task]: A common task on Free-Response Questions is to "Identify the current condition of the economy shown in the graph." You must state whether the economy is in long-run equilibrium, a recessionary gap, or an inflationary gap, and often justify your answer by comparing the short-run equilibrium output to the full-employment output.

• [MCQ Task]: Multiple-Choice Questions will often test your ability to connect the graphical model to economic conditions. For example, a question might state, "If an economy is operating with an actual unemployment rate of 7% and a natural rate of unemployment of 5%, which of the following is true?" The correct answer would indicate the economy is in a recessionary gap.

• [Common Pitfall ①]: Focusing on the Price Level Instead of Output. Students sometimes mistakenly believe a high price level automatically means an inflationary gap. An economy can have a high price level but still be in a recession (a condition known as stagflation). The type of gap is always determined by comparing the short-run equilibrium output ((Y_E)) to the full-employment output ((Y_F)).

• [Common Pitfall ②]: Confusing Short-Run and Long-Run Equilibrium. Remember that the economy is always in a short-run equilibrium where AD meets SRAS. The key question is where that intersection occurs relative to the LRAS curve. Long-run equilibrium is the special case where the short-run equilibrium happens to be on the LRAS curve.

Key Vocabulary

• Short-Run Equilibrium: The price level and real output combination determined by the intersection of the aggregate demand and short-run aggregate supply curves.

• Long-Run Equilibrium: A state in which the economy's short-run equilibrium output is equal to its full-employment output; the AD, SRAS, and LRAS curves all intersect at the same point.

• Full-Employment Level of Output ((Y_F)): The level of real GDP produced when the economy's resources are fully employed (i.e., unemployment is at its natural rate). Also known as potential output.

• Recessionary Gap: The situation where the short-run equilibrium level of real GDP is below the full-employment level.

• Inflationary Gap: The situation where the short-run equilibrium level of real GDP is above the full-employment level.