3.5 Equilibrium in Aggregate Demand-Aggregate Supply (AD-AS) Model

Equilibrium in the AD-AS model is where the economy settles based on aggregate demand and aggregate supply. Short-run equilibrium is where AD crosses SRAS, while long-run equilibrium is where AD, SRAS, and LRAS all meet at the full-employment level of output.

Equilibrium in the AD-AS Model Summary

In AP Macro 3.5, short-run equilibrium in the AD-AS model occurs where aggregate demand intersects short-run aggregate supply. That point gives the short-run price level and real output.

Long-run equilibrium occurs when AD and SRAS intersect on LRAS, at full-employment output. If the short-run equilibrium is to the right of LRAS, the economy has a positive or inflationary output gap; if it is to the left, the economy has a negative or recessionary output gap.

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Why This Matters for the AP Macroeconomics Exam

The AD-AS model is one of the most tested tools in AP Macroeconomics, and equilibrium is the starting point for almost every question that uses it. Once you can find short-run and long-run equilibrium, you can interpret what is happening to output, employment, and the price level, and you can predict how the economy responds to shocks and policy.

This topic supports both multiple-choice and free-response thinking. You will be asked to read graphs, draw correct curves, and explain whether the economy is at full employment, in an inflationary gap, or in a recessionary gap. Getting equilibrium right is what makes the later topics on short-run changes, self-adjustment, and fiscal policy make sense.

Key Takeaways

• Short-run equilibrium is the intersection of AD and SRAS, where aggregate quantity demanded equals aggregate quantity supplied.
• Long-run equilibrium happens when AD and SRAS intersect right on the LRAS curve, at the full-employment level of real output.
• Short-run equilibrium output can be at, above, or below full employment.
• Output above full employment is a positive (inflationary) output gap; output below full employment is a negative (recessionary) output gap.
• Always label AD-AS axes as Price Level and Real GDP, not Price and Quantity.
• LRAS sits vertically at full-employment output, which is the benchmark you compare short-run equilibrium against.

Finding Equilibrium in the AD-AS Model

Equilibrium in the AD-AS model works a lot like supply and demand for a single product, but it describes the whole economy at once. Instead of price and quantity for one good, you track the overall price level and Real GDP.

Short-Run Equilibrium

Short-run equilibrium occurs where the aggregate quantity of output demanded equals the aggregate quantity of output supplied. On a graph, that is the intersection of the AD curve and the SRAS curve.

If the price level is above this intersection, the aggregate quantity supplied is greater than the aggregate quantity demanded. If the price level is below it, the aggregate quantity demanded is greater than the aggregate quantity supplied. Either way, the economy moves back toward the AD-SRAS intersection.

Long-Run Equilibrium

Long-run equilibrium occurs when the AD and SRAS curves intersect directly on the LRAS curve. At that point, current output equals potential output, which is the full-employment level of real output. The LRAS curve is vertical at full-employment output, so it acts as the marker for where the economy can produce sustainably.

Output Gaps

The short-run equilibrium where AD and SRAS cross does not have to land on LRAS. Comparing that intersection to LRAS tells you whether the economy has an output gap.

• If the AD-SRAS intersection is on LRAS, the economy is at full employment.
• If the intersection is to the right of LRAS, there is a positive (inflationary) output gap.
• If the intersection is to the left of LRAS, there is a negative (recessionary) output gap.

A simple way to keep it straight: an intersection to the left of LRAS is "falling behind," so it is recessionary. An intersection to the right is "moving ahead," so it is inflationary.

Inflationary Gap

An inflationary gap is when short-run Real GDP is above potential Real GDP at full employment. It might sound good to produce a lot, but it can lead to an overheating economy and rising prices. In an inflationary gap, output is above potential and unemployment is below the natural rate.

As an application, an economy in a strong boom with very low unemployment, rising wages, and high consumer spending would show up as a positive output gap on the graph.

Recessionary Gap

A recessionary gap is when short-run Real GDP is below potential Real GDP at full employment. The economy produces less than it could, unemployment rises above the natural rate, and incomes, consumption, and living standards tend to fall.

What Happens Next

How does an economy close these gaps? You have two main paths: the economy can self-correct over time through price and wage adjustments, or the government can use fiscal policy to shift aggregate demand. Those are the focus of later topics on long-run self-adjustment and fiscal policy, so treat this as a preview.

How to Use This on the AP Macroeconomics Exam

MCQ

• Identify equilibrium as the AD-SRAS intersection, and check its position relative to LRAS to name any output gap.
• Watch for questions that test direction: above the intersection price level means a surplus of output, below means a shortage.
• Remember that long-run equilibrium requires all three curves (AD, SRAS, LRAS) to meet at the same point.

Free Response

• Draw a correctly labeled AD-AS graph with axes labeled Price Level and Real GDP.
• Show the equilibrium price level and output level with dashed lines to both axes.
• When asked about gaps, clearly show whether the AD-SRAS intersection is left of, right of, or on LRAS, and name the gap.

Common Trap

Do not mix up the AD-AS model with the supply and demand model for a single good. The axes are Price Level and Real GDP, not Price and Quantity, and the curves describe the entire economy.

Common Misconceptions

• Thinking equilibrium always means full employment. Short-run equilibrium can sit above or below full-employment output.
• Labeling the axes "Price" and "Quantity." In AD-AS, they are "Price Level" and "Real GDP."
• Assuming an inflationary gap is purely good news. Producing above potential can overheat the economy and push prices up.
• Confusing left and right of LRAS. Left of LRAS is recessionary (below potential); right of LRAS is inflationary (above potential).
• Treating short-run and long-run equilibrium as the same thing. Long-run equilibrium specifically requires AD and SRAS to intersect on LRAS.

Related AP Macroeconomics Guides

• 3.1 Aggregate Demand
• 3.2 Spending and Tax Multipliers
• 3.3 Short-Run Aggregate Supply (SRAS)
• 3.4 Long-Run Aggregate Supply (LRAS)
• 3.8 Fiscal Policy
• 3.6 Changes in the AD-AS Model in the Short Run