Equilibrium in the AD-AS Model — AP Macroeconomics
1. Core Definition of AD-AS Equilibrium ★★☆☆☆ ⏱ 3 min
Equilibrium in the AD-AS model occurs at the intersection of the aggregate demand (AD) curve and the relevant aggregate supply curve. This intersection gives the economy’s equilibrium price level (PL) and equilibrium real output (real GDP, Y).
Equilibrium means the total quantity of goods and services demanded in the economy equals the total quantity supplied, so there is no unintended inventory accumulation or depletion, and no inherent upward or downward pressure on the overall price level. When the economy is out of equilibrium, market forces automatically push it back to equilibrium: if output is above equilibrium, unsold goods pile up, leading firms to cut production and prices; if output is below equilibrium, excess demand leads firms to increase production and raise prices.
According to the AP Macroeconomics CED, this topic accounts for 10-15% of Unit 3’s weighting, and it is tested on both multiple-choice (MCQ) and free-response (FRQ) sections of the exam. Mastery of equilibrium conditions is required for nearly all AD-AS related questions.
2. Short-Run Macroeconomic Equilibrium ★★★☆☆ ⏱ 4 min
Short-run equilibrium occurs at the intersection of the AD curve and the upward-sloping short-run aggregate supply (SRAS) curve. In the short run, nominal wages and input prices are sticky (slow to adjust to changing economic conditions), so firms respond to higher price levels by increasing output, resulting in the upward slope of SRAS.
AD(Y) = SRAS(Y)
Where $AD(Y)$ is the price level at which aggregate demand equals output $Y$, and $SRAS(Y)$ is the price level at which short-run aggregate supply equals output $Y$. Solving this condition gives two equilibrium values: $PL^*$ (equilibrium price level) and $Y^*$ (equilibrium short-run real GDP). Importantly, short-run equilibrium output does not need to equal full-employment output ($Y_F$, the output at which LRAS is vertical); it can fall above, below, or exactly at $Y_F$.
Exam tip: On FRQs, always explicitly label both equilibrium output $Y^*$ and equilibrium price level $PL^*$ on your graph. AP exam rubrics almost always award 1 full point for correctly labeling both values, and missing one label will cost you a free point.
3. Long-Run Equilibrium and Output Gaps ★★★☆☆ ⏱ 4 min
Long-run equilibrium occurs when short-run equilibrium output equals full-employment (potential) output $Y_F$, meaning AD, SRAS, and LRAS all intersect at the same point. LRAS is vertical at $Y_F$ because in the long run, all nominal wages and input prices are fully flexible, so changes in the price level do not change the economy’s maximum sustainable output.
AD(Y_F) = SRAS(Y_F) = PL^*
At long-run equilibrium, the economy is at full employment: the unemployment rate equals the natural rate of unemployment, there are no output gaps, and the economy is operating at its potential output. If short-run equilibrium is not at $Y_F$, the economy has an output gap.
Exam tip: Always remember that LRAS is vertical at $Y_F$, so long-run equilibrium requires all three curves to intersect at the same output level. A common exam mistake is only checking for intersection of AD and LRAS, but SRAS must also intersect at that point for the economy to be in long-run equilibrium.
4. Automatic Long-Run Adjustment ★★★★☆ ⏱ 3 min
In the absence of government or central bank intervention, the economy automatically adjusts back to long-run equilibrium through shifts in SRAS driven by changes in nominal wages and input prices:
- **Recessionary gap**: High unemployment leads workers to accept lower nominal wages, and input prices fall. Lower production costs shift SRAS right, lowering the price level and increasing output until $Y^* = Y_F$.
- **Inflationary gap**: Low unemployment leads workers to demand higher nominal wages, and input prices rise. Higher production costs shift SRAS left, raising the price level and decreasing output until $Y^* = Y_F$.
Exam tip: When the question asks for automatic (self-correcting) adjustment without policy intervention, always shift SRAS, not AD or LRAS. Automatic adjustment works through input price changes that shift SRAS, not through changes in potential output or aggregate demand.
Common Pitfalls
Why: Students mix up gap names with direction, confusing the label with the policy action needed to close the gap
Why: Students confuse automatic self-correction with fiscal/monetary policy, which shifts AD
Why: Students forget SRAS is always relevant even in the long run, and short-run equilibrium is required for the economy to be at rest
Why: Students confuse changes in potential output (which shifts LRAS) with the self-correction mechanism, which does not change potential output
Why: Students rush through algebra and make simple arithmetic errors that are easy to catch