AD-AS模型中的均衡 — AP 宏观经济学

1. AD-AS均衡的核心定义 ★★☆☆☆ ⏱ 3 min

AD-AS模型中的均衡出现在总需求（AD）曲线与对应总供给曲线的交点处。该交点决定了经济体的均衡价格水平（PL）和均衡实际产出（实际GDP，Y）。

均衡意味着经济体中总商品和服务需求量等于总供给量，因此不会出现非计划存货的积累或减少，整体价格水平也不存在内在的上升或下降压力。当经济体偏离均衡时，市场力量会自动将其推回均衡：如果产出高于均衡水平，未售出商品会堆积，导致企业削减产量和价格；如果产出低于均衡水平，过剩需求会促使企业提高产量和价格。

根据AP宏观经济学课程与考试描述（CED），本主题占第3单元分值权重的10-15%，在考试的选择题（MCQ）和自由问答题（FRQ）部分都会考查。几乎所有AD-AS相关题目都要求掌握均衡条件。

2. 短期宏观经济均衡 ★★★☆☆ ⏱ 4 min

短期均衡出现在AD曲线与向上倾斜的短期总供给（SRAS）曲线的交点处。短期中，名义工资和投入价格具有粘性（对经济形势变化调整缓慢），因此企业会随着价格水平上升提高产出，这导致SRAS向上倾斜。

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. 长期均衡与产出缺口 ★★★☆☆ ⏱ 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.