AD-AS模型中的均衡

AP 宏观经济学· AP 宏观经济学 CED — 国民收入与价格水平决定· 14 分钟阅读

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。

📘 定义

宏观经济均衡

AD-AS模型中的一种状态：总需求量等于总供给量，不存在改变产出或整体价格水平的内在压力。

2. 短期宏观经济均衡★★★☆☆⏱ 4 min

✓ 计算器

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 $A D (Y)$ is the price level at which aggregate demand equals output $Y$ , and $S R A S (Y)$ is the price level at which short-run aggregate supply equals output $Y$ . Solving this condition gives two equilibrium values: $P L^{*}$ (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 $Y_{F}$ .

📐 例题

Suppose an economy has aggregate demand given by $P L = 150 - 0.5 Y$ , and short-run aggregate supply given by $P L = 30 + 0.25 Y$ , where $P L$ is the price level and $Y$ is real GDP in billions of dollars. Calculate the equilibrium short-run output and price level.

1
Apply the short-run equilibrium condition: set AD equal to SRAS
$150 - 0.5Y = 30 + 0.25Y$
2
Rearrange terms to isolate $Y$
$150 - 30 = 0.25Y + 0.5Y \implies 120 = 0.75Y$
3
Solve for $Y$
$Y = 120 / 0.75 = 160$
4
Plug $Y$ back into the SRAS equation to find $P L$
$PL = 30 + 0.25(160) = 30 + 40 = 70$
5
Verify consistency with the AD equation
$150 - 0.5(160) = 150 - 80 = 70, which matches$

Exam tip:

On FRQs, always explicitly label both equilibrium output $Y^{*}$ and equilibrium price level $P L^{*}$ 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.

📘 定义

Output Gap

The difference between short-run equilibrium output $Y^{*}$ and full-employment output $Y_{F}$ , divided into two types: recessionary (contractionary) when $Y^{*} < Y_{F}$ , and inflationary (expansionary) when $Y^{*} > Y_{F}$ .

📐 例题

Suppose the economy from the previous worked example has a full-employment output $Y_{F} = 200$ billion. Is the economy in long-run equilibrium? If not, what type of output gap exists, and what is the size of the gap?

1
Recall we found short-run equilibrium output $Y^{*} = 160$ billion
2
Compare $Y^{*}$ to $Y_{F}$ : $160 < 200$ , so $Y^{*} \neq = Y_{F}$ , meaning the economy is not in long-run equilibrium
3
By definition, when short-run equilibrium output is less than full-employment output, the gap is a recessionary output gap
4
Calculate the gap size
$\text{Gap} = Y_F - Y^* = 200 - 160 = 40$

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 $Y^{*} = Y_{F}$ .

📐 例题

Suppose an economy initially in long-run equilibrium experiences a positive AD shock (e.g., a surge in consumer confidence that increases consumption spending) that shifts AD right. Describe the automatic adjustment back to long-run equilibrium step-by-step.

1
Initial long-run equilibrium: $A D_{1}$ intersects $S R A S_{1}$ and $L R A S$ at $Y_{F}$ , with initial price level $P L_{1}$
2
The positive AD shock shifts AD right to $A D_{2}$ . The new short-run equilibrium is at the intersection of $A D_{2}$ and $S R A S_{1}$ , with new output $Y_{2} > Y_{F}$ and new price level $P L_{2} > P L_{1}$ , creating an inflationary gap of $Y_{2} - Y_{F}$
3
Because unemployment is below the natural rate, workers negotiate higher nominal wages in new long-term contracts, and other input prices rise to reflect the higher price level
4
Higher production costs for firms shift SRAS left from $S R A S_{1}$ to $S R A S_{2}$
5
The new long-run equilibrium forms at the intersection of $A D_{2}$ , $S R A S_{2}$ , and $L R A S$ . Output returns to $Y_{F}$ , and the price level rises to $P L_{3} > P L_{2}$ , eliminating the inflationary gap

✓ 快速检测

Test your understanding with this AP-style multiple choice question:

An economy has AD given by $P L = 200 - Y$ , SRAS given by $P L = 50 + 0.5 Y$ , and LRAS at $Y_{F} = 120$ . Which of the following correctly describes the economy's current equilibrium?
- A) Short-run equilibrium at Y = 100, PL = 100, with a $20 billion recessionary gap
- B) Short-run equilibrium at Y = 120, PL = 80, with no output gap
- C) Short-run equilibrium at Y = 100, PL = 100, with a $20 billion inflationary gap
- D) Short-run equilibrium at Y = 140, PL = 60, with a $20 billion inflationary gap

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.

5. 常见陷阱

错误做法:

Calling a gap where $Y^{*} > Y_{F}$ a recessionary gap, and $Y^{*} < Y_{F}$ an inflationary gap

原因:

Students mix up gap names with direction, confusing the label with the policy action needed to close the gap

正确做法:

Memorize the rule: the name describes the state of the economy relative to full employment. If output is lower than full employment, the economy is in a recession, so it is a recessionary gap

错误做法:

Shifting AD instead of SRAS during automatic long-run adjustment

原因:

Students confuse automatic self-correction with fiscal/monetary policy, which shifts AD

正确做法:

If the question explicitly says "no policy intervention" or "automatic adjustment," always shift SRAS to close the gap. Only shift AD if the question asks for government or central bank policy

错误做法:

Claiming long-run equilibrium occurs at the intersection of only AD and LRAS, with no requirement that SRAS intersects at that point

原因:

Students forget SRAS is always relevant even in the long run, and short-run equilibrium is required for the economy to be at rest

正确做法:

Always confirm all three curves intersect at $Y_{F}$ when identifying long-run equilibrium, and draw SRAS through the intersection point on FRQ graphs

错误做法:

Shifting LRAS during automatic adjustment to a demand-side output gap

原因:

Students confuse changes in potential output (which shifts LRAS) with the self-correction mechanism, which does not change potential output

正确做法:

LRAS only shifts when there is a change in productivity, labor force, capital stock, or a permanent supply shock. It never shifts during automatic adjustment to a demand shock

错误做法:

Failing to verify equilibrium calculations by plugging $Y^{*}$ into both AD and SRAS

原因:

Students rush through algebra and make simple arithmetic errors that are easy to catch

正确做法:

Always plug your solved $Y^{*}$ into both equations to confirm you get the same $P L^{*}$ ; this catches 90% of common calculation mistakes

6. 速查表

Category	Formula / Rule	Notes
Short-run equilibrium condition	$A D (Y) = S R A S (Y)$	Gives equilibrium $Y^{}$ and $P L^{}$ ; $Y^{*}$ can be above, below, or equal to $Y_{F}$
Recessionary gap	$Gap = Y_{F} - Y^{}$ , $Y^{} < Y_{F}$	Unemployment above natural rate; downward pressure on wages and input prices
Inflationary gap	$Gap = Y^{} - Y_{F}$ , $Y^{} > Y_{F}$	Unemployment below natural rate; upward pressure on wages and input prices
Long-run equilibrium condition	$Y^{} = Y_{F}$ , $A D (Y_{F}) = S R A S (Y_{F}) = P L^{}$	All three curves (AD, SRAS, LRAS) intersect at the same point
Automatic adjustment: recessionary gap	Shift SRAS right	Lower input prices shift SRAS; closes gap without changing $Y_{F}$
Automatic adjustment: inflationary gap	Shift SRAS left	Higher input prices shift SRAS; closes gap without changing $Y_{F}$
Long-run equilibrium output	$Y = Y_{F}$	Potential output; no output gap; unemployment equals natural rate
Out-of-equilibrium adjustment	Excess supply → cut output/prices; excess demand → raise output/prices	Driven by unintended inventory changes that push the economy back to equilibrium

真题中的出现

AI 根据考纲规律估算的考点位置,请对照官方真题核实准确性。仅作复习重点参考。

2023 · MCQ
产出缺口识别
2022 · FRQ
自动调整分析
2021 · MCQ
均衡计算

深入阅读

unit overviewAP 宏观经济学第3单元概述本主题的所属父单元

下一步

Equilibrium in the AD-AS model is the foundational framework for analyzing all macroeconomic shocks and policy effects in AP Macroeconomics, and it is a required prerequisite for all subsequent topics in the unit. Mastery of equilibrium conditions and output gap analysis is essential for nearly all free-response questions on the exam, and it will allow you to correctly analyze how government and central bank policymakers respond to recessions and inflationary booms. This topic alone makes up a significant portion of Unit 3, and concepts from here are tested across multiple units of the AP exam. Next, you will apply this framework to analyze how fiscal and monetary policy shift aggregate demand to close output gaps.