Supply and Demand Curves and Market Equilibrium | 供需曲线与市场均衡

📚 Supply and Demand Curves and Market Equilibrium | 供需曲线与市场均衡

Understanding how supply and demand interact to determine market equilibrium is one of the most fundamental skills in A-Level Economics. This article breaks down the demand curve, the supply curve, the equilibrium condition, and the effects of shifts in both curves, with exam-ready examples and step-by-step reasoning.

理解供给与需求如何相互作用并决定市场均衡，是 A-Level 经济学的核心技能之一。本文将系统讲解需求曲线、供给曲线、均衡条件以及两条曲线变动的影响，并辅以贴近考试的例题与逐步推理。

1. The Demand Curve | 需求曲线

Demand is the quantity of a good that consumers are willing and able to buy at a given price over a given period of time. The “willing and able” condition is crucial because wanting a good without the ability to pay does not constitute effective demand.

需求是指在某一特定时期内，消费者在某一价格水平下愿意并且能够购买的商品数量。”愿意并且能够”这一条件至关重要，因为仅有购买愿望而没有支付能力并不构成有效需求。

The law of demand states that, ceteris paribus, a fall in price leads to an expansion of demand, and a rise in price leads to a contraction of demand. This inverse relationship produces a demand curve that slopes downward from left to right.

需求定律表明，在其他条件不变的情况下，价格下降导致需求量的扩张，价格上升导致需求量的收缩。这种反向关系使需求曲线自左向右向下倾斜。

Qd = 120 − 2P

Here, Qd is the quantity demanded and P is the price. When P = 10, Qd = 100; when P = 40, Qd = 40. A change in price causes a movement along the demand curve; a change in any non-price determinant shifts the entire curve.

其中 Qd 为需求量，P 为价格。当 P = 10 时，Qd = 100；当 P = 40 时，Qd = 40。价格变动引起沿需求曲线的移动；任何非价格因素的变化则使整条曲线发生平移。

2. The Supply Curve | 供给曲线

Supply is the quantity of a good that producers are willing and able to sell at a given price over a given period. The law of supply states that higher prices generally lead to a greater quantity supplied, because higher prices imply higher revenue and profit margins for firms.

供给是指在某一特定时期内，生产者在某一价格水平下愿意并且能够出售的商品数量。供给定律表明，价格上升通常带来供给量的增加，因为更高的价格意味着更高的收益与利润空间。

Qs = 3P

When P = 10, Qs = 30; when P = 40, Qs = 120. The supply curve slopes upward from left to right. A price change produces a movement along the supply curve, while changes in production costs, technology or taxes shift the curve itself.

当 P = 10 时，Qs = 30；当 P = 40 时，Qs = 120。供给曲线自左向右向上倾斜。价格变动引起沿供给曲线的移动，而生产成本、技术或税收的变化则使供给曲线整体平移。

3. Market Equilibrium | 市场均衡

Market equilibrium occurs where the quantity demanded equals the quantity supplied at the prevailing price. The equilibrium price, also called the market-clearing price, is the price at which the market clears, leaving no shortage and no surplus.

市场均衡出现在某一价格水平下需求量等于供给量的情形。均衡价格又称市场出清价格，是使市场出清、既无短缺也无剩余的价格。

To find the equilibrium, set Qd equal to Qs:

要求均衡价格，需要令 Qd = Qs：

120 − 2P = 3P → 5P = 120 → P* = 24

Q* = 120 − 2(24) = 72 units

At the equilibrium price of 24, the quantity bought and sold is 72 units. This is an efficient outcome in a competitive market because the quantity consumers wish to buy exactly matches the quantity firms wish to sell.

在均衡价格 24 时，市场成交量为 72 单位。在竞争性市场中，这是一种有效的结果，因为消费者愿意购买的数量恰好等于生产者愿意出售的数量。

4. Disequilibrium and Market Adjustment | 非均衡与市场调节

If the actual price is above the equilibrium, quantity supplied exceeds quantity demanded, creating excess supply, or a surplus. For example, at P = 30, Qd = 60 and Qs = 90, so there is a surplus of 30 units. Firms respond by cutting prices and reducing output until equilibrium is restored.

如果实际价格高于均衡价格，供给量将超过需求量，形成超额供给，即剩余。例如，当 P = 30 时，Qd = 60、Qs = 90，市场出现 30 单位的剩余。厂商会因此降价并减少产量，直到恢复均衡。

If the actual price is below equilibrium, quantity demanded exceeds quantity supplied, creating excess demand, or a shortage. At P = 10, Qd = 100 and Qs = 30, so there is a shortage of 70 units. Consumers compete for limited goods, pushing the price upward.

如果实际价格低于均衡价格，需求量将超过供给量，形成超额需求，即短缺。当 P = 10 时，Qd = 100、Qs = 30，市场出现 70 单位的短缺。消费者为争夺有限商品而竞相出价，推动价格上升。

5. Shifts of the Demand Curve | 需求曲线的移动

Non-price determinants of demand shift the whole demand curve. Key factors include income, tastes, the prices of substitutes and complements, expectations and the number of buyers.

需求的非价格决定因素会使整条需求曲线发生平移。关键因素包括收入、偏好、替代品与互补品的价格、预期以及购买者数量。

An increase in income raises demand for normal goods but reduces demand for inferior goods.

收入增加会提高正常品的需求，但会减少低档品的需求。
A rise in the price of a substitute, such as coffee, increases demand for the related good, such as tea; a rise in the price of a complement, such as sugar, reduces demand.

替代品价格上升（如咖啡）会增加关联商品（如茶叶）的需求；互补品价格上升（如糖）则会减少需求。
If consumers expect future prices to rise, they buy now, increasing current demand; if they expect prices to fall, they delay purchases, decreasing current demand.

如果消费者预期未来价格上涨，他们会现在购买，从而增加当前需求；如果预期价格下跌，他们会推迟购买，从而减少当前需求。
An increase in the number of consumers shifts demand rightward; a fall in population shifts it leftward.

消费者数量增加使需求曲线右移；人口减少则使其左移。

An outward (rightward) shift of demand raises both equilibrium price and equilibrium quantity. An inward (leftward) shift lowers both.

需求曲线向右（外移）移动会同时提高均衡价格与均衡数量；向左（内移）移动则同时降低两者。

6. Shifts of the Supply Curve | 供给曲线的移动

Non-price determinants of supply shift the whole supply curve. These include costs of inputs, technology, taxes, subsidies, weather and the number of firms.

供给的非价格决定因素会使整条供给曲线发生平移，包括投入成本、技术、税收、补贴、天气以及企业数量。

A fall in the cost of inputs such as wages or raw materials shifts supply rightward; rising costs shift it leftward.

投入品成本（如工资、原材料）下降使供给右移；成本上升则使供给左移。
Technological improvements reduce production costs and increase supply.

技术进步降低生产成本并增加供给。
An increase in indirect taxes, such as excise duty, reduces supply; a subsidy increases supply.

间接税（如消费税）增加会减少供给；补贴会增加供给。
Favourable weather boosts agricultural supply, while natural disasters reduce it; the entry of new firms also increases market supply.

有利的天气会促进农产品供给，自然灾害则减少供给；新企业进入市场同样会增加总供给。

An outward shift of supply lowers equilibrium price and raises equilibrium quantity. This pattern is typical of technological progress in industries such as consumer electronics.

供给右移使均衡价格下降、均衡数量上升。这一模式在消费电子产品等行业的技术进步中十分典型。

7. Simultaneous Shifts | 需求与供给同时变动

In the real world, both curves often shift at the same time. When the directions are mixed, one of the equilibrium outcomes becomes ambiguous; we can usually determine only the direction of price or the direction of quantity, not both.

在现实中，两条曲线经常同时移动。当移动方向不一致时，均衡结果中有一个变量无法唯一确定；我们通常只能判断价格或数量中某一方的变动方向。

Scenario	Equilibrium Price	Equilibrium Quantity
Demand ↑ and Supply ↑	Ambiguous	Increases
Demand ↓ and Supply ↓	Ambiguous	Decreases
Demand ↑ and Supply ↓	Increases	Ambiguous
Demand ↓ and Supply ↑	Decreases	Ambiguous

For example, if a new health report increases demand for avocados while a drought reduces supply, the equilibrium price will definitely rise, but the quantity could rise, fall or remain unchanged depending on the relative size of the two shifts.

例如，若一份新的健康报告增加了牛油果的需求，同时干旱减少了供给，则均衡价格必定上升，但均衡数量的变化方向取决于两条曲线相对移动幅度的大小。

8. Consumer and Producer Surplus | 消费者剩余与生产者剩余

Consumer surplus is the difference between the maximum price a consumer is willing to pay and the actual market price paid. On a diagram, it is the triangular area below the demand curve and above the price line.

消费者剩余是指消费者愿意支付的最高价格与实际支付价格之间的差额。在图形上，它表现为需求曲线之下、价格线之上的三角形区域。

Producer surplus is the difference between the market price a producer receives and the minimum price the producer would accept, which is read from the supply curve. It is the area above the supply curve and below the price line.

生产者剩余是指生产者实际获得的市场价格与其愿意接受的最低价格（由供给曲线读出）之间的差额。在图形上，它表现为供给曲线之上、价格线之下的区域。

At equilibrium, the sum of

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Supply and Demand Curves and Market Equilibrium | 供需曲线与市场均衡