Changes in the Equilibrium | 均衡的变动

📚 Changes in the Equilibrium | 均衡的变动

Market equilibrium is a core concept in Cambridge A-Level Economics. It shows how the price mechanism allocates scarce resources when demand and supply interact freely. This article explains how changes in demand and supply shift the equilibrium price and quantity, and why these changes matter for consumers, producers and the wider economy.

市场均衡是剑桥 A-Level 经济学的核心概念。它展示了当需求与供给自由相互作用时，价格机制如何配置稀缺资源。本文解释需求与供给的变化如何移动均衡价格和数量，以及这些变化为何对消费者、生产者和整体经济都很重要。

1. Market Equilibrium and the Price Mechanism | 市场均衡与价格机制

Equilibrium occurs at the price where quantity demanded equals quantity supplied. At this point, every buyer who is willing and able to pay the equilibrium price can buy the good, and every seller who is willing to accept that price can sell it. The market clears because there is no excess demand and no excess supply.

均衡发生在需求量等于供给量的价格水平。此时，每一个愿意并且能够支付均衡价格的买家都可以买到商品，每一个愿意接受该价格的卖家都可以卖出商品。市场达到出清状态，因为既没有超额需求，也没有超额供给。

The equilibrium price is also called the market-clearing price, and the equilibrium quantity is the amount traded at that price. Any change in demand or supply creates a new equilibrium, which the price mechanism brings about through the forces of competition.

均衡价格也被称为市场出清价格，均衡数量是在该价格下成交的数量。需求或供给的任何变化都会形成新的均衡，而价格机制通过竞争力量来实现这一调整。

2. Disequilibrium: Excess Demand and Excess Supply | 非均衡：超额需求与超额供给

If the market price is below equilibrium, quantity demanded is greater than quantity supplied. This creates excess demand, also called a shortage. Consumers who cannot obtain the good bid the price up, and as price rises, quantity demanded contracts while quantity supplied extends until equilibrium is restored.

如果市场价格低于均衡价格，需求量就会大于供给量。这会产生超额需求，也称为短缺。买不到商品的消费者会抬高价格，随着价格上升，需求量收缩，供给量扩大，直到恢复均衡。

If the market price is above equilibrium, quantity supplied is greater than quantity demanded. This creates excess supply, also called a surplus. Sellers reduce price to clear unsold stocks, causing quantity demanded to extend and quantity supplied to contract until the surplus disappears.

如果市场价格高于均衡价格，供给量就会大于需求量。这会产生超额供给，也称为过剩。卖者降低价格以清空积压库存，从而使需求量扩大、供给量收缩，直到过剩消失。

3. Movements Along the Curve versus Shifts of the Curve | 沿曲线移动与曲线移动

A change in the good’s own price causes a movement along the existing demand or supply curve. For demand, a fall in price causes an extension of quantity demanded, while a rise in price causes a contraction. This is different from a shift of the whole curve.

商品自身价格的变化会引起沿原有需求曲线或供给曲线的移动。对需求而言，价格下降导致需求量扩大，价格上升导致需求量收缩。这与整条曲线的移动不同。

A shift of the demand curve occurs when a non-price determinant changes, such as income, tastes, the price of substitutes or complements, population, or expectations. A shift of the supply curve occurs when production conditions change, such as costs of production, technology, taxes, subsidies, weather, or the number of sellers.

当非价格决定因素发生变化时，需求曲线会发生移动，例如收入、偏好、替代品或互补品价格、人口或预期。当生产条件发生变化时，供给曲线会发生移动，例如生产成本、技术、税收、补贴、天气或卖者数量。

4. An Increase in Demand | 需求增加

An increase in demand shifts the demand curve to the right. At the original equilibrium price, quantity demanded now exceeds quantity supplied, so excess demand appears. Buyers bid up the price, and the market moves to a new equilibrium with a higher price and a higher quantity.

需求增加使需求曲线向右移动。在原来的均衡价格下，需求量现在超过供给量，因此出现超额需求。买者抬高价格，市场移动到新的均衡，价格更高，数量也更大。

For example, if real incomes rise, consumers buy more normal goods such as restaurant meals or new cars. Both equilibrium price and equilibrium quantity increase, ceteris paribus. Cambridge questions often expect students to state both directions clearly: P rises and Q rises.

例如，如果实际收入上升，消费者会购买更多正常商品，如餐厅用餐或新车。在其他条件不变的情况下，均衡价格和均衡数量

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