Demand and Supply: IGCSE Economics Key Points | IGCSE 经济：需求与供给 考点精讲

📚 Demand and Supply: IGCSE Economics Key Points | IGCSE 经济：需求与供给 考点精讲

Demand and supply form the backbone of market economics. Understanding how buyers and sellers interact to determine prices and quantities is essential for IGCSE Economics. This article breaks down the key concepts, diagrams, and exam techniques you need to master the topic of demand and supply.

需求与供给是市场经济学的基石。理解买方与卖方如何互动以决定价格与数量，对 IGCSE 经济考试至关重要。本文将拆解你需要掌握的需求与供给核心概念、图示和解题技巧。

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1. The Law of Demand | 需求定律

The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, and as the price falls, the quantity demanded rises. There is an inverse relationship between price and quantity demanded.

需求定律指出，在其他条件不变的情况下，商品价格上升，需求量下降；价格下降，需求量上升。价格与需求量之间存在反向关系。

This inverse relationship is explained by the income effect and substitution effect. When the price falls, consumers’ real purchasing power increases (income effect), and they switch away from relatively more expensive substitutes (substitution effect).

这种反向关系可由收入效应与替代效应解释。当价格下跌，消费者实际购买力提高（收入效应），同时他们会转向相对更便宜的替代品，放弃那些相对更贵的商品（替代效应）。

The demand curve slopes downwards from left to right. On a diagram, price is on the vertical axis and quantity demanded on the horizontal axis. A typical demand curve is labelled D.

需求曲线从左向右下倾斜。在图形中，价格在纵轴，需求量在横轴。典型的需求曲线标注为 D。

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2. Movement vs Shift of Demand | 需求量的变动与需求的变动

It is crucial to distinguish between a movement along the demand curve and a shift of the entire demand curve. A movement occurs when the price of the good itself changes, leading to a change in quantity demanded.

区分沿需求曲线的移动与整条需求曲线的平移至关重要。当商品自身价格变化时，发生沿曲线的移动，导致需求量变化。

A shift of the demand curve happens when any non-price determinant of demand changes. An increase in demand shifts the curve to the right (from D to D₁); a decrease shifts it to the left (from D to D₂). At every price, consumers now wish to buy more or less.

当任何一个非价格决定因素改变时，需求曲线发生平移。需求增加使曲线右移（从 D 至 D₁），需求减少使曲线左移（从 D 至 D₂）。在任何价格水平下，消费者愿意购买的数量都增加或减少了。

Movement along demand curve	Shift of demand curve
Caused only by a change in the good’s own price	Caused by changes in non-price determinants (income, tastes, etc.)
仅由商品自身价格变化引起	由非价格决定因素的变化引起（收入、偏好等）
Referred to as ‘change in quantity demanded’	Referred to as ‘change in demand’
称为“需求量的变动”	称为“需求的变动”

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3. Factors Affecting Demand | 影响需求的因素

Several non-price factors can shift the demand curve. For IGCSE Economics, you need to explain how each determinant influences demand with clear reasoning.

多种非价格因素能使需求曲线平移。在 IGCSE 经济中，你需要用清晰的推理说明每一个决定因素如何影响需求。

Income: For normal goods, a rise in income increases demand (curve shifts right). For inferior goods, higher income reduces demand as consumers switch to superior alternatives.

收入：对正常商品，收入上升增加需求（曲线右移）。对低档商品，收入提高反而减少需求，因为消费者会转向更优质的替代品。

Price of substitutes: If the price of a substitute good rises, demand for the product increases, as consumers switch away from the now more expensive substitute.

替代品价格：如果替代品的价格上升，该产品的需求就会增加，因为消费者会放弃现在更贵的替代品。

Price of complements: If the price of a complementary good rises, demand for the product falls, since the combined cost of using both goods becomes higher.

互补品价格：若互补品价格上升，对该产品的需求下降，因为同时使用两种商品的组合成本变高了。

Tastes and preferences: Advertising, fashion trends, and seasonal changes can make a product more or less desirable, shifting demand rightwards or leftwards.

口味与偏好：广告、时尚潮流和季节性变化会增强或减弱产品的吸引力，使需求向右或向左平移。

Other determinants include population changes (more buyers increase demand), expectations of future price rises (current demand may increase), and government policies such as subsidies or taxes on related goods.

其他决定因素包括人口变化（购买者增多使需求增加）、对未来价格上涨的预期（当前需求可能增加），以及政府对相关商品补贴或征税等政策。

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4. The Law of Supply | 供给定律

The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied increases, and as the price falls, the quantity supplied decreases. There is a direct relationship between price and quantity supplied.

供给定律指出，在其他条件不变的情况下，价格上升，供给量增加；价格下降，供给量减少。价格与供给量之间存在正向关系。

Firms are motivated by profit. Higher prices provide an incentive to produce and sell more, as the potential profit per unit increases. Additionally, as output expands, firms may face rising marginal costs, so they require a higher price to justify extra production.

企业受利润驱动。较高的价格激励企业生产和销售更多商品，因为单位利润潜力增大。此外，随着产量扩大，企业可能面临递增的边际成本，因此需要更高的价格才愿意增产。

The supply curve slopes upwards from left to right. On a diagram, price is on the vertical axis and quantity supplied on the horizontal axis. The typical supply curve is labelled S.

供给曲线从左向右上倾斜。在图形中，价格在纵轴，供给量在横轴。标准供给曲线标注为 S。

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5. Movement vs Shift of Supply | 供给量的变动与供给的变动

A movement along the supply curve is caused solely by a change in the price of the good itself. This is called a change in quantity supplied.

沿供给曲线的移动仅由商品自身价格变化引起，这称为供给量的变动。

A shift of the supply curve occurs when any non-price determinant changes. A rightward shift (from S to S₁) indicates an increase in supply; firms are willing and able to sell more at each price. A leftward shift (S to S₂) shows a decrease in supply.

当任何非价格决定因素变化时，供给曲线发生平移。右移（从 S 至 S₁）表示供给增加；企业在每一价格下愿意且能够出售更多。左移（S 至 S₂）表示供给减少。

In exam answers, always label shifts clearly and use the correct terms: ‘increase in supply’ vs ‘increase in quantity supplied’.

考试作答时，务必清晰标注平移，并使用正确术语：“供给增加”与“供给量增加”不可混淆。

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6. Factors Affecting Supply | 影响供给的因素

Several determinants can shift the supply curve. Being specific about how each factor changes costs or capacity is key to high marks.

若干决定因素能使供给曲线平移。具体说明每个因素如何改变成本或产能，是拿高分的关键。

Production costs: A rise in wages, raw material prices, or energy costs decreases supply (leftward shift) because producing becomes less profitable.

生产成本：工资、原材料价格或能源成本上升会减少供给（左移），因为生产变得不那么有利可图。

Technology improvements: Better technology raises productivity and lowers unit costs, increasing supply (rightward shift).

技术进步：更好的技术提高生产率，降低单位成本，增加供给（右移）。

Taxes and subsidies: An indirect tax increases costs and reduces supply. A subsidy lowers costs and increases supply. These are common IGCSE diagram questions.

税收与补贴：间接税增加成本、减少供给。补贴降低生产成本、增加供给。这是常见的 IGCSE 作图题。

Number of producers: More firms entering the market increase total market supply (rightward shift). Fewer firms reduce supply.

厂商数量：更多企业进入市场会增加市场总供给（右移）。企业数量减少则供给下降。

Other factors include weather conditions (important for agricultural output), expectations of future price changes, and government regulations.

其他因素包括天气条件（对农产品尤为重要）、对未来价格的预期以及政府监管。

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7. Market Equilibrium & Disequilibrium | 市场均衡与非均衡

Market equilibrium occurs where the demand and supply curves intersect. At this point, the quantity demanded equals the quantity supplied, and there is no tendency for the price to change.

市场均衡发生在需求曲线与供给曲线的交点处。此时需求量等于供给量，价格没有变动的趋势。

The equilibrium price (also called the market-clearing price) balances the plans of buyers and sellers. The equilibrium quantity is the amount traded at that price.

均衡价格（也称市场出清价格）使买卖双方的计划平衡。均衡数量即为该价格下的交易量。

If the price is set above equilibrium, excess supply (surplus) arises. Firms find they cannot sell all they produce, putting downward pressure on price.

若价格设定在均衡价格之上，就会出现超额供给（过剩）。企业发现他们无法售出所有产品，价格面临下行压力。

If the price is below equilibrium, excess demand (shortage) occurs. Buyers compete for the limited goods, driving the price up. This automatic adjustment is the price mechanism.

若价格低于均衡水平，出现超额需求（短缺）。购买者竞相购买限量商品，推动价格上升。这种自动调整就是价格机制。

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8. Changes in Market Equilibrium | 市场均衡的变动

When demand or supply shifts, the equilibrium price and quantity change. You must be able to analyse these shifts in diagrams and explain the outcomes in exams.

当需求或供给发生平移，均衡价格与数量随之改变。你必须能够在图示中分析这些变动，并在考试中解释结果。

Increase in demand (rightward shift): Equilibrium price rises, equilibrium quantity rises. This often results from higher incomes or successful advertising.

需求增加（右移）：均衡价格上升，均衡数量增加。这通常源于收入提高或成功的广告宣传。

Decrease in demand (leftward shift): Equilibrium price falls, equilibrium quantity falls. A fall in the price of a substitute could cause this.

需求减少（左移）：均衡价格下降，均衡数量减少。替代品价格下跌可能导致此情形。

Increase in supply (rightward shift): Equilibrium price falls, equilibrium quantity rises. Improved technology or a subsidy are typical causes.

供给增加（右移）：均衡价格下降，均衡数量增加。技术进步或补贴是常见原因。

Decrease in supply (leftward shift): Equilibrium price rises, equilibrium quantity falls. Higher production costs or a new tax can trigger this.

供给减少（左移）：均衡价格上升，均衡数量减少。生产成本提高或新增税收可能引发此情况。

When both demand and supply shift simultaneously, the net effect on either price or quantity becomes ambiguous without knowing the relative magnitude of the shifts. IGCSE questions sometimes test this higher-level analysis.

当需求与供给同时平移时，若不知平移的相对幅度，价格或数量的净效应便不明确。IGCSE 考题有时会考查这种进阶分析。

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9. Price Elasticity of Demand (PED) | 需求价格弹性

Price elasticity of demand measures the responsiveness of quantity demanded to a change in the good’s own price. It is a vital extension of basic demand theory.

需求价格弹性衡量需求量对商品自身价格变化的反应程度。它是基本需求理论的重要延伸。

PED = % change in quantity demanded ÷ % change in price = %ΔQd ÷ %ΔP

PED = 需求量变动百分比 ÷ 价格变动百分比 = %ΔQd ÷ %ΔP

Demand is price elastic when PED > 1 (ignoring the minus sign): a percentage change in price leads to a larger percentage change in quantity demanded. Demand is price inelastic when PED < 1: quantity demanded changes proportionately less than price.

当 PED > 1（忽略负号）时，需求富有价格弹性：价格百分比变动导致需求量百分比变动更大。当 PED < 1 时，需求缺乏价格弹性：需求量变动比例小于价格变动比例。

Factors influencing PED include the availability of close substitutes, whether the good is a necessity or luxury, the proportion of income spent on it, and the time period considered.

影响 PED 的因素包括：近似替代品的可得性、商品是必需品还是奢侈品、消费占收入的比例，以及所考虑的时间范围。

The relationship between PED and total revenue is a common exam application. If demand is elastic, a price cut raises total revenue; if inelastic, a price rise raises total revenue.

PED 与总收入的关系是常考应用。若需求富有弹性，降价会增加总收入；若缺乏弹性，提价会增加总收入。

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10. Price Elasticity of Supply (PES) | 供给价格弹性

Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. It helps explain how quickly producers can react to market signals.

供给价格弹性衡量供给量对价格变化的反应程度。它有助于解释生产者多快能对市场信号做出反应。

PES = % change in quantity supplied ÷ % change in price = %ΔQs ÷ %ΔP

PES = 供给量变动百分比 ÷ 价格变动百分比 = %ΔQs ÷ %ΔP

Supply is price elastic when PES > 1: producers can increase output easily without a sharp rise in costs. Supply is price inelastic when PES < 1: output is difficult to adjust quickly.

当 PES > 1 时，供给富有价格弹性：生产者能轻易增加产量而成本不会急剧上升。当 PES < 1 时，供给缺乏价格弹性：产量难以迅速调整。

The main determinants of PES are the time period (supply is more elastic in the long run), availability of spare capacity, ease of storage, and the flexibility of the production process.

PES 的主要决定因素包括：时间期限（长期供给弹性更大）、闲置产能的可用性、储存的便利性，以及生产流程的灵活度。

Agricultural products often have price-inelastic supply in the short run because growing cycles cannot be sped up. Manufactured goods may have more elastic supply if spare factory capacity exists.

农产品短期供给常缺乏弹性，因为生长周期无法加速。若工厂有闲置产能，工业制成品的供给弹性可能较大。

Mastering demand and supply, together with the elasticity concepts, equips you to explain a wide range of real-world market outcomes and score highly on IGCSE Economics papers.

掌握需求与供给以及弹性概念，将使你有能力解释众多现实市场结果，并在 IGCSE 经济试卷中夺取高分。

Published by TutorHao | Economics Revision Series | aleveler.com

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