📚 IB CIE Economics: Demand and Supply Key Exam Points | IB CIE 经济：需求与供给 考点精讲
Demand and supply form the core of microeconomics. In IB and CIE AS/A-Level syllabuses, a solid grasp of these concepts is essential for analysing market outcomes, government policies, and real-world events. This revision guide breaks down every key point you need to master, from basic definitions to elasticity applications, with clear bilingual explanations and exam-focused diagrams.
需求与供给是微观经济学的核心。在IB和CIE AS/A-Level课程中，透彻理解这些概念对于分析市场结果、政府政策以及现实事件至关重要。本考点精讲分解了你需要掌握的每个关键点，从基本定义到弹性应用，配有清晰的双语解释和紧扣考试的图表。
1. The Concept of Demand | 需求的概念
Demand refers to the willingness and ability of consumers to purchase a good or service at various prices over a specific period of time. Effective demand requires both desire and purchasing power.
需求是指消费者在特定时期内，在不同价格水平下愿意并且能够购买某种商品或服务的数量。有效需求必须同时具备购买意愿和购买力。
The demand curve is a downward-sloping line showing the inverse relationship between price and quantity demanded, ceteris paribus (all other factors held constant).
需求曲线是一条向下倾斜的线，表示在其他条件不变（ceteris paribus）的情况下，价格与需求量之间的反向关系。
2. The Law of Demand | 需求定律
The law of demand states that as the price of a good increases, the quantity demanded decreases, and as the price falls, the quantity demanded increases. This is due to the income effect and substitution effect.
需求定律指出，当一种商品的价格上升时，其需求量下降；当价格下降时，需求量上升。这归因于收入效应和替代效应。
The income effect: when the price falls, consumers’ real income increases, so they can buy more of the good. The substitution effect: when the price falls, the good becomes relatively cheaper compared to substitutes, prompting consumers to switch towards it.
收入效应：价格下降时，消费者的实际收入提高，因此能够购买更多该商品。替代效应：价格下降时，该商品相对于替代品变得更便宜，促使消费者转而购买它。
3. Determinants of Demand | 需求的决定因素
Factors that shift the entire demand curve (change in demand) include: income (normal vs. inferior goods), tastes and preferences, the price of related goods (substitutes and complements), demographic changes, expectations of future prices, and government policies such as taxes and subsidies.
导致整条需求曲线移动（需求变动）的因素包括：收入（正常品与低档品）、偏好与口味、相关商品的价格（替代品和互补品）、人口结构变化、对未来价格的预期，以及税收和补贴等政府政策。
For normal goods, an increase in income increases demand (rightward shift). For inferior goods, an increase in income decreases demand (leftward shift). Substitutes: an increase in the price of good A increases demand for good B. Complements: an increase in the price of good C decreases demand for good D.
对于正常品，收入增加导致需求增加（曲线右移）。对于低档品，收入增加导致需求减少（曲线左移）。替代品：商品A价格上升会增加对商品B的需求。互补品：商品C价格上升会减少对商品D的需求。
4. The Concept of Supply | 供给的概念
Supply refers to the willingness and ability of producers to offer a good or service for sale at various prices in a given time period. The supply curve is upward-sloping, reflecting a positive relationship between price and quantity supplied.
供给是指生产者在特定时期内，在不同价格水平下愿意并且能够提供出售的商品或服务的数量。供给曲线向上倾斜，反映了价格与供给量之间的正向关系。
The law of supply states that as price increases, quantity supplied increases; as price falls, quantity supplied falls. This is because higher prices increase potential profit, incentivising firms to expand output.
供给定律指出，价格上升时供给量增加，价格下降时供给量减少。这是因为较高的价格增加了潜在利润，激励企业扩大产量。
5. Determinants of Supply | 供给的决定因素
Shifts in the supply curve (change in supply) are caused by: changes in production costs (wages, raw materials, energy), technological progress, indirect taxes and subsidies, prices of related goods in production (joint supply, competitive supply), the number of firms in the market, and climatic conditions (for agricultural products).
导致供给曲线移动（供给变动）的因素包括：生产成本的变化（工资、原材料、能源）、技术进步、间接税和补贴、生产中的相关商品价格（联合供给、竞争性供给）、市场中企业数量以及气候条件（对农产品而言）。
An improvement in technology shifts the supply curve to the right (more supplied at every price). An increase in an indirect tax shifts supply leftwards, while a subsidy shifts it rightwards.
技术进步使供给曲线向右移动（每个价格下供给量增加）。间接税增加使供给向左移动，而补贴使供给向右移动。
6. Market Equilibrium | 市场均衡
Market equilibrium occurs at the price where quantity demanded equals quantity supplied. There is no excess demand (shortage) or excess supply (surplus). The equilibrium price is Pe and equilibrium quantity is Qe.
市场均衡发生在需求量等于供给量的价格水平上。此时没有超额需求（短缺）或超额供给（过剩）。均衡价格为Pe，均衡数量为Qe。
When price is set above equilibrium, a surplus occurs, putting downward pressure on price. When price is below equilibrium, a shortage occurs, putting upward pressure on price. The price mechanism automatically restores equilibrium in a free market.
当价格高于均衡价格时，出现过剩，对价格产生下行的压力。当价格低于均衡价格时，出现短缺，对价格产生上行的压力。在自由市场中，价格机制会自动恢复均衡。
7. Shifts versus Movements Along the Curve | 曲线的移动与沿线变动
A movement along a demand or supply curve is caused solely by a change in the good’s own price. This is called a change in quantity demanded or quantity supplied.
需求或供给曲线的沿线变动仅由商品本身价格的变动引起。这称为需求量或供给量的变动。
A shift of the entire curve occurs when any non-price determinant changes. This is called a change in demand or supply. It is crucial to distinguish these in exam diagrams and explanations.
当任何非价格决定因素发生变化时，整条曲线发生移动。这称为需求或供给的变动。在考试图表和解释中区分两者至关重要。
For example, an increase in demand shifts the demand curve rightwards, raising both equilibrium price and quantity. An increase in supply shifts the supply curve rightwards, reducing equilibrium price but raising equilibrium quantity.
例如，需求增加使需求曲线向右移动，同时提高均衡价格和均衡数量。供给增加使供给曲线向右移动，降低均衡价格但提高均衡数量。
8. Consumer and Producer Surplus | 消费者与生产者剩余
Consumer surplus is the difference between the price consumers are willing to pay and the market price they actually pay. It is the area below the demand curve and above the market price.
消费者剩余是消费者愿意支付的最高价格与实际支付的市场价格之间的差额。它是需求曲线下方、市场价格上方的面积。
Producer surplus is the difference between the market price producers receive and the minimum price they are willing to accept. It is the area above the supply curve and below the market price.
生产者剩余是生产者获得的市场价格与其愿意接受的最低价格之间的差额。它是供给曲线上方、市场价格下方的面积。
Total welfare (community surplus) is the sum of consumer and producer surplus. Market equilibrium maximises total welfare. Any deviation due to price controls or taxes creates deadweight loss.
总福利（社会剩余）是消费者剩余与生产者剩余之和。市场均衡使总福利最大化。任何因价格管制或税收导致的偏离都会产生无谓损失。
9. Price Elasticity of Demand (PED) | 需求价格弹性
PED measures the responsiveness of quantity demanded to a change in price. The formula is:
PED衡量需求量对价格变动的反应程度。其公式为：
PED = %Δ Quantity Demanded ÷ %Δ Price
If PED > 1, demand is elastic (luxuries, many substitutes). If PED < 1, demand is inelastic (necessities, addictive goods). If PED = 1, demand is unit elastic. PED = 0 is perfectly inelastic; PED = ∞ is perfectly elastic.
若PED > 1，需求富有弹性（奢侈品、有众多替代品）。若PED < 1，需求缺乏弹性（必需品、成瘾性商品）。若PED = 1，需求为单位弹性。PED = 0为完全无弹性；PED = ∞为完全弹性。
PED affects total revenue. For elastic demand, a price decrease raises total revenue; for inelastic demand, a price increase raises total revenue. This is a favourite exam calculation and evaluation point.
PED影响总收益。对于富有弹性的需求，降价会增加总收益；对于缺乏弹性的需求，提价会增加总收益。这是考试中备受青睐的计算与评估考点。
10. Applications and Exam Tips | 应用与应试技巧
Be ready to analyse the effects of indirect taxes and subsidies using demand and supply diagrams. An indirect tax shifts supply left, increasing price and reducing quantity. Consumer and producer share of the tax burden depends on relative elasticities.
准备好利用供求图分析间接税和补贴的影响。间接税使供给曲线左移，提高价格并减少数量。消费者和生产者的税收负担份额取决于相对弹性。
Similarly, a subsidy shifts supply right, reducing price and increasing quantity. Price floors (minimum prices) and price ceilings (maximum prices) create surpluses and shortages, respectively, and lead to government intervention or black markets.
类似地，补贴使供给右移，降低价格并增加数量。价格下限（最低价格）和价格上限（最高价格）分别造成过剩和短缺，并引发政府干预或黑市。
When writing exam answers, always explain the initial equilibrium, the shift, the new equilibrium, and the welfare implications. Use precise terminology: ‘demand’ not ‘want’, ‘quantity demanded’ not ‘demand’ when referring to a movement. Label your diagrams clearly and refer to them in your text.
在撰写考试答案时，务必解释初始均衡、移动、新均衡以及福利影响。使用精确术语：提及沿线变动时要用“需求量”而非“需求”。清晰标注图表并在文中引用它们。
| Key Term | Definition |
| Demand | Quantity consumers are willing and able to buy at various prices |
| Supply | Quantity producers are willing and able to sell at various prices |
| Equilibrium | Point where quantity demanded equals quantity supplied |
| PED | Responsiveness of quantity demanded to price changes |
| Consumer Surplus | Difference between willingness to pay and market price |
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