Demand | 需求

📚 Demand | 需求

Demand is one of the foundational concepts in IGCSE Economics, shaping how markets operate from the consumer's side. Without a clear understanding of demand, it would be impossible to analyse price determination or predict market outcomes. This article unpacks the meaning of demand, the law of demand, movements versus shifts, and the many factors that influence how much buyers are willing and able to purchase. Each section is designed to help Cambridge IGCSE students build confidence with both explanation and application.

需求是 IGCSE 经济学的基础概念之一，它从消费者一侧决定了市场如何运作。不理解需求，就无法分析价格决定或预测市场结果。本文详细解读需求的定义、需求定律、沿着需求曲线的移动与需求曲线的平移，以及影响购买意愿和能力的各种因素。每个小节都旨在帮助剑桥 IGCSE 学生掌握解释与应用的方法。

1. What is Demand? | 什么是需求？

In economics, demand refers to the quantity of a good or service that consumers are both willing and able to buy at each possible price over a specific time period. The key phrase is 'willing and able'. Desire alone is not enough; effective demand requires purchasing power. For example, a student may want a new smartphone, but without the money to pay for it, this desire does not count as demand in the economic sense.

在经济学中，需求指的是消费者在特定时期内、在每一个可能的价格水平下既愿意又能够购买的商品或服务的数量。关键词是'愿意并且能够'。只有渴望是不够的；有效需求需要支付能力。例如，一个学生可能想要一部新智能手机，但如果没有钱支付，这个渴望在经济意义上并不算作需求。

2. The Law of Demand | 需求定律

The law of demand states that, ceteris paribus (all other things being equal), as the price of a good rises, the quantity demanded falls, and as the price falls, the quantity demanded rises. This inverse relationship between price and quantity demanded is one of the most reliable and universal principles in economics. It reflects both the substitution effect and the income effect: when a good becomes cheaper, consumers switch away from relatively more expensive alternatives, and their real income increases, allowing them to buy more.

需求定律指出，在其他条件不变的情况下，一种商品的价格上升，需求量就会下降；价格下降，需求量就会上升。价格与需求量之间的这种反向关系是经济学最可靠、最普遍的原理之一。它反映了替代效应和收入效应：当一种商品变得更便宜时，消费者会放弃相对更贵的替代品，同时实际收入增加，从而能购买更多。

3. Demand Schedule and Demand Curve | 需求表和需求曲线

A demand schedule is a table that shows the quantity demanded at different price levels, holding other influences constant. The data can be plotted on a graph to create a demand curve, which slopes downward from left to right, reflecting the law of demand. Here is an example:

需求表是显示在不同价格水平下需求量的一张表格，并假定其他影响因素不变。将这些数据在坐标图上描点，便可画出需求曲线。需求曲线从左向右向下倾斜，反映了需求定律。下面是一个例子：

Price ($)	Quantity Demanded (units)
10	100
8	150
6	210
4	280

The demand curve is typically labelled D, with price on the vertical axis and quantity demanded on the horizontal axis. A change in the good's own price causes a movement along this same curve, not a shift.

需求曲线通常标为 D，纵轴表示价格，横轴表示需求量。商品自身价格的改变引起的是沿着同一条需求曲线的移动，而不是曲线的平移。

4. Individual vs Market Demand | 个人需求与市场需求

Individual demand refers to the quantity that a single consumer is willing and able to buy at each price. Market demand is the horizontal summation of all individual demands in the market. If Alice demands 5 units at $10 and Ben demands 3 units at the same price, market demand at $10 is 8 units. This aggregation is essential for understanding whole-market behaviour and is always shown as the total demand curve in market diagrams.

个人需求指的是单个消费者在每个价格下愿意且能够购买的数量。市场需求是市场中所有个人需求的水平加总。比如，在价格为10美元时，Alice需求5单位，Ben需求3单位，则市场价格10美元时的市场需求为8单位。这种加总对于理解整体市场行为至关重要，在市场图示中总是以总需求曲线的形式展现。

5. Movements Along the Demand Curve | 需求曲线的移动（沿曲线移动）

When the price of the good itself changes, there is a movement along the existing demand curve. A fall in price causes an extension (or expansion) of demand, shown as a move down and to the right along the curve. A rise in price causes a contraction of demand, shown as a move up and to the left. This is purely a response to a change in the own price, while all other determinants of demand remain unchanged.

当商品本身的价格发生变化时，就会发生沿着现有需求曲线的移动。价格下降引起需求的延伸（或扩展），表现为沿着曲线向右下方移动。价格上升引起需求的收缩，表现为沿曲线向左上方移动。这纯粹是对自身价格变化的反应，而需求的所有其他决定因素保持不变。

Students often confuse 'extension' with 'increase' in demand. Extension means a higher quantity demanded due to a lower price, staying on the same curve. An increase in demand means the entire curve shifts rightwards at every price, triggered by a non-price factor.

学生经常混淆需求的'延伸'和'增加'。延伸是指由于价格降低导致需求量上升，仍在同一条曲线上。需求增加则是指由于非价格因素，整条曲线在每一个价格水平下都向右平移。

6. Shifts of the Demand Curve | 需求曲线的平移

A shift of the demand curve occurs when a factor other than the good's own price changes. When demand increases, the curve shifts to the right (D to D₁), meaning more is demanded at each and every price. When demand decreases, the curve shifts to the left (D to D₂), meaning less is demanded at each price. These shifts are critical for analysing how market equilibrium changes when incomes, tastes, or the prices of related goods alter.

当商品自身价格以外的因素发生变化时，需求曲线就会发生平移。需求增加时，曲线向右平移（D 到 D₁），意味着在每个价格上需求量都增加。需求减少时，曲线向左平移（D 到 D₂），意味着在每个价格上需求量都减少。这些平移对于分析收入、偏好或相关商品价格变化时市场均衡如何改变至关重要。

7. Factors Affecting Demand (Demand Shifters) | 影响需求的因素（需求平移因素）

The main non-price determinants that shift the demand curve include:

使需求曲线平移的主要非价格决定因素包括：

• Income – For normal goods, higher income increases demand; for inferior goods, higher income reduces demand.

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• 收入 – 对于正常品，收入上升需求增加；对于劣等品，收入上升需求减少。

• Prices of related goods – Substitutes (e.g. tea and coffee): a rise in the price of one increases demand for the other. Complements (e.g. cars and petrol): a rise in the price of one reduces demand for the other.

• 相关商品的价格 – 替代品（如茶和咖啡）：一种商品价格上升会使另一种的需求增加。互补品（如汽车和汽油）：一种商品价格上升会使另一种的需求减少。

• Tastes and preferences – Favourable changes in fashion, advertising or health recommendations can boost demand.

• 品味与偏好 – 时尚、广告或健康建议的有利变化会提高需求。

• Population structure and size – A growing population or a change in age distribution increases demand for certain goods.

• 人口结构与规模 – 人口增长或年龄分布变化会增加对某些商品的需求。

• Expectations of future prices – If consumers expect prices to rise, they may buy more now, shifting current demand to the right.

• 对未来价格的预期 – 如果消费者预期未来价格上涨，他们现在可能会买更多，当前需求曲线右移。

• Government policies (indirect effects) – Taxes and subsidies on goods can affect demand, though these often operate through price changes unless they directly alter desirability.

• 政府政策（间接影响） – 对商品征税和补贴会影响需求，尽管通常是通过价格变化起作用，除非政策直接改变商品的吸引力。

8. Normal Goods and Inferior Goods | 正常品与劣等品

Goods can be classified by how demand responds to changes in income. A normal good is one for which demand increases when consumer income rises. Examples include restaurant meals, branded clothing and new cars. An inferior good is one for which demand falls as income rises, because consumers switch to higher-quality substitutes. Examples include basic supermarket own-brand products, second-hand clothes, and public bus travel when people can afford cars.

商品可以根据需求如何随收入变化而变化进行分类。正常品是指当消费者收入增加时需求上升的商品，例如餐馆用餐、品牌服装和新汽车。劣等品是指当收入增加时需求下降的商品，因为消费者会转向质量更好的替代品，例如超市自有品牌基本产品、二手服装和人们买得起汽车后逐渐减少乘坐的公共交通。

It is important to note that 'inferior' does not mean poor quality in an absolute sense; it simply describes the negative relationship between income and demand. A good can be normal for one income group and inferior for another.

需要注意的是，'劣等'并不是绝对意义上的质量差；它只是描述了收入与需求之间的负相关关系。一种商品对某收入群体可能是正常品，对另一收入群体可能是劣等品。

9. Substitutes and Complements | 替代品与互补品

Substitute goods satisfy similar wants. An increase in the price of one substitute shifts the demand curve for the other substitute to the right. For instance, if the price of coffee rises, many consumers will buy more tea, increasing the demand for tea. The cross-price elasticity of demand in this case is positive.

替代品满足相似的需求。一种替代品价格上升会使另一种替代品的需求曲线右移。例如，如果咖啡价格上升，许多消费者会买更多的茶，增加对茶的需求。这种情况下需求的交叉价格弹性为正。

Complementary goods are consumed together. A rise in the price of one complement reduces the demand for the other, shifting its demand curve leftwards. If the price of printers increases, fewer people will buy printer ink cartridges, decreasing the demand for ink. The cross-price elasticity here is negative.

互补品被一起消费。一种互补品价格上升会减少对另一种的需求，使其需求曲线左移。如果打印机价格上涨，买墨盒的人就会减少，墨盒需求下降。这种情况下交叉价格弹性为负。

10. Price Elasticity of Demand (Introduction) | 需求的价格弹性简介

Price elasticity of demand (PED) measures how responsive the quantity demanded is to a change in price. It is calculated as:

需求的价格弹性（PED）衡量需求量对价格变化的反应程度。它的计算公式为：

PED = %Δ Quantity Demanded ÷ %Δ Price

When PED is greater than 1, demand is price elastic, meaning consumers are relatively sensitive to price changes. When PED is less than 1, demand is price inelastic, meaning quantity demanded changes little when price changes. IGCSE students should be able to interpret PED values and link them to total revenue, but the deeper analysis is developed further in the price elasticity topic.

当 PED 大于 1 时，需求富有价格弹性，说明消费者对价格变化相对敏感。当 PED 小于 1 时，需求缺乏价格弹性，说明价格变化时需求量变化不大。IGCSE 学生需要能够解读 PED 数值并将其与总收益联系起来，但更深入的分析会在价格弹性专题中展开。

11. Exceptions to the Law of Demand | 需求定律的例外情况

Though the law of demand holds in most situations, there are rare exceptions. One is Giffen goods – very inferior goods for which a price rise leads to an increase in quantity demanded because the income effect (the feeling of being poorer) overwhelms the substitution effect. The classic 19th-century example is bread when it was a staple food for the poor: as bread prices rose, people could not afford meat and thus bought more bread. Another exception is Veblen goods, where higher prices make the good more desirable as a status symbol, so demand increases with price – often seen with luxury watches or designer handbags.

尽管需求定律在绝大多数情况下成立，但也有罕见的例外。一种是吉芬商品——属于极端劣等品，价格上升时需求量反而增加，因为收入效应（感觉更贫穷）压倒了替代效应。经典的 19 世纪例子是作为穷人主食的面包：面包价格上涨时，人们买不起肉，只好买更多面包。另一种例外是凡勃伦商品，价格越高，商品作为地位象征的吸引力越强，因此需求随价格上升而增加——常见于奢侈手表或设计师手袋。

12. Summary and Exam Tips | 总结与应试技巧

To master the topic of demand for Cambridge IGCSE Economics, focus on the precision of language: always distinguish between a movement along the demand curve (caused by a change in own price) and a shift of the curve (caused by a change in any other determinant). Draw and label demand diagrams accurately, clearly indicating whether a movement is an extension or contraction, or whether the shift is an increase or decrease in demand. Use relevant, real-world examples whenever possible – like referencing how streaming service subscriptions respond to income changes or how a rise in coffee prices affects tea demand. This not only strengthens your analysis but also meets the assessment objectives for application and evaluation.

要想在剑桥 IGCSE 经济学中掌握需求专题，必须注重用词精确：始终区分沿着需求曲线的移动（由自身价格变化引起）和需求曲线的平移（由其他任何一个决定因素的变化引起）。准确绘制并标注需求图示，清楚地标出是延伸还是收缩，或是需求的增加还是减少。尽可能使用相关的现实例子——比如提到流媒体订阅量如何随收入变化而变化，或咖啡价格上涨如何影响对茶的需求。这样做不仅能增强你的分析力，也符合考试中应用和评价的评分目标。

Published by TutorHao | Economics Revision Series | aleveler.com

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