Demand | 需求

📚 Demand | 需求

Demand is one of the most fundamental concepts in economics. It describes how consumers behave when they decide what to buy at different prices. In this article, we will explore the meaning of demand, the law of demand, the demand curve, and the key factors that can change demand in a market.

需求是经济学中最基本的概念之一。它描述了消费者在不同价格下决定购买什么、购买多少的行为。在本文中，我们将深入探讨需求的含义、需求定律、需求曲线，以及能够使市场需求发生变化的关键因素。

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1. What is Demand? | 什么是需求？

In economics, demand refers to the quantity of a good or service that consumers are willing and able to buy at each possible price over a given period of time. The word ‘able’ is just as important as ‘willing’. A person may want a luxury yacht, but if they cannot afford it, their wanting does not create demand.

在经济学中，需求是指在特定时期内，消费者在不同价格水平上愿意并且能够购买的商品或服务数量。“能够”与“愿意”同样重要。一个人可能想要一艘豪华游艇，但如果买不起，这种欲望并不能构成需求。

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2. The Law of Demand | 需求定律

The law of demand states that, ceteris paribus (all other things remaining equal), there is an inverse relationship between price and quantity demanded. When the price of a good rises, the quantity demanded falls; when the price falls, the quantity demanded rises.

需求定律指出，在其它条件不变（ceteris paribus）的情况下，价格与需求量呈反向变动关系。当商品价格上升时，需求量下降；当价格下降时，需求量上升。

This relationship exists for two main reasons. First, when price rises, the good becomes less attractive compared with substitutes. Second, a higher price reduces the real purchasing power of consumers, so they can afford less of the good.

这种关系存在两个主要原因。第一，当价格上升时，商品相对于替代品变得不那么有吸引力。第二，价格上升会降低消费者的实际购买力，因此他们能负担购买的数量更少。

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3. The Demand Schedule and Demand Curve | 需求表与需求曲线

A demand schedule is a table that shows the quantity demanded of a good at a range of different prices. For example, consider the following demand schedule for apples:

需求表是一种表格，它显示在若干不同价格水平下某种商品的需求量。例如，考虑以下苹果的需求表：

Demand Schedule | 需求表

Price ($) | 价格（美元）	Quantity Demanded | 需求量
1	100
2	80
3	60
4	40
5	20

When these points are plotted on a graph with price on the vertical axis and quantity on the horizontal axis, they produce a demand curve. The curve slopes downward from left to right, showing the inverse relationship between price and quantity demanded.

将这些点绘制在以价格为纵轴、数量为横轴的坐标图上，就得到了一条需求曲线。该曲线从左向右下方倾斜，表明价格与需求量之间的反向关系。

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4. Individual and Market Demand | 个体需求与市场需求

An individual demand curve shows the quantity demanded by one consumer at each price. Market demand is the total quantity demanded by all consumers in the market at each price. To obtain the market demand, we add together the quantity demanded by every individual consumer at each price.

个体需求曲线表示某个消费者在各个价格水平上的需求量。市场需求则是在每个价格水平上市场上所有消费者的需求量总和。要得到市场需求，只需把每个价格上所有消费者的需求量相加即可。

For example, if there are only two consumers in a market and at $3 one buys 10 units and the other buys 5 units, the market demand at $3 is 15 units. As prices change, the same horizontal summation is repeated at every possible price.

例如，假设市场上只有两个消费者，在价格为 3 美元时，一人购买 10 单位，另一人购买 5 单位，那么市场上该价格下的需求量为 15 单位。随着价格变化，我们可以在每个可能的价格上重复同样的水平加总。

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5. Movements along the Demand Curve | 沿需求曲线的移动

A change in the price of the good itself will cause a movement along the existing demand curve. This is called a ‘change in quantity demanded’. A fall in price leads to an extension of demand, while a rise in price leads to a contraction of demand.

商品本身价格的变化会引起曲线上的点沿着同一条需求曲线移动，这被称为“需求量的变化”。价格下降导致需求扩展，价格上升导致需求收缩。

• Extension of demand | 需求扩展：Price falls, quantity demanded rises. This is a movement downwards along the same demand curve.

  价格下降，需求量上升。这是沿同一条需求曲线向下的移动。

• Contraction of demand | 需求收缩：Price rises, quantity demanded falls. This is a movement upwards along the same demand curve.

  价格上升，需求量下降。这是沿同一条需求曲线向上的移动。

Notice that in this case the demand curve itself does not move. The whole curve remains unchanged; only the point on the curve changes.

注意在这种情形下，需求曲线本身并没有移动。整条曲线保持不变，只是曲线上的点发生了变化。

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

A shift of the demand curve occurs when there is a change in any non-price factor that affects demand. The whole curve moves to the right or to the left. A shift to the right means that at every price, consumers are willing and able to buy a larger quantity. A shift to the left means the opposite.

当影响需求的非价格因素发生变化时，整条需求曲线会发生移动，即向右或向左平移。向右移动表示在每一个价格水平上，消费者愿意而且能够购买的数量增加了。向左移动则相反。

Rightward shift → increase in demand | 右移 → 需求增加
Leftward shift → decrease in demand | 左移 → 需求减少

It is important to distinguish between a movement along the curve and a shift of the curve. A price change causes a movement along the curve, while a change in income, tastes, or other non-price factors causes the whole curve to shift.

区分曲线的移动与沿着曲线的移动非常重要。价格变化引起沿曲线的移动，而收入、偏好等非价格因素的变化则引起整条曲线的移动。

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7. Determinants of Demand | 影响需求的因素

Several non-price factors can shift the demand curve. These are often called the conditions of demand or determinants of demand. The most important ones are listed in the table below.

许多非价格因素都会使需求曲线发生移动。这些因素通常被称为“需求条件”或“需求的影响因素”。下表列出了其中最重要的几个。

Factor | 因素	Effect on Demand | 对需求的影响
Income | 收入	Higher income usually increases demand for normal goods; demand for inferior goods may fall.
收入	收入增加通常增加对正常品的需求；对低档品的需求可能下降。
Tastes and preferences | 口味与偏好	If a good becomes more fashionable, demand shifts right; if it becomes unfashionable, demand shifts left.
口味与偏好	如果商品变得更时尚，需求右移；如果变得过时，需求左移。
Prices of substitutes | 替代品价格	A rise in the price of a substitute increases demand for the good; a fall decreases it. For example, if tea becomes dearer, demand for coffee may rise.
替代品价格	替代品价格上升会增加对该商品的需求；替代品价格下降会减少该商品的需求。例如茶叶涨价，咖啡的需求可能上升。
Prices of complements | 互补品价格	A fall in the price of a complement increases demand for the good. For example, cheaper printers increase demand for ink cartridges.
互补品价格	互补品价格下降会增加对该商品的需求。例如打印机降价会增加对墨盒的需求。
Population and demographics | 人口与人口结构	A larger population or a larger number of buyers in the target age group shifts demand right.
人口与人口结构	人口增加或目标年龄组的买方数量增加会使需求右移。
Advertising | 广告	Successful advertising raises consumer awareness and shifts demand right; negative publicity shifts demand left.
广告	成功的广告提高消费者认知度，使需求右移；负面新闻使需求左移。
Expectations of future prices | 对未来价格的预期	If consumers expect prices to rise in the future, they may buy more now, shifting current demand right.
对未来价格的预期	如果消费者预期未来涨价，他们可能现在多买，使当前需求右移。

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8. Normal Goods, Inferior Goods and Exceptions | 正常品、低档品及需求定律的例外

A normal good is a good whose demand increases when consumer income rises. Most goods, such as restaurant meals and new cars, are normal goods. In contrast, an inferior good is one whose demand falls when income rises, because consumers switch to better-quality substitutes. Examples include cheap instant noodles or used clothing.

正常品是指当消费者收入上升时需求增加的物品。大多数商品，如餐厅用餐和新汽车，都是正常品。相反，低档品是指当收入上升时需求下降的商品，因为消费者会转向质量更好的替代品。例如廉价方便面或二手服装。

In nearly all cases, the law of demand holds. However, there are two famous exceptions. A Giffen good is a very inferior good for which the income effect is so strong that when its price falls, consumers buy less of it, and when its price rises, consumers buy more of it. This happens mainly for staple foods in very poor communities. A Veblen good is a luxury, status symbol. Some consumers may demand more of it at a higher price because a higher price makes it more exclusive.

在绝大多数情况下，需求定律成立。然而有两个著名的例外。吉芬商品是一种非常低档的商品，其收入效应极强，以至于价格下降时消费者反而购买更少，价格上升时反而购买更多。这主要发生在贫困群体赖以生存的主食上。凡勃伦商品是一种具有炫耀性、象征身份地位的奢侈品。某些消费者在价格更高时反而需求更多，因为高价格使其显得更独特。

For the Cambridge IGCSE exam, you should remember that the demand curve is usually drawn with a negative slope, but you should also be able to mention these exceptions as a note of realism.

对于剑桥 IGCSE 考试，你应该记住需求曲线通常画成负斜率，但也应当提及这些例外，使理解更加符合现实。

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9. Why Demand Analysis Matters | 需求分析为何重要

Understanding demand helps consumers, firms and governments make better decisions. Firms use demand information to choose what prices to charge and how much to produce. If they know that a price rise will cause a large fall in quantity demanded, they will be careful before raising prices.

理解需求有助于消费者、企业和政府作出更好的决策。企业利用需求信息来决定定价和生产数量。如果企业知道提高价格会导致需求量大幅下降，他们就会谨慎涨价。

Governments also use demand analysis to predict the effects of taxes and price controls. For example, if a government taxes sugary drinks, the demand curve shifts to the left because the effective price paid by consumers rises. Knowing the shape of the demand curve helps policy-makers estimate how much consumption will fall.

政府同样利用需求分析来预测税收和价格管制的影响。例如，如果政府对含糖饮料征税，消费者实际支付的价格上升，需求曲线就会左移。了解需求曲线的形状，可以帮助政策制定者估算消费量会下降多少。

In a market economy, demand is one of the two forces that determine price and output. Without a clear understanding of demand, it would be impossible to explain how resources are allocated in the production of goods and services.

在市场经济中，需求是决定价格和产出的两大力量之一。如果不能清晰理解需求，就不可能解释商品和服务的生产中资源是如何配置的。

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Published by TutorHao | Economics Revision Series | aleveler.com

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