📚 Elasticity: Key Concepts Explained | 弹性：考点精讲
Elasticity measures how much one variable responds to changes in another. In GCSE Economics, you need to understand price elasticity of demand (PED), price elasticity of supply (PES), and income elasticity of demand (YED). This guide covers all the key formulas, interpretations, and exam-style applications.
弹性衡量一个变量对另一个变量变化的反应程度。在GCSE经济学中，你需要掌握需求价格弹性（PED）、供给价格弹性（PES）和需求收入弹性（YED）。本指南涵盖了所有关键公式、解读和考试应用。
1. What is Elasticity? | 什么是弹性？
Elasticity is a numerical measure of responsiveness. If a small change in price causes a large change in quantity, we say the good is elastic. If quantity barely changes, it is inelastic. Elasticity allows economists and businesses to predict how markets react to shifts in price or income.
弹性是一个衡量反应程度的数值。如果价格的小幅变化引起数量的大幅变化，我们说该商品富有弹性。如果数量几乎不变，则是缺乏弹性的。弹性使经济学家和企业能够预测市场对价格或收入变化的反应。
The three main types you will study are PED, PES, and YED. Each follows a similar logic but focuses on different relationships. Memorising the formulas is essential, but understanding what the numbers mean in real-world situations is even more important for exam success.
你将学习的三种主要弹性是PED、PES和YED。每一种遵循相似的逻辑，但关注不同的关系。记住公式是必要的，但理解数字在现实世界中的含义对考试成功更为重要。
2. 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 tells us how sensitive consumers are to price changes. PED is always negative because price and quantity demanded move in opposite directions, but for simplicity we often ignore the minus sign and focus on the absolute value.
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。它告诉我们消费者对价格变化的敏感度。PED总是负数，因为价格和需求量反向变动，但为了简化，我们通常忽略负号而关注绝对值。
Goods with many close substitutes, luxury items, or products that take up a large share of income tend to have more elastic demand. Necessities, addictive products, and items with few alternatives tend to have inelastic demand. In exams, you will often be asked to calculate PED and explain its implications.
拥有许多近似替代品、奢侈品或占收入较大份额的产品往往需求更有弹性。必需品、上瘾性产品以及替代品少的商品往往需求缺乏弹性。考试中常要求你计算PED并解释其含义。
3. Calculating PED | 计算 PED
The formula for PED is: percentage change in quantity demanded divided by percentage change in price. Always use the midpoint method unless the question specifies otherwise. Write the formula clearly before substituting numbers.
PED的公式是：需求量变动的百分比除以价格变动的百分比。除非题目另有说明，始终使用中点法。在代入数字之前清晰地写出公式。
PED = %Δ Qd ÷ %Δ P
For example, if a 10% price cut leads to a 20% increase in quantity demanded, PED = 20% ÷ -10% = -2. Ignoring the sign, the value is 2, indicating elastic demand. If a 10% price rise causes only a 5% fall in quantity demanded, PED = -5% ÷ 10% = -0.5, which is inelastic.
例如，如果降价10%导致需求量增加20%，PED = 20% ÷ -10% = -2。忽略符号，数值为2，表明需求富有弹性。如果涨价10%仅导致需求量下降5%，PED = -5% ÷ 10% = -0.5，表明需求缺乏弹性。
Remember: percentage change = (new value – old value) / old value × 100. In exam calculations, show your working step by step to earn method marks even if the final answer is slightly off.
记住：百分比变化 = （新值 – 旧值）/ 旧值 × 100。在考试计算中，逐步展示计算过程，以便即使最终答案略有偏差也能获得方法分。
4. Interpreting PED Values | 解读 PED 数值
Economists classify PED into five categories based on the absolute value. This classification is crucial for multiple-choice and data-response questions. Always refer to the number, not just the label.
经济学家根据绝对值将PED分为五类。这种分类对于选择题和数据分析题至关重要。始终引用具体数字，而不仅仅是标签。
| Absolute PED | Classification / 分类 | Meaning / 含义 |
|---|---|---|
| 0 | Perfectly inelastic 完全无弹性 |
Quantity demanded does not change at all when price changes. A vertical demand curve. |
| Between 0 and 1 | Inelastic 缺乏弹性 |
Percentage change in Qd is smaller than percentage change in price. |
| 1 | Unit elastic 单位弹性 |
Percentage change in Qd equals percentage change in price. Total revenue stays constant. |
| Greater than 1 | Elastic 富有弹性 |
Percentage change in Qd is larger than percentage change in price. |
| Infinity / ∞ | Perfectly elastic 完全弹性 |
Any price rise causes quantity demanded to drop to zero. A horizontal demand curve. |
Perfectly inelastic and perfectly elastic are extreme cases rarely found in real markets, but they help illustrate the concept. Most real-world goods fall somewhere between elastic and inelastic.
完全无弹性和完全弹性是极端情况，现实中罕见，但有助于说明概念。大多数现实商品介于富有弹性和缺乏弹性之间。
5. Factors Affecting PED | 影响需求价格弹性的因素
The main factors that determine whether demand is elastic or inelastic include the availability of substitutes, the proportion of income spent on the good, whether the good is a necessity or luxury, the time period considered, and brand loyalty. Understanding these helps explain why some products react sharply to price changes while others do not.
决定需求是富有弹性还是缺乏弹性的主要因素包括：替代品的可获得性、商品支出占收入的比例、商品是必需品还是奢侈品、所考虑的时间长短以及品牌忠诚度。理解这些有助于解释为什么一些产品对价格变化反应剧烈而另一些则不然。
If there are many close substitutes, consumers can easily switch, so PED is high. Addictive goods like cigarettes have few substitutes and are inelastic. Goods that take up a small fraction of income, such as salt, tend to be inelastic because a price rise is barely noticed. Over a longer time period, demand becomes more elastic as consumers find alternatives.
如果有许多近似替代品，消费者可以轻易转换，因此PED较高。像香烟这类成瘾品替代少，需求缺乏弹性。占收入极小部分的商品，如食盐，往往缺乏弹性，因为涨价几乎不被察觉。在较长时期内，当消费者找到替代方案，需求会变得更有弹性。
Exam tip: always link the factor to the ability or willingness of consumers to switch. Use phrases like ‘consumers can easily switch to…’ or ‘it is difficult for consumers to reduce consumption in the short run’.
考试技巧：始终将因素与消费者的转换能力或意愿联系起来。使用诸如’消费者可以轻松转向……’或’在短期内消费者难以减少消费’等短语。
6. PED and Total Revenue | 需求价格弹性与总收入
Total revenue (TR) is the amount a firm receives from sales: price × quantity. The relationship between PED and total revenue is one of the most frequently tested applications. If demand is elastic (PED > 1), a price cut raises total revenue, and a price rise lowers it. If demand is inelastic (PED < 1), a price rise raises total revenue, and a price cut lowers it.
总收入（TR）是企业从销售中获得的金额：价格 × 数量。PED与总收入的关系是最常见的考点之一。如果需求富有弹性（PED > 1），降价会提高总收入，涨价则降低总收入。如果需求缺乏弹性（PED < 1），涨价会提高总收入，降价则降低总收入。
This explains why businesses that sell elastic products often use discounts and promotions, while producers of inelastic goods, like energy companies, can increase prices and see revenue rise. For unit elastic demand, total revenue remains unchanged when price changes. Remember to show the direction of change in exams.
这解释了为什么销售弹性产品的企业经常使用折扣和促销活动，而生产无弹性商品的企业，如能源公司，可以提高价格并看到收入增加。对于单位弹性需求，价格变化时总收入保持不变。考试中记得标明变化方向。
A common mistake is to confuse total revenue with profit. PED analysis only tells us about revenue, not costs. You must state that higher revenue does not automatically mean higher profit unless costs are controlled.
一个常见错误是将总收入与利润混淆。PED分析只告诉我们收入情况，而非成本。你必须说明，更高的收入并不自动意味着更高的利润，除非成本得到控制。
7. 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 shows how easily producers can expand output when the market price rises. Unlike PED, PES is usually positive because a higher price encourages firms to supply more.
供给价格弹性衡量供给量对价格变化的反应程度。它表明当市场价格上升时，生产者扩大产出的难易程度。与PED不同，PES通常为正，因为更高的价格鼓励企业供给更多。
PES is critical for understanding how quickly markets can adjust to shortages or surpluses. In the short run, supply is often inelastic because firms face capacity constraints. In the long run, supply becomes more elastic as firms can build new factories, hire more workers, and adopt new technology.
PES对于理解市场如何快速调整短缺或过剩至关重要。短期中，供给通常缺乏弹性，因为企业面临产能约束。长期中，当企业可以建设新工厂、雇佣更多工人和采用新技术时，供给会变得更有弹性。
8. Calculating and Interpreting PES | 计算与解读 PES
The formula mirrors PED: PES = percentage change in quantity supplied divided by percentage change in price. Use the same percentage change method and always present the formula before substituting numbers.
公式与PED相似：PES = 供给量变动的百分比 ÷ 价格变动的百分比。使用相同的百分比变动方法，并在代入数字之前始终先展示公式。
PES = %Δ Qs ÷ %Δ P
For example, if price rises by 15% and quantity supplied increases by 30%, PES = 30% ÷ 15% = 2, which is elastic. If price rises by 20% but quantity supplied only increases by 10%, PES = 10% ÷ 20% = 0.5, which is inelastic.
例如，如果价格上涨15%，供给量增加30%，PES = 30% ÷ 15% = 2，富有弹性。如果价格上涨20%但供给量仅增加10%，PES = 10% ÷ 20% = 0.5，缺乏弹性。
Interpreting PES: a value greater than 1 means supply is elastic (producers can respond strongly to price signals). A value between 0 and 1 indicates inelastic supply. A value of 0 means perfectly inelastic supply (vertical supply curve), while infinity means perfectly elastic supply (horizontal).
解读PES：数值大于1表示供给富有弹性（生产者能强烈响应价格信号）。数值介于0和1之间表示供给缺乏弹性。数值为0意味着完全无弹性（垂直供给曲线），无穷大意味着完全弹性（水平曲线）。
9. Factors Affecting PES | 影响供给价格弹性的因素
Key determinants of PES include the time period, the availability of stocks (inventories), spare production capacity, the ease of substituting resources between products, and the complexity of the production process. Time is often the most important factor in exam answers.
PES的关键决定因素包括时间长短、库存可获得性、闲置产能、资源在商品之间转换的难易程度以及生产过程的复杂性。时间通常是考试答案中最重要的因素。
In the immediate run (momentary period), supply is often perfectly inelastic because firms cannot change output at all. In the short run, at least one factor of production is fixed, so PES is inelastic but greater than zero. In the long run, all factors are variable, so PES tends to be elastic. Agricultural products often have inelastic supply in the short run due to growing seasons.
在瞬时（即期），供给通常完全无弹性，因为企业根本来不及改变产出。短期中，至少有一种生产要素固定，因此PES缺乏弹性但大于零。长期中，所有要素均可变，因此PES趋于富有弹性。农产品由于生长季节，短期供给往往缺乏弹性。
If a firm holds large inventories, it can quickly increase supply when price rises, making PES more elastic. Similarly, if a factory has spare capacity, supply can respond quickly. Complex production processes, such as shipbuilding, tend to make PES inelastic because ramping up output takes years.
如果企业持有大量库存，它可以在价格上涨时迅速增加供给，使PES更富弹性。同样，如果工厂有闲置产能，供给也能快速响应。复杂的生产过程，如造船，往往使PES缺乏弹性，因为扩大产出需要数年时间。
10. Income Elasticity of Demand (YED) | 需求收入弹性
Income elasticity of demand measures how quantity demanded changes in response to a change in consumers’ incomes. YED is vital for classifying goods as normal or inferior and for predicting how demand patterns shift during economic growth or recession.
需求收入弹性衡量需求量如何随消费者收入的变化而变化。YED对于将商品分类为正常品或劣等品，以及预测经济增长或衰退期间需求模式如何变化至关重要。
YED = %Δ Qd ÷ %Δ Y
If YED is positive, the good is a normal good – demand rises when income rises. If YED is negative, it is an inferior good – demand falls when income rises (e.g. bus travel may fall as people buy cars). Among normal goods, a YED between 0 and 1 indicates a necessity; a YED greater than 1 indicates a luxury.
如果YED为正，该商品是正常品——收入增加时需求上升。如果YED为负，则是劣等品——收入增加时需求下降（例如，随着人们购车，公交出行需求可能下降）。在正常品中，YED介于0和1之间的是必需品；YED大于1的是奢侈品。
For example, if income increases by 5% and demand for organic vegetables rises by 10%, YED = 10% ÷ 5% = 2, marking it as a luxury. If demand for basic bread rises by only 2%, YED = 0.4, a necessity. If demand for instant noodles falls by 3%, YED = -0.6, an inferior good.
例如，如果收入增加5%，有机蔬菜需求上升10%，YED = 10% ÷ 5% = 2，属于奢侈品。如果基本面包需求仅上升2%，YED = 0.4，属于必需品。如果方便面需求下降3%，YED = -0.6，属于劣等品。
11. YED and Business Decisions | 收入弹性与企业决策
Businesses use YED to forecast sales growth during different phases of the economic cycle. Firms producing luxury goods benefit more during a boom but suffer during a recession. Producers of necessities experience more stable demand. Those making inferior goods may see sales rise during economic downturns.
企业使用YED预测经济周期不同阶段的销售增长。生产奢侈品的公司在繁荣期受益更多，但在衰退期受损。必需品生产商面临更稳定的需求。生产劣等品的企业在经济下滑时可能看到销售上升。
Knowing YED helps a firm plan its product portfolio and marketing strategy. A company might add luxury lines when forecasts show rising incomes, or emphasise value ranges during lean times. In exams, you could be asked to advise a business based on given YED values.
了解YED有助于企业规划产品组合和营销策略。当预测显示收入增长时，公司可能增加奢侈产品线；而在拮据时期强调超值系列。考试中可能会要求你根据给定的YED数值向企业提出建议。
Remember that YED can change over time. As a country develops, goods that were once luxuries may become necessities. Mobile phones, for instance, have shifted from luxury to necessity in many countries, lowering their YED.
请记住，YED会随时间改变。随着国家发展，曾经是奢侈品的商品可能变为必需品。例如，手机在许多国家已从奢侈品转变为必需品，其YED随之降低。
12. Exam Tips for Elasticity | 弹性考点的考试技巧
In GCSE Edexcel Economics papers, elasticity questions often come as multiple-choice, short-answer, or part of a longer data-response task. Always define the elasticity, give the formula, calculate correctly, and interpret the number in context. Use economic vocabulary precisely: ‘responsive’, ‘sensitive’, ‘elastic’, ‘inelastic’.
在GCSE爱德思经济学试卷中，弹性问题常以选择题、简答题形式出现，或作为较长的数据分析题的一部分。始终定义弹性，给出公式，正确计算，并结合上下文解读数字。准确使用经济学术语：’有反应的’、’敏感的’、’富有弹性’、’缺乏弹性’。
Common pitfalls include forgetting the minus sign for PED, confusing total revenue with profit, mixing up PED and YED formulas, and failing to mention time periods when discussing PES. Double-check which type of elasticity the question is asking for before you start writing.
常见陷阱包括忘记PED的负号、混淆总收入与利润、搞混PED和YED公式，以及在讨论PES时没有提及时间阶段。在动笔前，先仔细确认题目要求的是哪一种弹性。
Practice drawing linked diagrams: an elastic demand curve is relatively flat, an inelastic curve is steep. Label axes ‘Price’ and ‘Quantity’ and show the effect of a tax or subsidy using the appropriate elasticities. Revise the link between tax incidence and PED/PES: the more inelastic side of the market bears a greater burden of a tax.
练习绘制相关图表：富有弹性的需求曲线相对平缓，缺乏弹性的曲线较陡。坐标轴标注’价格’和’数量’，并运用恰当的弹性展示税收或补贴的效果。复习税收归宿与PED/PES之间的联系：市场中弹性较小的一方承担更大的税收负担。
Finally, use real-world examples in long-answer questions. Mention brands or products you know – for instance, Netflix subscriptions (elastic demand), petrol (inelastic), farm produce (inelastic supply), and luxury holidays (high YED). This shows application and adds depth to your answer.
最后，在长答题中使用现实例子。提及你熟悉的品牌或产品——例如，Netflix订阅（富有弹性需求）、汽油（缺乏弹性）、农产品（缺乏弹性供给）和豪华假期（高YED）。这展示应用能力并使你的答案更有深度。
Published by TutorHao | Economics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导