📚 Price Elasticity of Supply | 供给价格弹性
Price elasticity of supply (PES) is one of the core elasticity measures in the Cambridge A-Level Economics syllabus. It explains how producers react to price signals and why some markets can expand output quickly while others cannot.
供给价格弹性（PES）是剑桥 A-Level 经济学大纲中的核心弹性指标之一。它解释了生产者如何对价格信号做出反应，以及为什么有些市场能够迅速扩大产出，而另一些市场则不能。
1. Definition and Core Concept | 定义与核心概念
PES is defined as the percentage change in quantity supplied divided by the percentage change in price. Since the law of supply states that quantity supplied usually increases when price rises, the value of PES is normally positive.
PES 的定义是供给量变动百分比除以价格变动百分比。由于供给定律表明当价格上升时供给量通常增加，因此 PES 值通常为正。
PES = %ΔQS ÷ %ΔP
This positive relationship means that we do not need to ignore a negative sign, as we do with price elasticity of demand. A positive PES value simply shows that quantity supplied moves in the same direction as price.
这种正相关关系意味着我们不需要像处理需求价格弹性那样忽略负号。PES 为正值仅表明供给量与价格同方向变动。
2. Calculating PES | 计算 PES
To calculate PES, first work out the percentage change in price and the percentage change in quantity supplied. Then divide the quantity change by the price change. The result is a numerical value that can be interpreted as elastic or inelastic.
要计算 PES，首先要算出价格变动百分比和供给量变动百分比，然后用供给量变动百分比除以价格变动百分比。计算结果是一个可以用来判断富有弹性或缺乏弹性的数值。
For example, if the market price of a good rises from $10 to $12, that is a 20% increase. If quantity supplied rises from 100 units to 130 units, that is a 30% increase. PES equals 30% ÷ 20% = 1.5.
例如，如果一种商品的市场价格从 10 美元上升到 12 美元，价格上升了 20%。如果供给量从 100 单位增加到 130 单位，供给量增加了 30%。PES 等于 30% ÷ 20% = 1.5。
PES = 30% ÷ 20% = 1.5
Always show the formula and your working in structured questions, even if the arithmetic is simple. Examiners award method marks for the formula, the substitution and the final answer.
即使计算很简单，在结构化题目中也要写出公式和计算步骤。考官会为公式、代入过程和最终答案分别给分。
3. Interpreting PES Values | 解释 PES 数值
PES can range from zero to infinity. A value of zero means supply is perfectly inelastic: quantity supplied does not respond at all to price changes. A value between zero and one means supply is inelastic: the percentage change in quantity supplied is smaller than the percentage change in price.
PES 的取值范围从 0 到无穷大。数值为 0 表示供给完全无弹性：供给量对价格变化完全无反应。数值在 0 和 1 之间表示供给缺乏弹性：供给量变动的百分比小于价格变动的百分比。
A value of one is unit elastic, while a value above one means supply is elastic. Perfectly elastic supply has an infinite PES: any amount is supplied at one price but nothing below it.
数值为 1 表示单位弹性，数值大于 1 表示供给富有弹性。完全弹性供给的 PES 为无穷大：在某一价格下可以供给任意数量，但低于该价格时供给为零。
| PES value | Description | 中文解释 |
|---|---|---|
| 0 | Perfectly inelastic | 完全无弹性 |
| Between 0 and 1 | Inelastic | 缺乏弹性 |
| 1 | Unit elastic | 单位弹性 |
| Above 1 | Elastic | 富有弹性 |
| Infinity | Perfectly elastic | 完全弹性 |
4. Determinants of PES: Time Period | 决定因素：时间周期
Time is the most important determinant of PES. In the momentary period, supply is often fixed because firms cannot change output immediately. In the short run, firms can use existing capacity more intensively, so supply becomes more elastic. In the long run, all inputs are variable, so supply is even more elastic.
时间是 PES 最重要的决定因素。在瞬时期间，供给通常是固定的，因为企业无法立即改变产出。在短期内，企业可以更充分地利用现有产能，因此供给变得更有弹性。在长期内，所有投入都是可变的，因此供给弹性更大。
Agricultural goods have a long production lag. If the price of wheat rises today, farmers cannot harvest more wheat immediately; the supply response occurs only after the next planting cycle. This makes short-run PES in agricultural markets very low.
农产品有较长的生产时滞。如果今天小麦价格上涨，农民无法立即收获更多小麦；供给反应要到下一个种植周期才会出现。这使得农产品市场的短期 PES 非常低。
5. Determinants: Spare Capacity and Stocks | 决定因素：闲置产能与库存
If a firm is operating below full capacity, it can increase output quickly when price rises, so PES is elastic. If a firm is already at full capacity, extra output may require new machinery or factories, making supply inelastic.
如果企业在低于满负荷产能的状态下运营，价格上升时它可以迅速增加产出，因此 PES 富有弹性。如果企业已经满负荷生产，增加产量可能需要新机器或新工厂，从而使供给缺乏弹性。
The ability to hold stocks also affects PES. Goods that can be stored cheaply allow producers to release inventories when prices rise, making supply more flexible. Perishable goods, by contrast, cannot be stored for long, so their supply is less responsive.
持有库存的能力也影响 PES。能够低成本储存的商品使生产者可以在价格上涨时释放库存，从而使供给更灵活。相比之下，易腐坏商品无法长期储存，因此其供给反应较弱。
6. Determinants: Factor Mobility and Production Period | 决定因素：要素流动性与生产周期
If resources can be moved easily from one use to another, supply tends to be elastic. Labour may need retraining, and capital may be specific to one industry, which reduces factor mobility and makes PES lower.
如果资源可以轻易地从一种用途转移到另一种用途，供给往往富有弹性。劳动力可能需要再培训，资本可能专用于某一行业，这会降低要素流动性，使 PES 降低。
A long production period also makes supply inelastic. Products such as houses, ships and aged wine cannot be supplied quickly even when prices rise, because the production process takes years. In these industries, higher prices may not lead to a rapid increase in quantity supplied.
生产周期长也会使供给缺乏弹性。房屋、船舶和陈年葡萄酒等产品即使在价格上涨时也无法快速供应，因为生产过程需要数年时间。在这些行业中，价格上涨可能不会带来供给量的迅速增加。
7. PES and Supply Curve Diagrams | PES 与供给曲线图形
The shape and angle of a supply curve illustrate PES. A vertical supply curve represents perfectly inelastic supply. A horizontal supply curve represents perfectly elastic supply. The steeper the upward-sloping supply curve, the more inelastic the supply; the flatter it is, the more elastic the supply.
供给曲线的形状和角度可以说明 PES。垂直的供给曲线代表完全无弹性供给。水平的供给曲线代表完全弹性供给。向上倾斜的供给曲线越陡峭，供给越缺乏弹性；越平坦，供给越富有弹性。
In exam diagrams, label the axes, draw the initial and new price and quantity, and clearly indicate whether the supply curve is steep or flat when explaining PES. A diagram showing a movement along a given supply curve is usually the correct way to illustrate a change in price.
在考试作图中，要标注坐标轴，画出初始和新的价格与数量，并在解释 PES 时明确指出供给曲线是陡峭还是平坦。表示价格变化时，通常应画出沿给定供给曲线的移动，而不是整条曲线的移动。
8. PES, Tax Incidence and Market Adjustment | PES、税负分担与市场调节
PES interacts with price elasticity of demand (PED) to determine how the burden of an indirect tax is shared between producers and consumers. When supply is more elastic than demand, producers can adjust output easily, so consumers tend to bear a larger share of the tax burden.
PES 与需求价格弹性（PED）相互作用，决定间接税负担如何在生产者和消费者之间分担。当供给比需求更有弹性时，生产者可以轻易调整产出，因此消费者往往承担更大份额的税收负担。
When supply is inelastic and demand is elastic, producers cannot reduce output easily, so they bear more of the tax. This is why governments often consider PES when taxing goods such as cigarettes, alcohol or fuel.
当供给缺乏弹性而需求富有弹性时，生产者难以减少产出，因此承担更多税收。这就是政府在征收香烟、酒精或燃料等商品税时常常考虑 PES 的原因。
9. PES in Commodity Markets and Manufactured Goods | 初级产品市场与制成品中的 PES
Primary commodities such as coffee, cocoa and oil often have inelastic supply in the short run because of long biological or geological production lags. Their supply cannot respond quickly to price changes.
咖啡、可可和石油等初级产品在短期内通常供给缺乏弹性，因为生物或地质生产时滞较长。它们的供给无法迅速对价格变化做出反应。
Manufactured goods may have more elastic supply, especially when firms have spare capacity and can switch production lines. Electronic goods, clothing and standardised components can often be supplied more quickly when prices rise.
制成品的供给往往更具弹性，尤其是当企业有闲置产能并且可以转换生产线时。电子产品、服装和标准化零部件通常能在价格上涨时更快地增加供应。
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
In the Cambridge A-Level exam, you should define PES, state the formula, interpret the value and apply at least two determinants to the context. Use diagrams where the question asks for analysis.
在剑桥 A-Level 考试中，你应该定义 PES，写出公式，解释数值，并结合背景至少运用两个决定因素。当题目要求分析时使用图形。
Common mistakes include confusing the sign of PES with PED, forgetting that supply is positive, ignoring the time period, and drawing shifts in supply instead of movements along the supply curve when describing price changes.
常见错误包括将 PES 与 PED 的符号混淆，忘记供给弹性为正值，忽略时间周期，以及在描述价格变化时错误地画出供给曲线移动而不是沿供给曲线移动。
11. Summary Table: Determinants and PES | 总结表：决定因素与 PES
| Determinant | Effect on PES | 中文解释 |
|---|---|---|
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