需求、供给与政策
国际文凭组织 · IB Diploma · 经济学 · HL 知识点 2 9:55 英语旁白 · 英中内嵌字幕
章节
文本转录
One market model runs through this whole topic.
整个主题使用同一个市场模型。
The demand curve slopes down and the supply curve slopes up, and they cross at a price of 20 and a quantity of 100.
需求曲线向下倾斜，供给曲线向上倾斜，两者在价格 20、数量 100 处相交。
Every worked number that follows, from elasticity to taxes and price controls, comes from this one diagram, so each calculation checks against the others.
接下来从弹性到税收和价格管制的每个计算都来自这张图，所以各个计算可以互相核对。
Demand is the quantity consumers are willing and able to buy at each price.
需求是消费者在每个价格上愿意并且能够购买的数量。
The curve slopes down, the law of demand, because of diminishing marginal utility: each extra unit gives less satisfaction, so a lower price is needed to justify it.
需求曲线向下倾斜，即需求定律，原因是边际效用递减：每多一个单位带来的满足更少，所以需要更低的价格。
A change in the good's own price moves along the curve.
商品自身价格变化引起沿曲线移动。
Income, the prices of substitutes and complements, tastes, expectations and the number of consumers shift the whole curve.
收入、替代品和互补品的价格、偏好、预期和消费者数量则使整条曲线移动。
The rational, utility-maximising consumer is a simplification.
理性的、追求效用最大化的消费者是一种简化。
Real choices bend: bounded rationality means people use rules of thumb; herd behaviour means they copy others; framing means the same choice described differently gets a different answer; and information is imperfect.
真实的选择会偏离：有限理性指人们使用经验法则；从众行为指模仿他人；框架效应指同一个选择换种说法就得到不同答案；而且信息是不完全的。
Behavioural economics supplies the evidence.
行为经济学提供了证据。
Supply is the quantity producers are willing and able to sell at each price.
供给是生产者在每个价格上愿意并且能够出售的数量。
Higher prices make production more profitable, so the curve slopes up.
价格越高，生产越有利可图，所以曲线向上倾斜。
Own-price changes move along it.
自身价格变化引起沿曲线移动。
Costs of production, technology, indirect taxes, subsidies and the number of sellers shift it.
生产成本、技术、间接税、补贴和卖方数量使曲线移动。
The profit-maximising producer is also a simplification: firms may satisfice, or pursue social responsibility and stakeholder aims.
追求利润最大化的生产者也是一种简化：企业可能只求满意，或追求社会责任和利益相关者的目标。
Equilibrium is where the curves cross: a price of 20 and a quantity of 100.
均衡是两条曲线的交点：价格 20，数量 100。
At a price of 12, quantity demanded is 116 but quantity supplied is only 68: excess demand of 48, and the price is bid up.
价格为 12 时，需求量是 116，供给量只有 68：超额需求 48，价格被抬高。
At a price of 25, quantity supplied is 120 but quantity demanded is 90: excess supply of 30, and the price is pushed down.
价格为 25 时，供给量是 120，需求量是 90：超额供给 30，价格被压低。
This self-correction is the invisible hand: price signals, rationing and incentives.
这种自我修正就是“看不见的手”：价格信号、配给和激励。
Consumer surplus is the area below the demand curve and above the price: here a triangle of half times 100 times 50, which is 2,500.
消费者剩余是需求曲线以下、价格以上的面积：这里是二分之一乘 100 乘 50 的三角形，等于 2500。
Producer surplus is the area above the supply curve and below the price: a rectangle of 400 plus a triangle of 800, which is 1,200.
生产者剩余是供给曲线以上、价格以下的面积：400 的长方形加 800 的三角形，等于 1200。
Allocative efficiency holds where price equals marginal cost, at the competitive equilibrium, and the sum of the two surpluses is then as large as it can be.
配置效率在价格等于边际成本处实现，即竞争均衡，此时两种剩余之和最大。
Any other price creates a deadweight loss.
任何其他价格都会造成无谓损失。
Price elasticity of demand is the percentage change in quantity demanded divided by the percentage change in price.
需求价格弹性是需求量变化的百分比除以价格变化的百分比。
On our model, the price rises from 20 to 22, a rise of 10 percent, and quantity demanded falls from 100 to 96, a fall of 4 percent.
在我们的模型中，价格从 20 升到 22，上升百分之 10；需求量从 100 降到 96，下降百分之 4。
So PED is minus 4 divided by 10, which is minus 0.4.
所以需求价格弹性是负 4 除以 10，等于负 0.4。
Its size is below 1, so demand is inelastic.
绝对值小于 1，所以需求缺乏弹性。
The total revenue test confirms it.
总收益检验可以证实这一点。
Revenue is price times quantity: 20 times 100 is 2,000; at 22 it is 22 times 96, which is 2,112.
总收益等于价格乘数量：20 乘 100 等于 2000；价格为 22 时是 22 乘 96，等于 2112。
Revenue rose with the price, so demand is inelastic here.
收益随价格上升，所以这里的需求缺乏弹性。
Revenue peaks at 2,450, at a price of 35 and a quantity of 70, where elasticity is exactly 1.
总收益在价格 35、数量 70 处达到峰值 2450，此时弹性正好为 1。
Demand is more elastic when there are close substitutes, when the good is a luxury rather than a necessity, when it takes a large share of income, and over a longer time.
当有相近替代品、商品是奢侈品而不是必需品、在收入中占比大，以及时间更长时，需求更有弹性。
Elasticity also changes along a straight-line demand curve: it is elastic on the high-price stretch and inelastic on the low-price stretch, so the revenue test gives different answers at different points.
沿着一条直线需求曲线，弹性也会变化：高价段富有弹性，低价段缺乏弹性，所以在不同的点上，收益检验会给出不同答案。
Price elasticity of supply is the percentage change in quantity supplied divided by the percentage change in price.
供给价格弹性是供给量变化的百分比除以价格变化的百分比。
When the price rises from 20 to 24, quantity supplied rises from 100 to 116, so PES is plus 0.8.
价格从 20 升到 24 时，供给量从 100 升到 116，所以供给弹性为正 0.8。
It depends on spare capacity, stocks, how easily factors move, and time.
它取决于闲置产能、库存、要素流动的难易和时间。
Primary commodities tend to have inelastic supply, because crops take a season to grow, and often inelastic demand too.
初级商品的供给往往缺乏弹性，因为作物要一季才能长成，需求也常常缺乏弹性。
So small shifts cause large price swings: the volatility that commodity exporters suffer.
所以小的变动就会引起价格大幅波动：这正是商品出口国遭受的不稳定。
Income elasticity of demand is the percentage change in quantity demanded divided by the percentage change in income.
需求收入弹性是需求量变化的百分比除以收入变化的百分比。
It is positive for normal goods, negative for inferior goods, and above 1 for luxuries.
正常品为正，低档品为负，奢侈品大于 1。
Cross elasticity links two goods: positive for substitutes, negative for complements.
交叉弹性把两种商品联系起来：替代品为正，互补品为负。
Firms use income elasticity to forecast over the business cycle, and cross elasticity to price against rivals.
企业用收入弹性预测经济周期中的需求，用交叉弹性针对竞争对手定价。
Governments intervene when markets fail, or for equity.
政府在市场失灵或出于公平考虑时进行干预。
A specific tax of 6 per unit shifts supply up by 6.
每单位 6 的从量税使供给曲线上移 6。
The new equilibrium has consumers paying 24 and a quantity of 92, while producers keep 18.
新的均衡是消费者支付 24，数量为 92，生产者得到 18。
So consumers pay 4 more and producers receive 2 less.
所以消费者多付 4，生产者少得 2。
Tax revenue is 6 times 92, which is 552.
税收收入是 6 乘 92，等于 552。
Demand is the inelastic side here, so consumers bear more of the tax.
这里需求一方缺乏弹性，所以消费者承担更多税负。
A subsidy of 6 per unit reverses the shift: supply moves down by 6.
每单位 6 的补贴方向相反：供给曲线下移 6。
Consumers now pay 16, quantity rises to 108, and the subsidy costs the government 6 times 108, which is 648.
消费者现在支付 16，数量升至 108，补贴使政府花费 6 乘 108，等于 648。
A maximum price of 16, below equilibrium, creates excess demand: 108 minus 84, a shortage of 24, with rationing, queues and black markets.
低于均衡的最高价格 16 造成超额需求：108 减 84，短缺 24，并带来配给、排队和黑市。
A minimum price of 24, above equilibrium, creates excess supply: 116 minus 92, a surplus of 24, which the government may have to buy.
高于均衡的最低价格 24 造成超额供给：116 减 92，过剩 24，政府可能不得不收购。
Quotas restrict supply directly.
配额直接限制供给。
Every intervention answer weighs the correction against its unintended consequences.
每个关于干预的答案都要权衡纠正作用和意外后果。
The market sits at equilibrium and the price rises to 25.
市场处于均衡，价格升到 25。
Quantity demanded falls to 90.
需求量降到 90。
That is minus 10 percent against a price rise of 25 percent, so the size of PED is 0.4: inelastic.
这是下降百分之 10，而价格上升百分之 25，所以需求价格弹性的绝对值为 0.4，缺乏弹性。
Total revenue moves from 20 times 100, 2,000, to 25 times 90, 2,250.
总收益从 20 乘 100 即 2000，变为 25 乘 90 即 2250。
With elasticity below 1, the price rise raises revenue: the total revenue test in one step.
弹性小于 1，提价使收益增加：一步完成总收益检验。
Now a data response.
下面是一道数据题。
In July 2023 India, which supplied about 40 percent of world rice exports, banned exports of non-basmati white rice to keep food prices at home low.
2023 年 7 月，供应全球约百分之 40 大米出口的印度禁止出口非巴斯马蒂白米，以压低国内食品价格。
The world rice price index rose by almost 10 percent in one month, and in August 2023 reached its highest level since 2008.
世界大米价格指数一个月内上涨近百分之 10，并在 2023 年 8 月达到 2008 年以来的最高水平。
Prices jumped in importing countries in Asia and Africa.
亚洲和非洲进口国的米价随之大涨。
India eased the ban in September 2024.
印度于 2024 年 9 月放宽禁令。
Country R imports most of its rice.
R 国的大米大多依靠进口。
The price rose from 80 cents to one dollar a kilogram, up 25 percent, while purchases fell from 50 to 48 thousand tonnes a month, down 4 percent.
价格从每公斤 80 美分升到 1 美元，上涨百分之 25；购买量从每月 5 万吨降到 4.8 万吨，下降百分之 4。
PED is minus 4 divided by 25, which is minus 0.16.
需求价格弹性是负 4 除以 25，等于负 0.16。
Demand is very inelastic: rice is a necessity, so households keep buying it and spend more of their income on it.
需求非常缺乏弹性：大米是必需品，家庭继续购买，并把更多收入花在大米上。
The government of Country R is considering a maximum price of 85 cents a kilogram, below the new equilibrium.
R 国政府考虑把大米最高价格定为每公斤 85 美分，低于新的均衡价格。
At that price sellers would supply 44 thousand tonnes a month, but consumers would want 49.5 thousand.
在这个价格下，卖方每月只供应 4.4 万吨，而消费者想买 4.95 万吨。
That is a shortage of 5.5 thousand tonnes, about 11 percent of what consumers want.
短缺 5500 吨，约占消费者需求的百分之 11。
The price cannot rise to clear the market, so rice is shared out by queues, first come first served, or rationing, and some is sold illegally on a black market.
价格不能上涨来出清市场，所以大米通过排队、先到先得或配给来分配，还有一部分在黑市上非法出售。
Evaluate the cap.
评价这项限价。
For it: demand is very inelastic, poor households spend a large share of income on rice, and the cap is cheap to announce.
支持：需求非常缺乏弹性，贫困家庭把很大一部分收入花在大米上，而且宣布限价几乎没有成本。
Against it: the shortage falls on whoever cannot queue, black-market prices may rise above a dollar, and importers paying the world price will import less, so the shortage grows.
反对：短缺落在无法排队的人身上，黑市价格可能超过 1 美元，而按世界价格进货的进口商会减少进口，使短缺扩大。
Alternatives: pay importers the gap, about 7.2 million dollars a month; release a buffer stock; or give targeted cash transfers.
替代方案：向进口商补足差价，每月约 720 万美元；释放缓冲库存；或发放定向现金补贴。
Judgement: a cap alone is unlikely to protect the poorest.
结论：单靠限价不太可能保护最贫困的人。
If it is used, it should be temporary and backed by government imports.
如果采用，应当是临时的，并由政府进口作为支撑。
Check yourself.
自我检查。
Can you tell a movement from a shift, and explain the law of demand?
你能区分沿曲线移动和曲线移动，并解释需求定律吗？
Find the shortage at 12 and the surplus at 25?
能找出价格 12 时的短缺和 25 时的过剩吗？
Explain consumer surplus of 2,500 and producer surplus of 1,200?
能解释 2500 的消费者剩余和 1200 的生产者剩余吗？
Calculate PED of minus 0.4, run the revenue test, and calculate PES of 0.8?
能算出负 0.4 的需求弹性，做总收益检验，并算出 0.8 的供给弹性吗？
Explain why commodity prices swing?
能解释商品价格为何波动吗？
Work through the tax of 6, the subsidy of 6, and the maximum and minimum prices?
能分析 6 的税、6 的补贴以及最高价和最低价吗？
And in a data response, use the extract's own numbers and end with a judgement?
在数据题中，能使用材料中的数字，并以判断结尾吗？