The price system and the microeconomy
A-Level Economics Topic 2 16:11 English narration · English + 中文 subtitles burned in
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Transcript
Nobody in this market is in charge.
这个市场里没有人在指挥。
No official decides what a kilo of tomatoes should cost.
没有哪个官员决定一公斤番茄该卖多少钱。
Yet every stall has a price, and by evening almost everything is sold.
可是每个摊位都有价格，到了傍晚，几乎所有东西都卖光了。
Who chose those numbers?
这些数字是谁定的呢？
Buyers and sellers, together.
是买家和卖家一起定的。
Here is the plan.
本节的安排是这样的。
First demand, and the idea behind it.
先讲需求，以及它背后的道理。
Then supply.
然后是供给。
Then price elasticity of demand — the calculation this paper asks for again and again.
接着是需求价格弹性——这门考试反复要考的计算。
Then income and cross elasticity, and the elasticity of supply.
再讲需求收入弹性、需求交叉弹性，以及供给弹性。
Then equilibrium.
然后是市场均衡。
And last, surplus.
最后是剩余。
Demand is the quantity buyers are willing and able to buy at each price — willing, and also able to pay.
需求是买家在每个价格上愿意并且有能力购买的数量——既要愿意，也要买得起。
The law of demand says that when the price rises, the quantity demanded falls, so the curve slopes downward.
需求定律说：价格上升，需求量下降，所以曲线向下倾斜。
Now the distinction the exam tests every year.
接下来是考试每年都要考的区分。
A change in the good's own price moves you along the curve.
商品自身价格的变化，让你沿着曲线移动。
A change in income, tastes, or a related price shifts the whole curve.
而收入、偏好或相关商品价格的变化，会让整条曲线移动。
Let's make that difference impossible to forget.
我们把这个区别弄到永远不会忘。
On the left, the good's own price falls.
左边，商品自身的价格下降。
The dot slides down the same curve to a larger quantity, and the curve itself never moves.
那个点沿着同一条曲线滑到更大的数量，曲线本身完全没有动。
On the right, income rises instead.
右边，变的是收入。
Every price now has a new quantity, so the whole curve moves right.
现在每个价格都对应新的数量，所以整条曲线向右移动。
Here is the trap: a price change never shifts the curve.
陷阱在这里：价格变化绝不会让曲线移动。
Look at the full picture of demand shifts — the determinants of demand.
看需求移动的完整图景。
Higher income, a fashion wave, or advertising moves demand right to D one.
收入提高、流行风潮或广告，会把需求向右移到迪一。
Lower income, or a good going out of fashion, moves it left to D two.
收入下降，或者商品过时，会把它向左移到迪二。
Related prices matter too.
相关价格也很重要。
If a substitute gets dearer, people switch and demand for this good rises.
如果替代品变贵，人们会转过来买这个商品，需求上升。
If a complement gets dearer, people buy less of both and demand falls.
如果互补品变贵，人们会两种都少买，需求下降。
A larger population also raises demand.
人口增加也会提高需求。
One consumer has an individual demand.
一个消费者有个人需求。
Market demand is every buyer's quantity added up at each price.
市场需求是每个价格上把所有买家的数量加总。
Why does demand slope down?
需求曲线为什么向下倾斜？
Marginal utility is the extra satisfaction from one more unit.
边际效用是多消费一单位所带来的额外满足。
The first cold drink on a hot day is wonderful.
大热天的第一杯冷饮非常爽。
The second is good.
第二杯还不错。
The fourth is worth almost nothing.
第四杯几乎没什么价值了。
That is diminishing marginal utility.
这就是边际效用递减。
Because later units are worth less to you, you will only pay less for them.
因为后面的单位对你价值更低，你只肯用更低的价格去买。
Utility is the satisfaction you get from consuming a good.
效用是你消费一种商品所得到的满足。
Marginal utility is only the extra satisfaction from one more unit.
边际效用只是多消费一单位带来的额外满足。
Watch the bars fall: the first unit is tall, the fifth is short.
看这些柱子在变矮：第一单位很高，第五单位很矮。
That is diminishing marginal utility on a chart.
这就是图上的边际效用递减。
Because later units add less satisfaction, you will only buy them if the price is lower.
因为后面的单位增加的满足更少，你只有在价格更低时才会买它们。
That is the deep reason the demand curve slopes down.
这就是需求曲线向下倾斜的深层原因。
Now the other side.
再看市场的另一边。
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有能力出售的数量。
A higher price means more profit, so firms make more, and the supply curve slopes upward.
价格越高，利润越多，企业就生产得更多，所以供给曲线向上倾斜。
Shifts work the same way.
曲线移动的道理也一样。
Lower costs, better technology, or a subsidy move supply right.
成本下降、技术进步或者政府补贴，会让供给向右移动。
Higher costs, or an indirect tax, move it left.
成本上升或者征收间接税，会让它向左移动。
Market demand and market supply are simply every buyer and every firm added up.
而市场需求和市场供给，就是把每一个买家、每一家企业加总起来。
Now the supply shifts on their own diagram — the determinants of supply.
现在单独看供给移动。
Lower costs of production, better technology, or a subsidy move supply right to S one.
成本下降、技术进步或补贴，会把供给向右移到艾斯一。
Higher costs of production — dearer wages, raw materials or rent — or an indirect tax move it left to S two.
成本上升或间接税，会把它向左移到艾斯二。
The number of firms matters: more firms mean more market supply.
企业数量也重要：企业越多，市场供给越大。
Weather matters for farm goods — a drought cuts harvests and shifts supply left.
对农产品来说，天气很重要——干旱会减少收成，让供给向左移动。
Remember the rule: the good's own price is only a movement along the supply curve.
记住这条规则：商品自身价格只是沿着供给曲线移动。
Costs, technology, tax and subsidy shift the whole curve.
成本、技术、税收和补贴才会移动整条曲线。
Market supply adds every firm at each price.
市场供给是每个价格上把所有企业加总。
Knowing the slope is not enough.
只知道曲线往哪边倾斜还不够。
A firm needs to know by how much.
企业需要知道变化有多大。
Price elasticity of demand measures how strongly quantity answers a price change.
需求价格弹性衡量的是：价格变动时，数量的反应有多强烈。
Watch two markets get exactly the same price rise.
看这两个市场受到完全相同的涨价。
On the left the curve is shallow and quantity collapses — elastic demand.
左边曲线比较平坦，数量大幅下跌——这是富有弹性的需求。
On the right the curve is steep and quantity barely moves — inelastic demand.
右边曲线很陡，数量几乎不动——这是缺乏弹性的需求。
Here is the same idea on a clean diagram.
这里用一张干净的图再看一遍。
Price elasticity of demand is the percent change in quantity demanded over the percent 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 the left the curve is steep, so the same price fall only brings a small rise in quantity — inelastic demand.
左边曲线很陡，同样的降价只带来数量的小幅上升——缺乏弹性。
On the right the curve is shallow, so the same price fall brings a large rise in quantity — elastic demand.
右边曲线比较平，同样的降价带来数量的大幅上升——富有弹性。
Steeper demand means more inelastic.
需求曲线越陡，越缺乏弹性。
The formula is on the top of the figure.
图上方就是公式。
Let's calculate one.
我们来算一道。
The price rises by ten percent, and the quantity demanded falls by twenty-five percent.
价格上涨百分之十，需求量下降百分之二十五。
Find the elasticity, and classify the demand.
求弹性，并判断需求属于哪一类。
Pause here and try it yourself.
先暂停，自己试一试。
Ready?
好了吗？
Start with the formula: percentage change in quantity, over percentage change in price.
先写公式：需求量变化的百分比，除以价格变化的百分比。
Put the numbers in — minus twenty-five over ten.
代入数字——负二十五除以十。
That gives minus two point five.
得到负二点五。
The minus sign is only the law of demand, so ignore it here.
负号只是需求定律的体现，这里可以不管它。
Two point five is bigger than one, so demand is elastic.
二点五大于一，所以需求富有弹性。
Now learn every label.
现在把每个名称都学会。
If the size is more than one, demand is elastic — quantity responds a lot.
如果绝对值大于一，需求富有弹性——数量反应很大。
If it is less than one, demand is inelastic — quantity responds a little.
如果小于一，需求缺乏弹性——数量反应很小。
If it is equal to one, demand is unit elastic — the percent changes match.
如果等于一，需求是单位弹性—— 两边百分比变化一样大。
If it is equal to zero, demand is perfectly inelastic — quantity does not change at all.
如果等于零，需求完全无弹性——数量完全不变。
If it goes to infinity, demand is perfectly elastic — any tiny price rise sends quantity to zero.
如果趋向无穷大，需求完全有弹性——价格只要略涨，数量就变成零。
Think of petrol.
想想汽油。
Drivers need it for work and school, and close substitutes are few in the short run.
上班上学都要用，短期内又几乎没有接近的替代品。
So demand is price-inelastic: when the pump price rises, the quantity bought falls only a little.
所以需求价格缺乏弹性：油价上涨时，购买量只下降一点点。
That is why a tax on petrol raises a lot of revenue for the government without cutting sales much.
这就是为什么对汽油征税能给政府带来大量税收，而销量减少得不多。
The same logic helps firms: if demand is inelastic, a higher price can raise total revenue.
同样的道理也帮到企业：如果需求缺乏弹性，提价可以提高总收益。
So what decides elasticity?
那么弹性由什么决定？
One question: can the buyer go without, or find something else?
问一个问题：买家能不能不买，或者能不能找到别的选择？
Substitutes come first — few substitutes, inelastic; many close ones, elastic.
首先是替代品——替代品少，缺乏弹性；替代品多而且接近，富有弹性。
Next, necessity or luxury.
其次，是必需品还是奢侈品。
Then the share of income: small share inelastic, big share elastic.
然后是它占收入的比重：比重小，缺乏弹性；比重大，富有弹性。
And time: with no time to adjust demand is inelastic, but given time buyers find other options.
还有时间：来不及调整时，需求缺乏弹性；给足时间，买家就能找到别的办法。
Habit and addiction also make demand inelastic.
习惯和成瘾也会让需求缺乏弹性。
Here is why firms care.
企业为什么这么在意弹性？
Total revenue is price times quantity.
总收益等于价格乘以数量。
If demand is inelastic, a higher price loses only a few sales, so total revenue rises.
如果需求缺乏弹性，提价只会失去很少的销量，所以总收益上升。
If demand is elastic, a higher price loses many sales, so total revenue falls.
如果需求富有弹性，提价会失去大量销量，所以总收益下降。
Governments use the same idea: a tax on petrol raises a lot of money without cutting sales much.
政府用的是同一个道理：对汽油征税能收到很多钱，而销量减少得不多。
Price is not the only thing that changes.
会变的不只是价格。
Income elasticity compares demand with income.
需求收入弹性把需求和收入放在一起比较。
For a normal good, income rises and demand rises, so the answer is positive.
正常品：收入上升，需求上升，所以结果是正的。
For an inferior good, demand falls instead, so the answer is negative.
低档品：收入上升，需求反而下降，所以结果是负的。
Cross elasticity compares demand with the price of another good: positive for substitutes, negative for complements.
需求交叉弹性把一种商品的需求和另一种商品的价格放在一起比较： 替代品为正，互补品为负。
Restaurant meals give plus one point five — normal, and a luxury.
餐馆用餐算出来是正一点五——正常品，而且是奢侈品。
Tea and coffee give plus zero point four, so they are substitutes.
茶和咖啡算出来是正零点四，所以它们是替代品。
Income elasticity of demand compares the percent change in quantity with the percent change in income.
需求收入弹性把数量变化的百分比和收入变化的百分比放在一起比较。
A normal good has a positive answer: as income rises, demand rises — the solid line on the chart.
正常品的结果是正的：收入上升，需求上升——图上那条实线。
Necessities have a value between zero and one.
必需品的数值在零到一之间。
Luxuries have a value above one, so demand grows faster than income.
奢侈品大于一，所以需求比收入涨得更快。
An inferior good has a negative answer: as income rises, demand falls — the dashed line.
低档品的结果是负的：收入上升，需求反而下降——那条虚线。
Cheap instant noodles are a classic inferior good.
便宜的方便面就是典型的低档品。
Firms care because, as incomes grow, demand for luxuries grows fastest.
企业在意这一点，因为收入增长时， 奢侈品的需求增长最快。
Let's work the handout numbers.
我们用讲义上的数字来算。
A person's income rises by twenty percent, and their demand for restaurant meals rises by thirty percent.
一个人的收入上升百分之二十， 对餐馆用餐的需求上升百分之三十。
Find the income elasticity and classify the good.
求收入弹性，并判断商品类型。
Pause here and try it. Ready?
先暂停，自己试一试。
Start with the formula: percentage change in quantity demanded, over percentage change in income.
好了吗？ 先写公式：需求量变化的百分比，除以收入变化的百分比。
Put the numbers in — plus thirty over plus twenty.
代入数字——正三十除以正二十。
That gives plus one point five.
得到正一点五。
Positive and bigger than one, so it is a normal good, and a luxury.
符号是正的，所以是正常品。 一点五大于一，所以它还是奢侈品。
Cross elasticity of demand compares demand for one good with the price of another.
需求交叉弹性把一种商品的需求和另一种商品的价格放在一起比较。
For substitutes the answer is positive: tea gets dearer, so coffee demand rises.
替代品的结果是正的：茶变贵了，咖啡需求上升。
For complements the answer is negative: printers get dearer, so ink demand falls — people use them together.
互补品的结果是负的：打印机变贵了，墨水需求下降——因为人们一起用它们。
For unrelated goods the answer is about zero: bread and bicycles do not move together.
不相关商品的结果接近零：面包和自行车不会一起动。
Firms watch rivals with this number.
企业用这个数字盯着对手。
If cross elasticity with a rival is high and positive, a rival price cut will hurt your sales.
如果和对手的交叉弹性又高又为正， 对手降价就会伤害你的销量。
One more calculation.
再算一道。
When the price of tea rises by ten percent, the quantity of coffee demanded rises by four percent.
茶的价格上涨百分之十时，咖啡的需求量上升百分之四。
Find the cross elasticity and say how the two goods are related.
求交叉弹性，并说明这两种商品的关系。
Pause here and try it.
先暂停，自己试一试。
Ready?
好了吗？
Start with the formula: percentage change in quantity of A, over percentage change in price of B.
先写公式：甲商品需求量变化的百分比，除以乙商品价格变化的百分比。
Put the numbers in — plus four over plus ten.
代入数字——正四除以正十。
That gives plus zero point four.
得到正零点四。
The sign is positive, so tea and coffee are substitutes.
符号是正的，所以茶和咖啡是替代品。
Supply has an elasticity too.
供给也有弹性。
It measures how strongly the quantity supplied answers a price change, and it is always positive.
它衡量价格变动时供给量的反应有多强烈，而且它总是正的。
Time matters most.
其中时间最重要。
In the momentary period supply is fixed, so the answer is zero.
在瞬时期，供给是固定的，所以结果是零。
In the short run one factor is still fixed, so supply rises a little.
在短期，至少有一种要素还是固定的，所以供给只上升一点。
In the long run everything can change, and supply is most elastic.
在长期，一切都能改变，供给的弹性最大。
Spare capacity and stocks help too.
闲置产能和库存也有帮助。
Price elasticity of supply is the percent change in quantity supplied over the percent 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。
It is always positive.
它总是正的。
On the left the supply curve is steep: the same price rise brings only a small rise in quantity — inelastic supply, value below one.
左边供给曲线很陡：同样的涨价只带来数量的小幅上升——缺乏弹性，数值小于一。
On the right the curve is flatter: the same price rise brings a large rise in quantity — elastic supply, value above one.
右边曲线比较平：同样的涨价带来数量的大幅上升——富有弹性，数值大于一。
Flatter supply means more elastic.
供给曲线越平，越富有弹性。
Spare capacity, stocks, and mobile factors all make supply more elastic.
闲置产能、库存和要素容易流动，都会让供给更有弹性。
Work one for supply.
再为供给算一道。
The price of a good rises by eight percent, and the quantity supplied rises by four percent.
一种商品的价格上涨百分之八，供给量上升百分之四。
Find the elasticity of supply, and say whether supply is elastic or inelastic.
求供给弹性，并判断供给是否富有弹性。
Pause here and try it.
先暂停，自己试一试。
Ready?
好了吗？
Start with the formula: percentage change in quantity supplied, over percentage change in price.
先写公式：供给量变化的百分比，除以价格变化的百分比。
Put the numbers in — four over eight.
代入数字——四除以八。
That gives zero point five.
得到零点五。
Zero point five is less than one, so supply is inelastic.
零点五小于一，所以供给缺乏弹性。
Now put demand and supply on one diagram.
现在把需求和供给画在同一张图上。
They cross at a single point, and that point is the equilibrium.
它们相交于一点，这一点就是均衡。
The price there is the equilibrium price.
那里的价格是均衡价格。
The quantity there is the equilibrium quantity.
那里的数量是均衡数量。
At that price, what buyers want to buy is exactly what sellers want to sell.
在这个价格上，买家想买的量正好等于卖家想卖的量。
The market clears.
市场出清。
What if the price sits somewhere else?
如果价格停在别的地方呢？
Then the market is in disequilibrium.
那市场就处于非均衡状态。
Set the price too high, and firms want to sell more than buyers will take.
价格定得太高，企业想卖的就比买家愿意买的多。
That gap is excess supply, so sellers cut the price.
这个缺口就是超额供给，于是卖家降价。
Set the price too low, and buyers want more than firms will supply.
价格定得太低，买家想买的就比企业愿意供给的多。
That is excess demand, a shortage, and buyers bid the price up.
这就是超额需求，也就是短缺，买家会把价格抬上去。
Either way the price is pushed back to the middle.
不管哪一种，价格都会被推回中间。
That is the price mechanism: it signals, it rations, it rewards.
这就是价格机制：它发出信号、进行配给、给出激励。
Now shift a curve and watch the equilibrium move.
现在让一条曲线移动，看看均衡怎么变。
Incomes rise, so demand for coffee shifts right.
收入上升，所以对咖啡的需求向右移动。
Follow the new crossing point: the price rises, and the quantity rises with it.
跟着新的交点看：价格上升，数量也跟着上升。
If demand fell instead, both would fall.
如果需求下降，两者都会下降。
A demand shift moves price and quantity the same way.
需求移动会让价格和数量同向变化。
Supply is the mirror.
供给这边正好相反。
Better technology cuts costs in the wheat market, so supply shifts right.
技术进步降低了小麦市场的成本，所以供给向右移动。
The new crossing point is lower and further along: the price falls, and the quantity rises.
新的交点更低、更靠右：价格下降，数量上升。
A supply shift moves price and quantity opposite ways.
供给移动会让价格和数量反向变化。
The same diagram sets wages in the labour market, and the exchange rate in the currency market.
同一张图也决定劳动市场上的工资，以及外汇市场上的汇率。
Memorise the four cases.
把四种情况记牢。
When demand rises and supply stays fixed, both the equilibrium price and the equilibrium quantity rise — that is this figure.
需求上升而供给不变时，均衡价格和均衡数量都上升——就是这张图。
When demand falls, both fall.
需求下降时，两者都下降。
When supply rises, the price falls and the quantity rises.
供给上升时，价格下降、数量上升。
When supply falls, the price rises and the quantity falls.
供给下降时，价格上升、数量下降。
So a demand shift moves price and quantity the same way.
所以需求移动让价格和数量同向变化。
A supply shift moves them opposite ways.
供给移动让它们反向变化。
Always draw the new crossing, then read both axes.
永远先画新的交点，再读两个轴上的变化。
The same demand and supply diagram works in many places.
同一张需求和供给的图可以用在很多地方。
In product markets it sets the price of goods and services.
在产品市场，它决定商品和服务的价格。
In factor markets it sets the price of factors — the demand for and supply of labour set the wage.
在要素市场，它决定要素的价格——劳动的需求和供给决定工资。
In the foreign exchange market, the demand for and supply of a currency set the exchange rate: the price of one currency in terms of another.
在外汇市场，一种货币的需求和供给决定汇率：也就是一种货币用另一种货币表示的价格。
Learn one diagram, and you can read all three markets.
学会这一张图，你就能读懂这三种市场。
One last idea: who gains?
最后一个概念：谁得到了好处？
Some buyers would have paid far more than the market price.
有些买家本来愿意出比市场价高得多的钱。
What they keep is consumer surplus — the blue area below demand and above the price.
他们省下来的部分就是消费者剩余——需求曲线以下、价格线以上的蓝色面积。
Some sellers would have accepted far less.
有些卖家本来愿意接受低得多的价格。
Their gain is producer surplus — the orange area above supply and below the price.
他们多得到的部分就是生产者剩余—— 供给曲线以上、价格线以下的橙色面积。
Together they are total welfare, largest at the free-market equilibrium.
两者加起来就是总福利， 而它在自由市场均衡时最大。
How do the two areas change?
这两块面积怎么变？
Watch if demand rises: the price and quantity rise, so producer surplus rises and consumer surplus usually rises too.
如果需求上升，价格和数量都上升， 所以生产者剩余上升，消费者剩余通常也上升。
Next, if supply rises: the price falls and quantity rises, so consumer surplus rises.
如果供给上升，价格下降、数量上升， 所以消费者剩余上升。
Now if only the price rises while the curves stay fixed, producer surplus goes up but consumer surplus goes down.
如果曲线不动而只有价格上升， 生产者剩余增加，消费者剩余减少。
Together, consumer surplus plus producer surplus is total welfare.
消费者剩余加生产者剩余就是总福利。
Total welfare is largest at the free-market equilibrium — that is why the exam asks you to shade both areas on the diagram.
总福利在自由市场均衡时最大——这就是考试要你在图上同时标出两块面积的原因。
Three marks students throw away.
三个学生常丢的分。
First, movement or shift — the good's own price moves you along the curve; anything else shifts it.
第一，沿线移动还是曲线移动——商品自身价格只是沿线移动， 其他原因才让曲线移动。
Second, quote the formula, then interpret the number: below one inelastic, above one elastic, and say what that does to revenue.
第二，先写公式，再解释算出来的数值： 小于一是缺乏弹性，大于一是富有弹性，并说明它对总收益的影响。
Third, get the signs right — substitutes positive, complements negative, an inferior good negative.
第三，注意正负号——替代品为正，互补品为负，低档品为负。