Supply and Demand
AP Microeconomics Topic 2 10:39 English narration · English + 中文 subtitles burned in
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Transcript
Why does a cup of coffee cost what it costs?
一杯咖啡为什么是这个价钱？
Nobody decided it.
没有人决定过它。
There is no committee and no official price list.
既没有委员会，也没有官方价目表。
There are only buyers, who want more when it is cheap, and sellers, who offer more when it is dear.
只有买者——东西便宜时他们想要更多；还有卖者——价格高时他们愿意多供给。
The two push against each other until the price settles where they agree.
双方相互推挤，直到价格停在他们一致同意的地方。
That settling point is what this whole unit is about.
这个停下来的点，正是整个单元要讲的内容。
Today: supply and demand.
今天讲供给与需求。
How each curve is built, what moves it, where the two meet, how strongly people react to a price, and what happens when a government steps in.
每条曲线是怎么来的、什么会让它移动、两者在哪里相遇、 人们对价格的反应有多强，以及政府介入时会发生什么。
This is the model the exam uses more than any other.
这是考试用得最多的一个模型。
Let's begin.
让我们开始吧。
A market only works where property rights are clear — owners can use, exclude others from, and trade what they own, or prices cannot coordinate anything.
市场只有在产权清晰时才能运作——所有者可以使用自己拥有的东西、可以排除别人使用、 也可以拿去交易；否则价格无法协调任何事情。
Start with demand.
先看需求。
Demand is the relationship between the price of a good and the quantity buyers are willing and able to buy.
需求是一种物品的价格与买者愿意且有能力购买的数量之间的关系。
The market demand curve is the horizontal summation of every buyer's: at each price, add up the quantities they all want.
市场需求曲线是把每个买者的需求横向相加：在每一个价格上，把他们想买的数量全部加起来。
The law of demand says price and quantity move in opposite directions, so the curve slopes downward.
需求定律说，价格和数量朝相反方向变动，所以曲线向下倾斜。
Why?
为什么？
Two reasons.
有两个原因。
The substitution effect: when a good gets more expensive, buyers switch to the alternatives.
替代效应：当一种物品变贵，买者就转向替代品。
And the income effect: a higher price shrinks what your money can actually buy.
收入效应：更高的价格会让你的钱实际能买到的东西变少。
Now the distinction that decides exam marks.
接下来是决定考试得分的区分。
If the good's own price changes, you move along the curve — that is a change in quantity demanded.
如果是这种物品自己的价格变了， 你就沿着曲线移动——这叫需求量的变化。
If anything else changes, the whole curve shifts — that is a change in demand.
如果变的是别的东西，整条曲线就移动—— 这叫需求的变化。
Five things shift it, and you can remember them as TRIBE.
有五样东西会让它移动，可以记成 TRIBE。
Tastes.
口味偏好。
Related goods: a dearer substitute raises demand, a dearer complement lowers it.
相关物品：替代品涨价会提高需求，互补品涨价会降低需求。
Income: up for a normal good, down for an inferior good.
收入：正常商品增加，低档商品减少。
Buyers, meaning how many there are.
买者，也就是人数多少。
And expectations about future prices.
以及对未来价格的预期。
Market demand is not one buyer's curve.
市场需求不是某一个买者的曲线。
It is the horizontal summation of every buyer's curve: at each price, add up the quantities they all want.
它是把每个买者的需求横向相加： 在每一个价格上，把他们想买的数量全部加起来。
That is why more buyers — the B in TRIBE — shifts the whole market demand curve to the right.
所以买者变多——TRIBE 里的 B——会把整条市场需求曲线向右移动。
Market supply works the same way.
市场供给也一样。
It is the horizontal sum of every firm's supply curve.
它是把每家企业的供给横向相加。
Add the quantities all firms offer at each price, so more sellers shifts market supply to the right.
在每一个价格上，把所有企业愿意提供的数量加起来， 所以卖者变多会把市场供给向右移动。
Supply is the mirror image.
供给是它的镜像。
It is the relationship between price and the quantity sellers will offer.
它是价格与卖者愿意提供的数量之间的关系。
The law of supply says price and quantity move in the same direction, so the curve slopes upward: a higher price covers higher costs and rewards more output.
供给定律说，价格和数量朝相同方向变动，所以曲线向上倾斜： 更高的价格能覆盖更高的成本，也奖励更多产出。
The same rule applies: the good's own price gives a change in quantity supplied — a movement along the curve — while anything else shifts the curve.
关于移动与移位的规则同样适用。
Input prices, technology, taxes and subsidies, the number of sellers and expectations all shift supply.
投入品价格、技术、税收与补贴、 卖者数量以及预期，都会让供给移动。
Cheaper inputs or better technology shift the whole curve to the right.
更便宜的投入或更好的技术，会把整条曲线向右移动。
Put the two together.
把两者放在一起。
Where they cross is equilibrium: the price at which the quantity buyers want equals the quantity sellers offer, so the market clears.
它们相交的地方就是均衡：在这个价格上， 买者想买的数量等于卖者愿意提供的数量，市场出清。
Away from it, the market corrects itself.
偏离这一点，市场会自我修正。
Set the price above equilibrium and sellers offer more than buyers want — a surplus — and the price gets pushed down.
把价格定在均衡之上， 卖者提供的比买者想要的多——这是过剩——价格就会被压下去。
Set it below and buyers want more than sellers offer — a shortage — and the price gets pushed up.
把价格定在均衡之下，买者想要的比卖者提供的多——这是短缺——价格就会被推上去。
Watch what happens when a determinant moves a curve.
看看某个决定因素让曲线移动时会发生什么。
Incomes rise, so demand shifts to the right.
收入上升，于是需求向右移动。
The new crossing point sits higher and further right: both the price and the quantity go up.
新的交点更高、更靠右：价格和数量都上升了。
A single shift always gives a definite answer for both.
单条曲线移动时，两者的结果总是确定的。
But if both curves shift at once, one of the two — price or quantity — is indeterminate: you cannot predict it without knowing which shift is bigger.
但如果两条曲线同时移动，价格和数量中就会有一个无法预测， 除非你知道哪一次移动更大。
That is a favourite exam trap.
这是考试最爱设的陷阱。
Equilibrium is not only where the market lands; it is where the gains are largest.
均衡不仅是市场最终停下的地方，也是收益最大的地方。
Consumer surplus is the area below the demand curve and above the price — the gap between what buyers would have paid and what they do pay.
消费者剩余是需求曲线以下、价格以上的面积—— 买者原本愿意支付的钱与实际支付的钱之间的差额。
Producer surplus is the area above the supply curve and below the price.
生产者剩余是供给曲线以上、价格以下的面积。
Add them together and you get total surplus, and the competitive equilibrium maximizes it.
把两者相加就是总剩余，而竞争均衡让它达到最大。
That is what economists mean by calling a free market allocatively efficient.
这就是经济学家所说的配置有效率。
Knowing the direction is not enough; the exam wants the size.
只知道方向还不够，考试要的是幅度。
Price elasticity of demand is the percentage change in quantity demanded, divided by the percentage change in price, taken as a positive number.
需求价格弹性是需求量的变动百分比 除以价格的变动百分比，取正数。
Above one is elastic.
大于一是富有弹性。
Below one is inelastic.
小于一是缺乏弹性。
Exactly one is unit elastic.
正好等于一是单位弹性。
Now look at the two pictures.
现在看这两幅图。
On the left the curve is steep, so the same price fall barely changes quantity.
左边的曲线很陡，所以同样的降价几乎不改变数量。
On the right the curve is flat, so the same price fall moves quantity a lot.
右边的曲线很平，所以同样的降价让数量变动很大。
Elasticity earns its keep here.
弹性在这里派上大用场。
Total revenue is price times quantity.
总收入等于价格乘以数量。
If demand is elastic, quantity reacts more than price does, so cutting the price actually raises revenue.
如果需求富有弹性，数量的反应比价格更大，所以降价反而提高总收入。
If demand is inelastic, quantity barely reacts, so cutting the price lowers revenue.
如果需求缺乏弹性，数量几乎不反应，所以降价会降低总收入。
One detail the exam loves: along a straight-line demand curve elasticity is not constant.
有一个考试很爱的细节：沿着一条直线需求曲线，弹性并不是常数。
It is elastic near the top and inelastic near the bottom.
靠上的一段富有弹性，靠下的一段缺乏弹性。
And demand is more elastic when there are more substitutes, when the good is a luxury, and when buyers have more time.
而当替代品更多、物品是奢侈品、买者有更多时间时，需求就更富有弹性。
Three more elasticities, and each is quick.
还有三个弹性，每个都很快。
Price elasticity of supply measures how strongly sellers react.
供给价格弹性衡量卖者的反应有多强。
It is larger when firms have spare capacity, and above all when they have more time.
当企业有闲置产能时它更大，而最重要的是当他们有更多时间时。
Cross-price elasticity divides the percentage change in one good's quantity by the percentage change in another good's price.
交叉价格弹性是用一种物品数量的变动百分比， 除以另一种物品价格的变动百分比。
Read the sign: positive means substitutes, negative means complements.
看符号：正号表示替代品，负号表示互补品。
Income elasticity divides the change in quantity by the change in income.
收入弹性是用数量的变动除以收入的变动。
Positive means a normal good, negative means an inferior good, and above one means a luxury.
正号是正常商品， 负号是低档商品，大于一则是奢侈品。
Demand is more elastic when the good takes a large share of income — a car, not a pencil.
当这种物品占收入的份额很大时，需求更富有弹性——汽车是，铅笔不是。
Supply is more elastic when the good is storable, and it is more elastic in the long run than in the short run: given time, firms can add capacity.
当物品可以储存时，供给更富有弹性；长期也比短期更富有弹性： 给企业时间，它们就能增加产能。
When the price of coffee rises ten percent, the quantity of tea demanded rises six percent.
咖啡价格上涨百分之十时，茶叶的需求量上升百分之六。
Cross-price elasticity is six divided by ten, which is plus zero point six.
交叉价格弹性是六除以十，等于正的零点六。
The positive sign confirms coffee and tea are substitutes.
正号确认咖啡和茶是替代品。
Pause here and try the next one.
先暂停，试下一题。
A five percent rise in incomes raises restaurant-meal demand ten percent.
收入上升百分之五，餐厅餐点的需求上升百分之十。
Income elasticity is ten divided by five, which is plus two.
收入弹性是十除以五，等于正的二。
That is a normal good, and a luxury because it exceeds one.
这是正常商品， 而且因为大于一，所以是奢侈品。
Now let a government step in.
现在让政府介入。
A price ceiling is a legal maximum, like rent control.
价格上限是法定的最高价，比如租金管制。
Set below the equilibrium, it holds the price down and creates a lasting shortage — watch the gap open between what buyers want and what sellers offer.
设在均衡之下，它把价格压住，造成持续的短缺—— 看买者想要的数量和卖者愿意提供的数量之间裂开的缺口。
A price floor is the opposite: a legal minimum, like a minimum wage.
价格下限正相反：法定的最低价，比如最低工资。
Set above the equilibrium, it creates a lasting surplus.
设在均衡之上，它造成持续的过剩。
Both stop trades that the two sides wanted, so total surplus falls.
两者都阻止了双方本来都愿意做的交易，所以总剩余下降。
An excise tax works differently.
消费税的作用方式不同。
It adds a cost to every unit sold, so the whole supply curve shifts upward by the amount of the tax.
它给每一单位销售都加上一笔成本， 所以整条供给曲线按税额向上移动。
The new crossing point gives a higher price for buyers and a smaller quantity traded.
新的交点意味着买者面对更高的价格， 成交量也更小。
Now the question examiners ask: who really pays it?
接下来是考官会问的问题：到底是谁在承担这笔税？
The side that cannot walk away.
是那个走不掉的一方。
Whichever side is more inelastic carries more of the burden.
哪一方更缺乏弹性，就承担更多的负担。
And between the curves sits a wedge of trades that were worth doing and now never happen.
而在两条曲线之间，还留下了一块本来值得做、如今却不再发生的交易。
That is deadweight loss.
这就是无谓损失。
A subsidy is the tax in reverse.
补贴是税收的反向操作。
The government pays sellers for each unit, so the whole supply curve shifts down.
政府按每一单位向卖者付钱， 所以整条供给曲线向下移动。
Quantity traded rises, and the new price is lower for buyers.
成交量上升，买者面对的新价格更低。
Shade the graph the same way you shade a tax: consumer surplus, producer surplus, the government's outlay, and the deadweight-loss triangle of extra units that cost more to make than they are worth.
图画法和征税时一样：标出消费者剩余、生产者剩余、政府支出， 以及那些生产成本高于价值的多余产量所形成的无谓损失三角形。
Incidence still falls on the more inelastic side.
负担仍然落在更缺乏弹性的一方。
Last, open the market to the world.
最后，把市场向世界开放。
If the world price is below the domestic price, the country imports: domestic sellers supply less, buyers want more, and imports fill the gap.
如果世界价格低于国内价格，这个国家就进口： 国内卖者供给减少，买者想要更多，进口填补了缺口。
Consumers gain more than producers lose, so total surplus rises.
消费者的收益大于生产者的损失，所以总剩余上升。
If the world price is higher, the country exports instead, and producers gain more than consumers lose.
如果世界价格更高，这个国家就转为出口，生产者的收益大于消费者的损失。
A tariff is a tax on imports, and a quota is a limit on them.
关税是对进口征的税。
Either one raises the domestic price, helps domestic producers, hurts consumers by more, and creates deadweight loss.
它抬高国内价格，帮助国内生产者， 但对消费者的伤害更大，并造成无谓损失。
Let's do the calculation.
我们来算一算。
A price rise from ten to twelve dollars is a twenty percent rise, and quantity demanded falls from one hundred to eighty-five — a fifteen percent fall.
价格从十美元涨到十二美元，是上涨百分之二十； 需求量从一百降到八十五，是下降百分之十五。
Find the elasticity, and say what happens to total revenue.
求弹性，并说明总收入会怎样变化。
Pause here and try it.
先暂停，自己试一试。
Ready?
好了吗？
Elasticity is fifteen divided by twenty, which is zero point seven five.
弹性等于十五除以二十，也就是零点七五。
That is less than one, so demand is inelastic.
它小于一，所以需求缺乏弹性。
And because it is inelastic, revenue moves with the price.
正因为缺乏弹性，总收入与价格同向变动。
Check it: ten times one hundred is one thousand, and twelve times eighty-five is one thousand and twenty.
验算一下： 十乘以一百是一千，十二乘以八十五是一千零二十。
Three marks students give away.
三个学生白白丢掉的分。
First, decide whether the event changes the good's own price — that moves you along the curve — or a determinant, which shifts the whole curve.
第一，判断这件事改变的是物品自己的价格—— 那是沿着曲线移动——还是某个决定因素，那会让整条曲线移位。
Second, when both curves shift, one of price or quantity is simply unknown; say so, and do not guess.
第二，当两条曲线同时移动时，价格和数量中有一个就是未知的； 要说出来，不要瞎猜。
Third, tax incidence falls on the more inelastic side.
第三，税收负担落在更缺乏弹性的一方。
Get those three right, and this unit is yours.
把这三点做对，这个单元就是你的了。