Our main objectives in this video are
我们在本视频中的主要目标是
• to understand what the law of demand is
• 了解什么是需求定律
• learn how to draw the demand curve and
• 学习如何绘制需求曲线
• to master the intuition or reasoning behind it.
• 掌握其背后的直觉或推理。
To start, I want you to think of the market for petrol. From the previous screencast, we know
首先，我希望您考虑一下汽油市场。从之前的截屏视频中，我们知道
that the existence of petrol brands means that this market is not EXACTLY perfectly competitive,
汽油品牌的存在意味着这个市场并不完全竞争激烈，
but if we abstract from brands and think of petrol in a GENERAL sense, then the market comes
但是，如果我们从品牌中抽象出来，从一般意义上考虑汽油，那么市场就来了
close enough to perfect competition.
接近完美的竞争。
This kind of simplification is frequently done in economics as a way to get a handle on reality.
这种简化在经济学中经常进行，作为处理现实的一种方式。
Markets can be quite complex, but if we begin by making judicious assumptions, then we can try
市场可能相当复杂，但如果我们从做出明智的假设开始，那么我们可以尝试
to understand a simpler version of reality, and we can then become more confident in analysing a
来理解更简单的现实版本，然后我们就可以更有信心地分析
more complicated version of it later on.
稍后会有更复杂的版本。
Now I want you to take the perspective of petrol buyers and think of the amount of petrol they
现在我希望您从汽油购买者的角度考虑一下他们的汽油数量
buy in a year. What are the factors that can lead them to buy more or less petrol?
一年内购买。哪些因素会导致他们购买或多或少的汽油？
Pause the video here while you think of as many factors as you can; otherwise, the discussion
在此处暂停视频，同时考虑尽可能多的因素;否则，讨论
will continue in 5 seconds.
将在 5 秒后继续。
(5-second pause)
（暂停 5 秒）
One factor that quickly comes to mind is the price of petrol. If petrol becomes cheaper, buyers
很快想到的一个因素是汽油的价格。如果汽油变得更便宜，买家
tend to buy more petrol, and if it becomes more expensive, they might economise and buy less.
倾向于购买更多的汽油，如果它变得更贵，他们可能会节省并购买更少。
There are other factors as well. If more cars are bought because they have become cheaper, then
还有其他因素。如果因为汽车变得更便宜而购买更多汽车，那么
more petrol would be bought, even though the price of petrol has not changed.
即使汽油价格没有变化，也会购买更多的汽油。
We can then split these factors into 2 columns.
然后我们可以将这些因素分为 2 列。
In the column here, we put the price of petrol, and in the column here we list all of the
在这里的列中，我们输入了汽油的价格，在这里的列中，我们列出了所有
factors that can cause more or less petrol to be bought even if the price of petrol has not
即使汽油价格尚未购买，也可能导致购买更多或更少汽油的因素
changed. To *this* column we add the price of natural gas, diesel, and electricity, which are
改变。在*此*列中，我们添加了天然气、柴油和电力的价格，它们是
petrol substitutes: clearly, if these products, which can be used in place of petrol, become
汽油替代品：显然，如果这些可以代替汽油使用的产品变成
cheaper, it might encourage petrol buyers to switch and buy less petrol. We also add population,
更便宜，它可能会鼓励汽油购买者改用并减少购买汽油。我们还添加了 population、
income, concerns about “clean” energy, and expectations about future petrol prices.
收入、对“清洁”能源的担忧以及对未来汽油价格的预期。
These two factors both involve prices but the difference is that this pertains to actual changes
这两个因素都涉及价格，但区别在于这与实际变化有关
in the price of petrol while this involves expected prices of petrol: people change how much
在汽油价格中，虽然这涉及到预期的汽油价格：人们改变多少
petrol they buy NOW not in response to a change in the price NOW but to a change in the price in
他们现在购买的汽油不是为了响应现在的价格变化，而是为了响应
the FUTURE.
未来。
How many of these factors did you think of? I hope you identified most, if not all of them.
您考虑了多少个因素？我希望你能找到大多数，如果不是全部的话。
3
Now suppose that none of these factors on the right side changed, then all we have left is the
现在假设右侧的这些因子都没有改变，那么我们只剩下
role of the price of petrol itself on the behavior of buyers. If none of these other factors
汽油价格本身对买家行为的作用。如果没有其他任何因素
changed, then there is a negative or inverse relationship between the price of petrol and the
changed，则 Petrol 的价格与
quantity of petrol that consumers would like to buy.
消费者想要购买的汽油数量。
This is none other than the “law of demand”: all else constant, the quantity demanded for a good
这就是“需求法则”：所有其他方面不变，商品所需的数量
or service decreases when its price increases, and increases when its price falls.
或者 service 在价格上涨时减少，当价格下跌时增加。
“All else constant” in Latin is ceteris paribus,
拉丁语中的“所有其他不变”是 ceteris paribus，
and quantity demanded is the amount of a good or a service that buyers are willing and able to
需求数量是买家愿意并能够购买的商品或服务的数量
purchase at a given price.
以给定的价格购买。
Since quantity demanded and price are inversely related, if we take a two-dimensional graph,
由于需求量和价格成反比，如果我们采用二维图，
with quantity demanded plotted on the x-axis and price on the y-axis, the law of demand
需求数量在 x 轴上绘制，价格在 y 轴上绘制，需求定律
corresponds to a downward sloping curve which we call the demand curve.
对应于一条向下倾斜的曲线，我们称之为需求曲线。
The main thing to remember here is the ceteris paribus assumption: when quantity demanded
这里要记住的主要事情是 ceteris paribus 假设：当需求量
changes along a demand curve, it is due only to changes in the price of the good or service. We
沿着需求曲线的变化，它只是由于商品或服务价格的变化。我们
are holding all other factors constant.
保持所有其他因素不变。
We can draw a demand curve for petrol for a single buyer, for a group of buyers, or for an
我们可以为单个买家、一组买家或
entire market.
整个市场。
*This* graph, which appears in the textbook, is for a single buyer, Chloe. So we see that as
*这张*图表出现在教科书中，是针对单个买家 Chloe 的。 所以我们看到
petrol becomes cheaper, Chloe’s quantity of petrol demanded increases.
汽油变得更便宜，Chloe 的汽油需求量增加。
*This* list of prices and corresponding quantities demanded on the right is the demand schedule
右侧*此*价格和相应需求量列表是需求时间表
— a demand schedule is just a table of price-quantity demanded pairs.
- 需求明细表只是价格-数量需求对的表。
The price at which quantity demanded is zero is the “choke price” and the quantity demanded when
需求量为零的价格是“阻塞价格”，需求量
price is zero is called the “freebie quantity”. So we see here that Chloe’s choke price is 7
价格为零称为 “免费赠品数量”。 所以我们在这里看到 Chloe 的扼流圈价格是 7
dollars per gallon, and her freebie quantity is 350 gallons a year.
美元/加仑，她的免费赠品数量为每年 350 加仑。
Let’s have a quick exercise in drawing a demand curve. Here is the first problem in the
让我们快速练习一下绘制需求曲线。这是
exercises for Chapter 4.
第 4 章的练习。
This is the hypothetical demand schedule of a country for laundry detergent. I want you to take
这是一个国家对洗衣粉的假设需求时间表。我要你拿
pen and paper and use the data in the table to draw the demand curve.
笔和纸，并使用表格中的数据绘制需求曲线。
Pause the video here while you work; otherwise, the answer will be displayed in 5 seconds.
在工作时在此处暂停视频;否则，答案将在 5 秒后显示。
(5-second pause)
（暂停 5 秒）
Let’s look at the answer.
让我们看看答案。
Your demand curve should look like *this*. We will do quite a bit of graphing in economics, and
您的需求曲线应如下所示。我们将在经济学中做相当多的绘图，并且
although this exercise is simple, it reminds us to pay attention to some important details.
这个练习虽然简单，但它提醒我们注意一些重要的细节。
First, in graphing a demand curve, price goes on the y axis and quantity demanded goes on the x
首先，在绘制需求曲线时，价格位于 y 轴上，需求数量位于 x 轴上
axis. It’s also good to indicate the units in which you are measuring quantity: here we say that
轴。指示您测量数量的单位也很好：这里我们说
it is in million ounces.
它以百万盎司为单位。
Another point is to make sure that your intervals are evenly spaced. In this graph here, price
另一点是确保您的间隔均匀分布。在此图表中，价格
is in 2-dollar increments while quantity is in increments of 10 million. Depending on the
以 2 美元为增量，而数量以 1000 万为增量。根据
increments you use, your own demand curve can be flatter or steeper than the curve we have here,
增量，您自己的需求曲线可能比我们这里的曲线更平坦或更陡峭，
but it should be linear.
但它应该是线性的。
You might be wondering whether demand curves have to be linear. And the answer is, they don’t
您可能想知道需求曲线是否必须是线性的。答案是，他们没有
have to be. For example, they can be curved inward, like Figure 4.3 in the textbook, or a bit
必须如此。例如，它们可以向内弯曲，如教科书中的图 4.3，或者一点
less tidy, like *this* one.
不那么整洁，就像*这个*一样。
This demand curve is based on one of my previous classes’ responses to *this* question. I called
这个需求曲线是基于我之前一节课对*这个*问题的回答。我打电话
out prices in descending order and asked the students to raise their hand when I called out the
按降序排列价格，并要求学生在我喊出
price that corresponds to the maximum price that they were willing to pay.
price 对应的 price 的 x 的 z 值。
I started from 5 dollars and called out numbers in descending order in 10-cent increments. When
我从 5 美元开始，以 10 美分的增量按降序叫出数字。什么时候
I got to $4.10, my student Rhea raised her hand.
我拿到了 4.10 美元，我的学生 Rhea 举起了手。
Four students then raised their hand when I reached $4, bringing the total number of people
当我达到 4 美元时，四名学生举起了手，使总人数达到
willing to buy a pack of KitKats at that price to 5. “Five” consists of the four new people who
愿意以 5 到 5 的价格购买一包 KitKats。“五”由四个新人组成，他们
4
raised their hands, PLUS Rhea — since Rhea was willing to pay as much as $4.10, she would also
举起了手，PLUS Rhea — 因为 Rhea 愿意支付高达 4.10 美元的费用，她也会
have been willing to pay less than $4.10. This is why Rhea is also included in the quantity
愿意支付不到 4.10 美元的费用。这就是为什么 Rhea 也包含在数量中的原因
demanded at $4, although she raised her hand before that.
要价 4 美元，尽管她在此之前举起了手。
None of the remaining students raised their hands UNTIL the price fell to $3.50: at this point,
剩下的学生都没有举手，直到价格跌至 3.50 美元：此时，
5 new people indicated their willingness to buy, for a quantity demanded of 10.
5 个新人表示愿意购买，需求量为 10 人。
All in all, I was able to collect 5 data points, as shown here in the demand schedule, which I
总而言之，我能够收集 5 个数据点，如需求计划所示，我
plot over here. Connecting these five points resulted in the demand curve in green.
这边的情节。将这 5 个点连接起来，需求曲线变为绿色。
Unlike the demand curve in Figure 4.1 which is linear throughout, this demand curve is not as
与图 4.1 中贯穿始终线性的需求曲线不同，这条需求曲线不是
smooth. But there’s nothing wrong with it. The way the demand curve looks depends on the
光滑。但这并没有错。需求曲线的外观取决于
behaviour of buyers. That said, we will frequently use linear demand curves in our analysis
买家的行为。也就是说，我们将在分析中经常使用线性需求曲线
because they are easier to work with without really changing the main predictions or results of
因为它们更容易使用，而不会真正改变
the models we will be looking at.
我们将要研究的模型。
Let me point out a couple of things, and you might want to take note:
让我指出几件事，您可能需要注意：
It might be that buyers would like to buy multiple units, buying more when the price falls. In
买家可能想购买多个单位，当价格下跌时购买更多。 在
contrast, the KitKat example is a case of “unit demand”: this is when consumers are interested
相比之下，KitKat 示例是 “单位需求” 的一个案例：这是消费者感兴趣的时候
in buying only one unit of the good. In such a case, quantity demanded is equal to the number of
只购买一个单位的商品。在这种情况下，需求量等于
buyers.
买家。
Next, let me use the KitKat example to illustrate the two ways of “interpreting” a demand curve.
接下来，让我使用 KitKat 示例来说明“解释”需求曲线的两种方法。
One way is to read it horizontally, where the demand curve indicates the quantity demanded at a
一种方法是水平读取它，其中需求曲线表示
given price.
给定价格。
The other way is to read it vertically, in which case the demand curve indicates the most that
另一种方法是垂直阅读它，在这种情况下，需求曲线最表明
somebody in the market is willing to pay for a particular unit. The height of the demand curve
市场上有人愿意为特定单元付费。需求曲线的高度
at that unit is called the market’s “willingness to pay” for that unit.
在该单位被称为市场对该单位的“支付意愿”。
So we see that on the 7th of January 2020 at around 9 am, Rhea was willing to pay as much as
所以我们看到，在 2020 年 1 月 7 日上午 9 点左右，Rhea 愿意支付
$4.10 for a KitKat. $4.10 is the market’s willingness to pay for the first unit.
KitKat 售价 4.10 美元。4.10 美元是市场为第一个单位支付的意愿。
Now think about this question: What is the market’s willingness to pay for the fifth pack of
现在想一想这个问题：市场为第五包买单的意愿是什么
KitKats? Pause the video here while you think about this; otherwise, the answer will be
奇巧？在思考这个问题时，请在此处暂停视频;否则，答案将是
displayed in 5 seconds.
在 5 秒后显示。
(5-second pause) – (View screencast for the answer to the concept check.)
（5 秒暂停）-（查看截屏视频以获取概念检查的答案。
We can say more – for example, that the most that somebody was willing to pay for the 6th pack
我们可以说得更多——例如，某人愿意为第 6 个包装支付的最高金额
of KitKats, according to the demand curve, was $3.50. You can also verify this from the demand
的 KitKat 为 3.50 美元。您也可以从需求中验证这一点
schedule.
附表。
Now, this brings us to this question: Why were some students willing to pay more than others?
现在，这给我们带来了一个问题：为什么有些学生愿意比其他学生支付更多费用？
It could be that some students were not hungry when I asked, or didn’t like chocolate (unlike
可能是我问的时候有些学生并不饿，或者不喜欢巧克力（不像
Rhea, who said that Kitkats were her favourite), or didn’t have the cash to pay for it. The
Rhea 说 Kitkats 是她的最爱），或者没有现金支付。这
benefits from a Kitkat were different for different students, and a demand curve reflects the
Kitkat 的好处 因不同的学生而异，需求曲线反映了
benefit of the good. Also, some didn’t have cash on them, so willingness to pay is influenced
善的好处。此外，有些人身上没有现金，因此会影响支付意愿
by incomes and budgets.
按收入和预算。
The point is that demand is not simply a matter of what consumers want, it’s also about their
关键是，需求不仅仅是消费者想要什么的问题，还与他们的需求有关
ability to pay and what they can afford.
支付能力和他们能负担得起的东西。
A buyer will compare her willingness to pay with the actual market price to decide whether to
买方将她的付款意愿与实际市场价格进行比较，以决定是否
buy the good or not. If her willingness to pay is greater than the market price, she will buy
买或不买好。如果她的支付意愿大于市场价格，她就会购买
the good; but if the market price is greater than her willingness to pay, she will not buy the
好的;但如果市场价格高于她愿意支付的价格，她就不会购买
good.
好。
Suppose that the price of a pack of Kitkats was 3 dollars and 25 cents. How many packs would
假设一包 Kitkats 的价格是 3 美元 25 美分。多少包会
have been bought?
已经被买走了吗？
If you said 10, you’re correct. Remember, a buyer purchases a product if her willingness to pay
如果你说 10 个，那你是对的。请记住，如果买家愿意付款，则买家会购买产品
for it is no lower than the market price. There are only 10 students for whom that is true if
因为它不低于市场价格。只有 10 名学生是正确的，如果
the market price is 3 dollars and 25 cents.
市场价格为 3 美元 25 美分。
Now suppose that a pack of Kitkats was 4 dollars and 10 cents. How many packs would have been
现在假设一包 Kitkats 是 4 美元 10 美分。本来会有多少包
bought?
买？
5
If you said 1, you’re correct. At that price, only Rhea would have considered it worthwhile to
如果你说 1，那你是对的。以这个价格，只有 Rhea 会认为值得
buy Kitkats.
购买 Kitkats。
Let’s recap:
让我们回顾一下：
First, we talked about the Law of demand, which says that there is an inverse relationship
首先，我们讨论了需求定律，它说存在反比关系
between price and quantity demanded. We also learned the importance of keeping all other factors
在价格和需求数量之间。我们还了解到保留所有其他因素的重要性
that can influence demand constant when thinking about this relationship.
在考虑这种关系时，这可能会影响 Demand Constant。
The law of demand corresponds to a downward sloping demand curve. Because the shape of the
需求定律对应于向下倾斜的需求曲线。因为
demand curve depends on the behaviour of buyers, it may not necessarily be linear.
需求曲线取决于 买家的行为，不一定是线性的。
Finally, we learned how to interpret the demand curve. The height of the demand curve indicates
最后，我们学习了如何解释需求曲线。需求曲线的高度表示
willingness to pay.
愿意支付。
Figure 4.1 shows the demand curve for gasoline of one buyer — Chloe. In the next video, we will
图 4.1 显示了一个买家 Chloe 的汽油需求曲线。在下一个视频中，我们将
look at how to aggregate individual buyers’ demand curves to get the market demand curve.
了解如何聚合单个买家的需求曲线以获得市场需求曲线。