工程经济学pfa公式怎么用（国际课程A-level经济）

Supply and demand 供应和需求

The demand curve 需求曲线

If you ask a taxi driver what he thinks economics is all about, the answer is invariably: "...it's all about supply and demand, guv'." And of course, there is the old joke: "teach a parrot to say supply and demand and you've got an economist!" Obviously there is a lot more to the subject than just supply and demand, but there is an element of truth in the layman's view. Much of what you learn at A' level involves an application of the basic laws of supply and demand that appear in these first few Learn-Its.

如果你问一个出租车司机他认为经济学是怎么回事，他的回答总是这样。"......都是关于供应和需求的，头儿"。当然，还有一个老笑话："教一只鹦鹉说供应和需求，你就有了一个经济学家！" 很明显，这个主题不仅仅是供应和需求，还有很多内容，但外行人的观点也有一定的道理。你在A-level考试中所学到的大部分内容都涉及到在这前几项学习内容中出现的基本供求规律的应用。

Before we look at the basic demand curve, it is important to understand that economists only recognise demand when it is effective demand. This means demand that is backed up with a willingness and ability to pay. I should think that many of you reading this page would demand a Ferrari, but let's face it, it's not really effective demand, is it!

在我们研究基本需求曲线之前，重要的是要明白，经济学家只承认有效需求。这意味着有支付意愿和能力支持的需求。我想，你们中的许多人在阅读这一页时都会要求一辆法拉利，但让我们面对现实吧，这并不是真正的有效需求，不是吗？

Now we will look at the theory of demand. At higher prices, a lower quantity will be demanded than at lower prices, ceteris paribus. At lower prices, a higher quantity will be demanded than at higher prices, ceteris paribus. Basically, when the price is high demand is low and vice versa. Ceteris paribus means 'all other things being equal'. It is very important that you state this condition when using demand curves. I will explain why under "determinants of demand". First, let's have a look at the normal downward-sloping demand curve:

现在我们来看看需求的理论。在较高的价格下，需求量会比在较低的价格下低，在其他条件不变的情况下。在较低的价格下，需求量将比在较高的价格下更多，其他情况下也是如此。基本上，当价格高时，需求量就低，反之亦然。其他条件不变的意思是 "所有其他条件都相同"。在使用需求曲线时，说明这一条件是非常重要的。我将在 "需求的决定因素 "中解释原因。首先，让我们看一下正常的向下倾斜的需求曲线。

Note that I've drawn it as a curve, which is probably a little more realistic. Many of you will use straight-line demand curves in the exam. This is perfectly OK, remember that these diagrams are only sketches that you are using to help you analyse a situation. Anyway, as you can see in the diagram above, the demand for CDs is fairly low at the relatively high price of fifteen pounds, but at the bargain price of five pounds demand is much higher.

请注意，我把它画成了一条曲线，这可能更现实一点。你们中的许多人将在考试中使用直线需求曲线。这是完全可以的，记住，这些图只是你用来帮助你分析情况的草图。总之，正如你在上图中所看到的，在相对较高的15英镑的价格下，对CD的需求相当低，但在5英镑的便宜价格下，需求就高得多。

It is fairly obvious so far that the price of a good is a pretty strong determinant of its demand, but there are many other things that will affect demand too.

到目前为止，相当明显的是，一种商品的价格是其需求的一个相当有力的决定因素，但还有许多其他东西也会影响需求。

1. Real income. If one's real income rose (real means 'allowing for inflation'), one should be able to afford more CDs.
2. The price of other goods. If the price of CD players rose then one would expect demand for CD players to fall, and so would the demand for CDs. These goods are complements. If the prices of rock concerts rose then one would expect the demand for these concerts to fall. Perhaps those who decided against the concert might buy a CD instead. These goods are substitutes.
3. Tastes and preferences. A slightly obscure but very important determinant. As you get older, you may lose Interest in the repetitive music currently in the charts and try some original sounds from the 60s, 70s or 80s. Changing preferences will affect your demand for a product regardless of its price.
4. Expectations of future prices. If you think that the price for CDs is likely to fall in the near future, perhaps because of reduced production costs or competition from the US, you may delay some purchases which will reduce demand in the current time period. Alternatively, you may feel that CD prices are likely to rise in the near future, perhaps due to the lack of competition in the retail market, so you may increase your demand in the current time period.
5. Advertising. Although many of you probably doubt the effectiveness of some of the appalling adverts on the TV, one assumes that these companies would not spend fortunes on these adverts if they did not expect to see a significant rise in demand for the product in question (Virgin and Our Price are always trying to sell you CDs via the TV.)
6. Population. Quite obviously, a significant rise in the number of people in a given area or country will affect the demand for a whole host of goods and services. Note that a change in the structure of the population (we have an ageing population) will increase the demand for some goods but reduce the demand for others.
7. Interest rates and credit conditions. If interest rates are relatively low then it is cheaper to borrow money that can then be spent. This is not so applicable to CDs, but will certainly affect the demand for 'big ticket' items such as cars and major electrical goods. In boom time (like the late 80s) it is often easier to obtain credit regardless of the rate of interest.

实际收入：如果一个人的实际收入增加（实际意味着 "考虑到通货膨胀"），他应该能够负担更多的CD。

其他商品的价格：如果CD机的价格上涨，那么人们会期望对CD机的需求下降，对CD的需求也会下降。这些商品是互补的。如果摇滚音乐会的价格上涨，那么人们会预期对这些音乐会的需求下降。也许那些决定不看音乐会的人可能会买一张CD来代替。这些商品是替代品。

品味和偏好：这是一个略显晦涩但非常重要的决定因素。随着年龄的增长，你可能对目前排行榜上的重复性音乐失去兴趣，而尝试一些60年代、70年代或80年代的原创声音。喜好的改变会影响你对产品的需求，而不管其价格如何。

对未来价格的预期：如果你认为CD的价格在不久的将来可能会下降，也许是因为生产成本降低或来自美国的竞争，你可能会推迟一些购买，这将减少当前时间段的需求。或者，你可能觉得CD的价格在不久的将来可能会上升，也许是由于零售市场缺乏竞争，所以你可能在当前时间段增加需求。

广告：尽管你们中的许多人可能会怀疑电视上一些骇人听闻的广告的有效性，但我们假设这些公司如果不期望看到有关产品的需求大幅上升，就不会在这些广告上花费巨资（维珍和我们的价格总是试图通过电视向你推销CD。）

人口：很明显，一个特定地区或国家的人口数量的显著增加将影响对一系列商品和服务的需求。请注意，人口结构的变化（我们有一个老龄化的人口）将增加对一些商品的需求，但减少对其他商品的需求。

利率和信贷条件：如果利率相对较低，那么借钱就比较便宜，然后可以花钱。这对CD不那么适用，但肯定会影响对汽车和主要电器产品等 "大票 "项目的需求。在经济繁荣时期（如80年代末），无论利率如何，往往更容易获得信贷。

Now that you understand that there are many things that affect the demand for a good other than its price, I hope you can see the importance of the ceteris paribus assumption. The normal downward-sloping demand curve shows the relationship between the price of the good and its demand, all other things being equal. Those 'all other things' are the list above: incomes, prices of other goods, etc. If you do not make this assumption, then you could have a situation when the price of CDs falls, but at the same time one's income falls by such a large amount that one actually demands fewer CDs. In other words, one does not want to confuse shifts in the demand curve and movements along a demand curve.

现在你明白了，除了价格之外，还有很多东西会影响商品的需求，我希望你能明白 "等价交换 "假设的重要性。正常的向下倾斜的需求曲线显示了在所有其他因素相同的情况下，商品的价格和它的需求之间的关系。这些 "所有其他事物 "就是上面列出的：收入、其他商品的价格等等。如果你不做这个假设，那么你可能会出现这样的情况：CD的价格下降，但同时一个人的收入下降了很多，以至于他实际上需要更少的CD。换句话说，我们不希望混淆需求曲线的变化和需求曲线上的移动。

It is very important that you understand the difference shifts of and movements along demand curves. Examiners often test your understanding of this point in multiple-choice questions.

了解需求曲线的移动和沿需求曲线的移动的区别是非常重要的。考官经常在多项选择题中测试你对这一点的理解。

A movement along a demand curve only occurs when there is a change in the price of the good in question. Some textbooks call these movements extensions and contractions. In the diagram below (note that it is a straight-line sketch), when the price of CDs falls (from P1 to P2) there is a rise in demand (from Q1 to Q2), ceteris paribus. The movement along the curve is from point A to point B. When the price rises (from P1 to P3) there is a fall in demand (from Q1 to Q3), ceteris paribus. The movement along the curve is from point A to point C.

只有当有关商品的价格发生变化时，才会出现需求曲线上的移动。一些教科书把这些运动称为延伸和收缩。在下图中（注意，这是一个直线草图），当CD的价格下降（从P1到P2）时，需求就会上升（从Q1到Q2），其他情况下也是如此。当价格上升时（从P1到P3），需求就会下降（从Q1到Q3），视情况而定。沿着曲线的移动是由A点到C点。

Note that we must say 'ceteris paribus'. If one of the other determinants of demand changes as well, then the curve would shift. Also note - always label all of your diagrams very well. Examiners hate poorly labelled diagrams.

请注意，我们必须说 "外在条件"。如果需求的其他决定因素之一也发生了变化，那么曲线就会移动。还要注意的是--一定要把你所有的图都标得非常清楚。考官最讨厌标签不全的图。

A shift in the demand curve occurs if one of the 'other' (i.e. non-price) determinants of demand change. This means that for a given price level the quantity demanded will change. This is illustrated in the diagram below:

如果需求的 "其他"（即非价格）决定因素之一发生变化，就会出现需求曲线的移动。这意味着，在给定的价格水平下，需求量将发生变化。这在下图中有所说明。

Note that the price has not changed (P1) and yet demand has increased (in the case of the shift to D2) to Q2. This could be due to a rise in real incomes (assuming the good is normal - see the required section in the 'Elasticities' topic), a rise in the price of a substitute good, a fall in the price of a complement, etc. (see 'determinants of demand' above).

In the case of the shift to D3, demand has fallen even though the price has remained constant.

This is quite a difficult idea to understand, so why don't you try the exercise below. Assume the product you are considering is a car. In the left column are a number of factors that may change. Can you click on the way the factor needs to move, in order to make the demand line move to D3?

请注意，价格没有变化（P1），但需求却增加了（在转移到D2的情况下），达到了Q2。这可能是由于实际收入的增加（假设该商品是正常的--见 "弹性 "主题中的必要部分），替代商品价格的上升，补充商品价格的下降，等等（见上文 "需求的决定因素"）。

在转向D3的情况下，尽管价格保持不变，但需求却下降了。

这是一个相当难理解的想法，所以你为什么不试试下面的练习。假设你所考虑的产品是一辆汽车。左边一栏是一些可能发生变化的因素。你能点击因素需要移动的方式，以使需求线移动到D3吗？