A-Level 经济备考：供给法则&生产者剩余理论

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讲解过Law of demand和Consumer surplus后，与其相对应的Law of supply和Producer surplus也准备了非常精细的知识点讲解给各位同学。两者之间既有着联系也有不同，需要同学们自己找寻好的方法同时理解和熟记这两部分内容。

Law of supply的基本内容：

When the price increases, the quantity supplied increases; When theprice decreases, the quantity supplied decreases.

前提是Under the condition of ceterisparibus.(All other factors being equal)，P和QD之间的关系也可以用P and QD are directly related来表述。

Producer surplus的定义：

The difference between what producers received and the minimumamount they are willing to accecpt.

what producers received=b+c

the amount willing to accecpt=c

同时我们也将consumer surplus和consumer surplus放到同一张图里比较一下。

consumer surplus=c

producer surplus=b

consumer expenditure=supplier revenue=a+b

与Demand相对应的，Supply需要注意在P和QS的图像中Movements along the curve与Shifts of the curve之间的区别。导致Change in quantity supplied改变的因素只有一个，就是Change in price，会导致在P-QS图像上点位置的移动。而Change in supply则是整条图像位置的变动，可能导致这种变化的factors如下：

1.Price of related products

(1)   Joint supply: For example ,mutton and sheepskin. When the price of mutton increases,the quantity supplied for mutton will increase too, which means there will be amovement along the curve. However, the supply of sheepskin increases too, sothere will be a shift in the curve.

(2) Competitive supply: Products which use the same factor of production. When the price ofapple increases, the farmer will decided to plant less pear trees, so thequantity supplied for apple will increase, and the supply for pears willdecrease.

2.When cost of production increases (such as the raw material costs and wages) ,the supply decreases.

3.When the productivity increases, the cost of production decreases, and the supply increases.

4.Government policies: When indirect tax increases, the cost of production increases too, and the supply decreases.

When the subsidy increases, the cost of production decreases, and the supply increases.

5.Supply can be influenced by weather and season too, for example, for agricultural products.

6.If the number of firms producing one product increases, the supply increases too.

在这里也提醒各位同学们，一定不要混淆Movements along the curve和Shifts of the curve，并且应该尽快背熟各个Factors，并不定期复习加深记忆。同时也要注意各个变化的方向性。

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