【Alevel 經濟精講】供給法則&生產者剩余理論

講解過Law of demand和Consumer surplus后，與其相對應的Law of supply和Producer surplus也準備了非常精細的知識點講解給各位同學。兩者之間既有著聯系也有不同，需要同學們自己找尋好的方法同時理解和熟記這兩部分內容。

Law of supply的基本內容：

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

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前提是Under the condition of ceterisparibus.(All other factors being equal)，P和QD之間的關系也可以用P and QD are directly related來表述。

Producer surplus的定義：

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

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what producers received=b+c

the amount willing to accecpt=c

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同時我們也將consumer surplus和consumer surplus放到同一張圖里比較一下。

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consumer surplus=c

producer surplus=b

consumer expenditure=supplier revenue=a+b

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與Demand相對應的，Supply需要注意在P和QS的圖像中Movements along the curve與Shifts of the curve之間的區別。導致Change in quantity supplied改變的因素只有唯一一個，就是Change in price，會導致在P-QS圖像上點位置的移動。而Change in supply則是整條圖像位置的變動，可能導致這種變化的factors如下：

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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.

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

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

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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.

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

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

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在這里也提醒各位同學們，一定不要混淆Movements along the curve和Shifts of the curve，并且應該盡快背熟各個Factors，并不定期復習加深記憶（小編當年基本每天看筆記都會默背一次）。同時也要注意各個變化的方向性。