📚 The Supply and Demand Model and Its Applications | 供求模型及其应用
The supply and demand model is the foundation of microeconomics. It explains how prices are determined in a market and how resources are allocated. Understanding this model is essential for analysing real-world economic issues, from government price controls to the impact of taxes and international trade.
供求模型是微观经济学的基础。它解释了市场价格如何决定、资源如何配置。理解这一模型对于分析现实经济问题至关重要，从政府价格管制到税收与国际贸易的影响，都离不开它。
1. The Law of Demand | 需求定律
The law of demand states that, ceteris paribus, as the price of a good falls, the quantity demanded rises, and as the price rises, the quantity demanded falls. This inverse relationship arises because consumers look for substitutes and experience diminishing marginal utility.
需求定律指出，在其他条件不变的情况下，当商品价格下降时，需求量上升；当价格上升时，需求量下降。这种反向关系源于消费者寻求替代品以及边际效用递减。
The demand curve is downward sloping from left to right. Each point on the curve shows the quantity consumers are willing to buy at a given price. The market demand curve is the horizontal sum of all individual demand curves.
需求曲线从左向右下方倾斜。曲线上的每一点表示在给定价格下消费者愿意购买的数量。市场需求曲线是所有个人需求曲线的水平加总。
2. The Law of Supply | 供给定律
The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied rises, and as the price falls, the quantity supplied falls. Higher prices provide an incentive for firms to produce and sell more, as they can earn higher revenue.
供给定律指出，在其他条件不变的情况下，当商品价格上升时，供给量上升；当价格下降时，供给量下降。较高的价格激励企业生产和销售更多产品，因为能获得更高收入。
The supply curve is upward sloping from left to right. It reflects rising marginal costs: to produce more, firms often need to use less efficient resources, increasing cost per unit. The market supply curve aggregates individual firms’ supply curves.
供给曲线从左向右上方倾斜。它反映了边际成本递增：为了生产更多，企业往往需要使用效率较低的资源，导致单位成本上升。市场供给曲线是各企业供给曲线的加总。
3. Market Equilibrium | 市场均衡
Equilibrium occurs where the quantity demanded equals the quantity supplied. At this point, the market clears and there is no tendency for price to change. The equilibrium price is also called the market-clearing price.
当需求量等于供给量时，市场达到均衡。此时市场出清，价格没有变动的趋势。均衡价格也称为市场出清价格。
Qₐ = Qₛ → Pₑ and Qₑ
If price is above equilibrium, a surplus exists, pushing price down. If price is below equilibrium, a shortage exists, pushing price up. This adjustment process is known as the rationing function of price.
如果价格高于均衡价格，则出现过剩，价格会下降；如果价格低于均衡价格，则出现短缺，价格会上升。这一调整过程被称为价格配给功能。
4. Movements Along the Curve vs. Shifts of the Curve | 曲线移动与曲线平移
A change in price causes a movement along the demand or supply curve, which is called a change in quantity demanded or quantity supplied. This is shown as point A to point B on the same curve.
价格变动引起需求曲线或供给曲线上的点移动，称为需求量或供给量的变动。这表现为同一曲线上从A点到B点的移动。
A shift of the curve occurs when a non-price determinant changes. For demand, these include income, tastes, the price of substitutes and complements, expectations, and the number of buyers. A rightward shift means more is demanded at every price.
当非价格决定因素发生变化时，曲线整体平移。需求的决定因素包括收入、偏好、替代品与互补品的价格、预期以及买者数量。右移表示在每一价格下需求量都增加。
For supply, non-price determinants include costs of production, technology, taxes and subsidies, and the number of sellers. A rightward shift means more is supplied at every price.
供给的非价格决定因素包括生产成本、技术、税收与补贴以及卖者数量。右移表示在每一价格下供给量都增加。
5. Changes in Equilibrium: Demand Shifts | 需求变动对均衡的影响
An increase in demand (rightward shift) will raise both equilibrium price and quantity, other things equal. For example, during a heatwave, the demand for ice cream rises, pushing up the price and the quantity sold.
需求增加（右移）会使均衡价格和均衡数量都上升，其他条件不变。例如，热浪期间，冰淇淋需求上升，推动价格和销量上涨。
A decrease in demand (leftward shift) will lower both equilibrium price and quantity. This could happen if a good becomes less fashionable or if a substitute good becomes cheaper.
需求减少（左移）会使均衡价格和均衡数量都下降。这可能发生在商品变得不流行或替代品变得更便宜时。
6. Changes in Equilibrium: Supply Shifts | 供给变动对均衡的影响
An increase in supply (rightward shift) will lower equilibrium price and raise equilibrium quantity. This commonly occurs due to technological improvements or falling input costs. For example, cheaper solar panels increase the supply of electricity from solar energy.
供给增加（右移）会降低均衡价格并提高均衡数量。这通常由技术进步或投入成本下降引起。例如，太阳能板价格下降使得太阳能电力供给增加。
A decrease in supply (leftward shift) will raise equilibrium price and lower equilibrium quantity. Natural disasters, higher wages, or government regulations can cause this. An example is the impact of a poor harvest on agricultural commodities.
供给减少（左移）会提高均衡价格并降低均衡数量。自然灾害、工资上涨或政府管制都可能导致这种变化。例如收成不佳对农产品的影响。
7. Simultaneous Shifts in Demand and Supply | 供求同时变动
When both demand and supply shift, we can determine the direction of either price or quantity, but not both — unless we know the relative size of the shifts.
当需求和供给同时变动时，我们能够确定价格或数量中的一个方向，但不能同时确定两个方向——除非我们知道变动的相对大小。
For example, if both demand and supply increase, quantity definitely increases, but the change in price is ambiguous: it depends on which shift is larger.
例如，如果需求和供给同时增加，数量一定增加，但价格变动方向不确定：取决于哪个变动更大。
| Demand | Supply | Price | Quantity |
| ↑ | ↓ | ↑ | ? |
| ↓ | ↑ | ↓ | ? |
| ↑ | ↑ | ? | ↑ |
| ↓ | ↓ | ? | ↓ |
8. Price Ceilings and Price Floors | 最高限价与最低限价
A price ceiling is a legal maximum price set below equilibrium to keep prices affordable. It causes a persistent shortage and creates problems such as queues, black markets, and lower quality. Rent control in many cities is a classic example.
最高限价是设定在均衡价格以下的法定最高价格，目的是让价格可负担。它会导致持续短缺，并引发排队、黑市、质量下降等问题。许多城市的租金管制是典型例子。
A price floor is a legal minimum price set above equilibrium to support producers. It creates a surplus. Agricultural price supports and minimum wages are examples. The government may need to buy the surplus or restrict output.
最低限价是设定在均衡价格以上的法定最低价格，用于支持生产者。它会产生过剩。农产品支持价格和最低工资就是例子。政府可能需要购买过剩产品或限制产量。
9. Elasticity of Demand | 需求弹性
Price elasticity of demand measures the responsiveness of quantity demanded to a change in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price.
需求价格弹性衡量需求量对价格变化的反应程度。其计算公式为需求量变动百分比除以价格变动百分比。
PED = %ΔQd ÷ %ΔP
If |PED| > 1, demand is elastic; if |PED| < 1, demand is inelastic; if |PED| = 1, unit elastic. Determinants include the availability of substitutes, the proportion of income spent, and the time period considered.
如果|PED| > 1，需求富有弹性；如果|PED| < 1，需求缺乏弹性；如果|PED| = 1，单位弹性。决定因素包括替代品可得性、支出占收入比例以及时间长短。
Elasticity affects total revenue. When demand is elastic, a decrease in price increases total revenue; when demand is inelastic, an increase in price increases total revenue.
弹性影响总收益。当需求富有弹性时，降价会增加总收益；当需求缺乏弹性时，提价会增加总收益。
10. Elasticity of Supply | 供给弹性
Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. It equals the percentage change in quantity supplied divided by the percentage change in price.
供给价格弹性衡量供给量对价格变化的反应程度。它等于供给量变动百分比除以价格变动百分比。
PES = %ΔQs ÷ %ΔP
Supply tends to be more elastic in the long run because firms can adjust their production capacity. In the short run, fixed factors limit supply response. For example, agricultural supply can increase only limitedly in a season.
长期中供给通常更有弹性，因为企业可以调整生产能力。短期内，固定要素限制了供给反应。例如，一个季节内农产品供给只能有限增加。
11. Consumer Surplus and Producer Surplus | 消费者剩余与生产者剩余
Consumer surplus is the difference between what buyers are willing to pay and what they actually pay. It is the area below the demand curve and above the price line, up to the quantity traded.
消费者剩余是买者愿意支付的价格与实际支付价格之间的差额。它是需求曲线以下、价格线以上、直至交易数量的面积。
Producer surplus is the difference between the price sellers receive and their marginal cost of production. It is the area above the supply curve and below the price line.
生产者剩余是卖者获得的价格与其边际生产成本之间的差额。它是供给曲线以上、价格线以下的面积。
Total surplus = consumer surplus + producer surplus. At market equilibrium without externalities, total surplus is maximised, achieving allocative efficiency.
总剩余 = 消费者剩余 + 生产者剩余。在没有外部性的市场均衡下，总剩余达到最大化，实现配置效率。
12. Applications: Taxes, Subsidies and Trade | 应用：税收、补贴与贸易
A per-unit tax shifts the supply curve leftward by the amount of the tax. The new equilibrium quantity is lower, buyers pay a higher price, and sellers receive a lower net price. The tax burden is split depending on price elasticities of demand and supply.
单位税收使供给曲线向左移动，移动幅度等于税额。新的均衡数量降低，买者支付更高价格，卖者获得的净价格更低。税负根据供求弹性的不同而分担。
When demand is more inelastic than supply, consumers bear a larger share of the tax. When supply is more inelastic, producers bear a larger share. Governments use taxes to raise revenue and correct negative externalities.
当需求比供给更缺乏弹性时，消费者承担较大的税负；当供给更缺乏弹性时，生产者承担较大税负。政府利用税收筹集收入并纠正负外部性。
Subsidies have the opposite effect: they shift supply rightward, lowering price and raising quantity. International trade alters domestic equilibrium by allowing imports and exports, often increasing total surplus through specialisation.
补贴则产生相反效果：它使供给右移，降低价格并提高数量。国际贸易通过允许进口和出口改变国内均衡，通常通过专业化提高总剩余。
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