📚 Supply and Demand Curves and Market Equilibrium Analysis | 供求曲线与市场均衡分析
Supply and demand are the twin pillars of economic analysis. In any competitive market, the interaction of buyers' willingness to pay and sellers' willingness to produce determines both the price and the quantity traded. Understanding how these forces reach equilibrium is essential for interpreting market outcomes and the effects of government policy.
供给与需求是经济分析的两大支柱。在任何竞争性市场中，买方的支付意愿与卖方的生产意愿相互作用，共同决定了交易价格与数量。理解这些力量如何达到均衡，是解读市场结果及政府政策影响的关键。
1. The Demand Curve | 需求曲线
The quantity demanded of a good is the amount consumers are willing and able to buy at a given price over a given period. The demand curve plots this relationship, with price on the vertical axis and quantity on the horizontal axis.
需求量是指在给定价格和时期内，消费者愿意且能够购买的商品数量。需求曲线描绘了这一关系，纵轴为价格，横轴为数量。
The law of demand states that, ceteris paribus, as price rises, quantity demanded falls. This inverse relationship produces a downward-sloping curve. It arises from the income effect and the substitution effect: a higher price reduces purchasing power and makes substitutes relatively cheaper.
需求定律表明，在其他条件不变的情况下，价格上升，需求量下降。这种反向关系形成向下倾斜的曲线。其原因包括收入效应与替代效应：价格提高削弱购买力，并使替代品显得相对便宜。
An individual consumer's demand curve reflects diminishing marginal utility. The market demand curve is the horizontal sum of all individual demand curves.
单个消费者的需求曲线反映了边际效用递减。市场需求曲线则是所有个人需求曲线的横向加总。
2. The Supply Curve | 供给曲线
The quantity supplied is the amount producers are willing and able to sell at a given price. The supply curve shows a positive relationship: higher prices offer greater profit incentives, so quantity supplied rises as price rises.
供给量是指在给定价格下，生产者愿意且能够出售的商品数量。供给曲线呈现正相关关系：价格越高，利润激励越大，因此供给量随价格上升而增加。
The upward slope of the supply curve is rooted in increasing marginal cost. As output expands, producers face higher opportunity costs and require higher prices to cover them. In the short run, diminishing returns cause marginal cost to rise.
供给曲线向上倾斜的根源在于边际成本递增。随着产量扩大，生产者面临更高的机会成本，因此需要更高的价格来补偿。在短期内，边际报酬递减使边际成本上升。
Market supply is the horizontal summation of all firms' supply curves. It is flatter when more firms can easily enter an industry.
市场供给是所有企业供给曲线的横向加总。当更多企业能够轻松进入行业时，市场供给曲线会更平坦。
3. Market Equilibrium | 市场均衡
Equilibrium occurs where quantity demanded equals quantity supplied. The equilibrium price is the price at which buyers' and sellers' plans are exactly consistent, and the equilibrium quantity is the amount traded at that price.
市场均衡发生在需求量等于供给量之处。均衡价格是买方与卖方计划完全一致时的价格，均衡数量则是该价格下的成交量。
At equilibrium: Qd = Qs
If the market price is above equilibrium, a surplus exists – supply exceeds demand. Suppliers compete to sell, driving the price down. If the price is below equilibrium, a shortage exists – demand exceeds supply, and competition among buyers drives the price up.
若市场价格高于均衡价格，则出现过剩，供给超过需求。供给者竞相出售，推动价格下降。若价格低于均衡价格，则出现短缺，需求超过供给，买方的竞争推动价格上升。
This self-correcting mechanism is the core of the price system. It automatically allocates resources toward consumer wants and away from surpluses.
这种自我修正机制是价格体系的核心。它自动将资源分配到消费者需求旺盛之处，并退出过剩领域。
4. Changes in Demand vs. Movements Along the Curve | 需求变动与沿曲线移动
It is crucial to distinguish a movement along a demand curve from a shift of the curve. A movement along the curve is caused only by a change in price. A shift occurs when a non-price determinant of demand changes.
区分沿需求曲线移动与曲线本身位移至关重要。沿曲线的移动仅由价格变化引起；当需求的非价格决定因素发生变化时，曲线才会位移。
Key determinants of demand include:
需求的关键决定因素包括：
Income: normal goods see increased demand with higher income, while inferior goods see decreased demand.
收入：正常商品随收入增加而需求增加，劣等商品则需求减少。
Prices of related goods: substitutes and complements affect demand in opposite directions.
相关商品价格：替代品与互补品对需求的影响方向相反。
Tastes and preferences: shifts in consumer preferences change demand.
偏好：消费者偏好的变化会改变需求。
Expectations of future prices and incomes.
对未来价格与收入的预期。
Number of buyers: more buyers increase market demand.
买方数量：买方增加会扩大市场需求。
An increase in demand shifts the curve to the right; a decrease shifts it to the left. At every price, consumers now wish to buy either more or less, which changes equilibrium price and quantity.
需求增加使曲线右移；需求减少使曲线左移。在每个价格水平下，消费者希望购买更多或更少，从而改变均衡价格与数量。
5. Changes in Supply | 供给变动
A supply curve shifts when non-price factors change production costs or seller intentions. Important supply shifters include:
当非价格因素改变生产成本或卖方意愿时，供给曲线会发生位移。重要的供给变动因素包括：
Technology: improved technology lowers costs and shifts supply rightward.
技术：技术进步降低成本，使供给曲线右移。
Input prices: higher costs of labour, raw materials or energy reduce supply.
投入品价格：劳动、原材料或能源成本上升会减少供给。
Taxes and subsidies: taxes reduce supply; subsidies increase it.
税收与补贴：税收减少供给；补贴增加供给。
Number of sellers: entry of new firms increases market supply.
卖方数量：新企业进入会扩大市场供给。
Expectations of future prices: producers may withhold supply if they expect higher future prices.
未来价格预期：若生产者预期未来价格更高，可能减少当前供给。
An increase in supply shifts the curve rightward, lowering equilibrium price and raising equilibrium quantity, assuming demand is unchanged.
在需求不变的前提下，供给增加使曲线右移，均衡价格下降，均衡数量增加。
6. Elasticity of Demand and Supply | 需求与供给弹性
Elasticity measures responsiveness of quantity to a change in price or another determinant. Price elasticity of demand (PED) is the percentage change in quantity demanded divided by the percentage change in price.
弹性衡量数量对价格或其他决定因素变化的反应程度。需求价格弹性（PED）等于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. Determinants include the availability of substitutes, the proportion of income spent on the good, the time period, and whether the good is a necessity or luxury.
若 PED > 1，需求富有弹性；若 PED < 1，需求缺乏弹性。其决定因素包括替代品的可得性、该商品占收入的比例、时间长短以及商品是必需品还是奢侈品。
Price elasticity of supply (PES) follows the same logic. Supply tends to be more elastic in the long run because firms can adjust production capacity.
供给价格弹性（PES）遵循同样的逻辑。由于企业能够调整生产能力，长期供给往往更富有弹性。
Elasticity is vital for analysing the effects of price changes on total revenue. A price cut increases revenue if demand is elastic, but decreases revenue if demand is inelastic.
弹性对于分析价格变化对总收益的影响至关重要。若需求富有弹性，降价会增加收益；若需求缺乏弹性，降价反而使收益下降。
7. Consumer and Producer Surplus | 消费者与生产者剩余
Consumer surplus is the difference between what consumers are willing to pay and what they actually pay. It is the area below the demand curve and above the equilibrium price.
消费者剩余是消费者愿意支付的价格与实际支付价格之间的差额，即需求曲线以下、均衡价格以上的面积。
Producer surplus is the difference between the price producers receive and their minimum acceptable price. It is the area above the supply curve and below the equilibrium price.
生产者剩余是生产者获得的价格与其最低可接受价格之间的差额，即供给曲线以上、均衡价格以下的面积。
Together, total surplus measures society's gains from trade. At a competitive market equilibrium, total surplus is maximised, meaning resources are allocated efficiently. Any deviation from equilibrium creates deadweight loss.
两者之和构成社会从交易中获得的总剩余。在竞争性市场均衡条件下，总剩余达到最大，资源配置有效率。任何偏离均衡都会造成无谓损失。
Total surplus = Consumer surplus + Producer surplus
8. Price Controls: Ceilings and Floors | 价格控制：上限与下限
Governments sometimes intervene to set prices. A price ceiling is a maximum price allowed by law. When set below equilibrium, it causes shortage because demand exceeds supply.
政府有时会干预价格。价格上限是法律允许的最高价格。若设于均衡价格之下，需求量超过供给量，导致短缺。
Examples include rent controls in cities. While they make housing affordable, they reduce the quantity supplied, create waiting lists, and may lower quality.
例如城市中的租金管制。租金管制虽使住房更可负担，但减少了供给量，造成排队等待，并可能降低住房质量。
A price floor is a minimum price set by law. When set above equilibrium, it creates a surplus. Agricultural price supports and minimum wages are common examples.
价格下限是法律设定的最低价格。若设于均衡价格之上，则会产生过剩。农产品价格支持与最低工资是常见例子。
Both types of intervention prevent the market from clearing and generate allocative inefficiency. But policy makers may accept inefficiency in pursuit of
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