Market Structures | 市场结构

📚 Market Structures | 市场结构

Market structures are a central topic in Cambridge A-Level Economics because they explain how firms behave, how prices are determined, and whether markets deliver efficient outcomes for consumers and society. A sound understanding of perfect competition, monopoly, monopolistic competition, oligopoly and contestable markets allows students to evaluate real-world industries and government policy.

市场结构是剑桥 A-Level 经济学核心主题，因为它解释了企业如何行为、价格如何决定，以及市场是否为消费者和社会带来有效率的结果。掌握完全竞争、垄断、垄断竞争、寡头垄断和可竞争市场，有助于学生评价现实行业与政府政策。

1. Overview of Market Structures | 市场结构概览

Market structure refers to the key characteristics of a market that shape the behaviour of firms. The main dimensions are the number and size of firms, the degree of product differentiation, the strength of barriers to entry, the availability of information, and the extent of price-setting power.

市场结构是指塑造企业行为的关键市场特征。主要维度包括企业数量与规模、产品差异程度、进入壁垒强度、信息可获得性以及定价权大小。

Economists usually arrange these structures along a spectrum. Perfect competition lies at one extreme with many small firms and no market power, while pure monopoly lies at the other with a single dominant seller. Between them sit monopolistic competition and oligopoly.

经济学家通常将这些结构排列成一个光谱。完全竞争位于一个极端，存在众多小企业且没有市场势力；纯粹垄断位于另一端，只有单一主导卖家。介于两者之间的是垄断竞争和寡头垄断。

Examiners expect candidates to use the structural characteristics to classify markets, draw cost and revenue diagrams, and evaluate efficiency and equity. The contestable markets model adds an important qualification: the number of firms matters less than the threat of entry.

考官希望考生运用结构特征对市场进行分类，绘制成本与收益图形，并评价效率与公平。可竞争市场模型补充了重要限定：企业数量不如进入威胁重要。

2. Perfect Competition: Characteristics and Outcomes | 完全竞争：特征与结果

Perfect competition is an idealised model based on strict assumptions. There are many buyers and sellers, all firms produce identical or homogeneous products, there is perfect information, buyers and sellers can enter or exit freely, and no single firm can influence the market price.

完全竞争是一个基于严格假设的理想化模型。市场上有大量买者和卖者，所有企业生产同质产品，信息完全，买者和卖者可以自由进入或退出，任何单个企业都无法影响市场价格。

Because each firm faces a perfectly elastic demand curve, it is a price taker. Average revenue equals marginal revenue equals price, so the firm can sell any quantity at the prevailing market price.

由于每个企业面对完全弹性的需求曲线，它是价格接受者。平均收益等于边际收益等于价格，因此企业可以按现行市场价格出售任意数量。

P = AR = MR

In the short run, a perfectly competitive firm may earn supernormal profit if price exceeds average total cost. However, the absence of barriers means new firms enter, shifting market supply to the right and lowering price until only normal profit remains.

在短期，如果价格高于平均总成本，完全竞争企业可能获得超额利润。然而，由于没有进入壁垒，新企业会进入，使市场供给右移并压低价格，直到只剩正常利润。

Long-run equilibrium therefore occurs where price equals minimum

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