📚 Characteristics and Equilibrium Analysis of Perfect Competition | 完全竞争模型的特征与均衡分析
Perfect competition is a fundamental market structure in microeconomics, used as a benchmark for evaluating real-world markets. This model assumes a highly idealized setting where no single buyer or seller can influence the market price.
完全竞争是微观经济学中的一种基础市场结构，常被用作衡量现实市场的基准。该模型假设了一个高度理想化的环境，其中没有任何单一买方或卖方能够影响市场价格。
1. Definition and Basic Assumptions | 定义与基本假设
Perfect competition is a market structure characterized by a very large number of small firms selling an identical product, with complete information and freedom of entry and exit. The market price is determined by the intersection of industry demand and supply.
完全竞争是一种市场结构，其特征是大量小型企业销售同质产品，具有完全信息以及自由进入和退出的条件。市场价格由行业需求与供给的交点决定。
The assumptions of perfect competition include: many buyers and sellers, homogeneous products, perfect information, freedom of entry and exit, and no transaction costs. These assumptions ensure that each firm is a price taker.
完全竞争的假设包括：众多买卖双方、产品同质、信息完全、自由进出以及无交易成本。这些假设确保每个企业都是价格接受者。
2. Key Features of Perfect Competition | 完全竞争的主要特征
The key features of perfect competition can be summarised in the following table:
完全竞争的主要特征可归纳如下表：
| Feature | 特征 | Implication | 含义 |
|---|---|
| Many buyers and sellers | 众多买卖双方 | No one can influence price | 无人能影响价格 |
| Homogeneous products | 同质产品 | Products are perfect substitutes | 产品完全可替代 |
| Perfect information | 完全信息 | All agents know prices and quality | 所有参与者了解价格与质量 |
| Freedom of entry and exit | 自由进入与退出 | Firms can enter or leave without barriers | 企业无壁垒地进入或退出 |
| Price taker | 价格接受者 | Firm faces a perfectly elastic demand curve | 企业面临完全弹性的需求曲线 |
These features imply that the firm cannot set its own price; it must accept the market-determined price. Advertising is unnecessary, and no firm has market power.
这些特征意味着企业不能自行定价，必须接受市场决定的价格。广告没有必要，任何企业都没有市场势力。
3. The Firm as a Price Taker | 作为价格接受者的企业
Because the firm is a price taker, its demand curve is a horizontal line at the market price P. This means the average revenue (AR) equals marginal revenue (MR) equals price (P).
由于企业是价格接受者，其需求曲线是在市场价格 P 处的水平线。这意味着平均收益（AR）等于边际收益（MR）等于价格（P）。
AR = MR = P
The horizontal demand curve indicates that the firm can sell any quantity at the prevailing market price, but selling above the price would result in zero sales.
水平需求曲线表明，企业可以按现行市场价格出售任意数量，但若定价高于市场价格，销量将变为零。
4. Short-Run Equilibrium of the Firm | 企业的短期均衡
In the short run, the firm maximises profit by producing where marginal cost (MC) equals marginal revenue (MR). The profit-maximising output is found at the intersection of MC and MR.
在短期内，企业通过使边际成本（MC）等于边际收益（MR）来实现利润最大化。利润最大化产量位于 MC 与 MR 的交点。
MC = MR = P
At this output, the firm can be in one of three situations: normal profit, supernormal profit, or loss. The outcome depends on the position of the average cost (AC) curve relative to price.
在这一产量水平上，企业可能面临三种情况：正常利润、超额利润或亏损。结果取决于平均成本（AC）曲线相对于价格的位置。
- If P > AC: supernormal profit | 如果 P > AC：超额利润
- If P = AC: normal profit | 如果 P = AC：正常利润
- If P < AC: loss, but may continue if P > AVC | 如果 P < AC：亏损，但若 P > AVC 可能继续生产
In the short run, the firm may earn supernormal profits, because new firms cannot enter the industry immediately.
在短期内，企业可能赚取超额利润，因为新企业无法立即进入该行业。
5. Short-Run Supply Curve | 短期供给曲线
The short-run supply curve of a perfectly competitive firm is the portion of its marginal cost curve that lies above the average variable cost (AVC) curve.
完全竞争企业的短期供给曲线是位于平均可变成本（AVC）曲线之上的那部分边际成本曲线。
This is because the firm will only produce if price covers average variable cost; otherwise, it would shut down. The MC curve shows the profit-maximising quantity at each price.
这是因为企业只有在价格覆盖平均可变成本时才会生产；否则它将停产。MC 曲线显示了在每个价格水平下的利润最大化产量。
The industry short-run supply curve is the horizontal sum of all firms’ short-run supply curves.
行业的短期供给曲线是所有企业短期供给曲线的水平加总。
6. Long-Run Equilibrium | 长期均衡
In the long run, there is full freedom of entry and exit. If firms earn supernormal profits, new firms enter the industry, increasing supply and driving the price down until profits are eliminated.
在长期内，进入和退出完全自由。如果企业赚取超额利润，新企业会进入该行业，增加供给并压低价格，直到利润消失。
If firms make losses, some firms exit, reducing supply and raising the price until remaining firms break even. Therefore, in long-run equilibrium, economic profit is zero.
如果企业出现亏损，部分企业会退出，减少供给并推高价格，直到剩余企业收支平衡。因此，在长期均衡中，经济利润为零。
P = MR = MC = Minimum AC
At long-run equilibrium, the firm produces at the minimum point of its average cost curve, achieving productive efficiency.
在长期均衡中，企业在其平均成本曲线的最低点生产，实现生产效率。
7. Long-Run Industry Supply | 长期行业供给
The long-run industry supply curve can be horizontal, upward-sloping, or downward-sloping, depending on whether the industry is a constant-cost, increasing-cost, or decreasing-cost industry.
长期行业供给曲线可以是水平的、向右上方倾斜的，或向右下方倾斜的，具体取决于行业是成本不变、成本递增还是成本递减行业。
- Constant-cost industry: LR supply is horizontal | 成本不变行业：长期供给曲线水平
- Increasing-cost industry: LR supply is upward-sloping | 成本递增行业：长期供给曲线向右上方倾斜
- Decreasing-cost industry: LR supply is downward-sloping | 成本递减行业：长期供给曲线向右下方倾斜
In an increasing-cost industry, entry of new firms raises input prices, so the long-run supply curve slopes upward.
在成本递增行业中，新企业进入会提高投入品价格，因此长期供给曲线向右上方倾斜。
8. Efficiency in Perfect Competition | 完全竞争中的效率
Perfect competition is often praised for achieving both allocative and productive efficiency in long-run equilibrium.
完全竞争通常被称赞能在长期均衡中同时实现配置效率和生产效率。
Allocative efficiency occurs when P = MC, meaning resources are allocated to produce the goods and services most valued by society.
当 P = MC 时实现配置效率，意味着资源被用于生产社会最重视的商品和服务。
Productive efficiency occurs when production is at the lowest point of the average cost curve, meaning goods are produced at the lowest possible cost.
当生产位于平均成本曲线的最低点时实现生产效率，意味着商品以尽可能低的成本生产。
However, perfect competition may not promote dynamic efficiency, as firms have little incentive to innovate due to zero long-run profits.
然而，完全竞争可能无法促进动态效率，因为长期利润为零，企业缺乏创新的动力。
9. Real-World Applications and Limitations | 现实中的应用与局限性
Perfect competition is rarely found in the real world. Agricultural markets sometimes approximate it, but most markets feature differentiated products or barriers to entry.
完全竞争在现实中很少见到。农产品市场有时近似于完全竞争，但大多数市场具有产品差异化或进入壁垒。
Despite its unrealistic assumptions, the model serves as a benchmark for evaluating market performance. It helps policymakers understand the consequences of market power and regulation.
尽管其假设不现实，该模型仍可作为评估市场表现的基准。它帮助政策制定者理解市场势力与监管的后果。
Limitations include the assumptions of perfect information and homogeneous products, which are rarely met. In addition, the model ignores transaction costs and externalities.
局限性包括完全信息和同质产品的假设，这些在现实中很少满足。此外，该模型忽略了交易成本和外部性。
10. Conclusion | 结论
Perfect competition is a powerful analytical tool in economics. Its key features — many buyers and sellers, homogeneous products, perfect information, and free entry and exit — lead to a price-taking firm with a horizontal demand curve.
完全竞争是经济学中强大的分析工具。其主要特征——众多买卖双方、同质产品、完全信息以及自由进出——使得企业成为价格接受者，面对水平需求曲线。
Short-run equilibrium is achieved where MC = MR, while long-run equilibrium occurs at P = MC = minimum AC, with zero economic profit. The model demonstrates the ideal outcomes of efficiency, though it remains an abstraction from reality.
短期均衡在 MC = MR 处实现，而长期均衡出现在 P = MC = 最低 AC 时，经济利润为零。该模型展示了效率的理想结果，尽管它仍是现实的一种抽象。
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