Perfect Competition

Perfect competition is the market structure in which there are many buyers and sellers, the product is homogeneous, firms can freely enter and exit, and every participant is a price taker — no single agent can influence the market price. Equilibrium is reached when quantity supplied equals quantity demanded; excess demand drives price up and excess supply drives price down until the market clears.

In the Topic 2 welfare analysis, perfect competition is the efficiency benchmark: the First Welfare Theorem states that a competitive equilibrium maximises total surplus (consumer surplus plus producer surplus). Every departure from it — monopoly, oligopoly, vertical chains — creates deadweight loss measured relative to the competitive outcome. For linear demand $P = A - Q$ with $MC = c$, the competitive price and quantity are $P^* = c$ and $Q^* = A - c$, the same outcome that Bertrand competition achieves with just two firms. See Topic 2 — Equilibrium in Different Market Structures.

When to use

Use perfect competition as the baseline against which all other market structures are compared. The Lerner Index $L = (P - MC)/P$ is zero under perfect competition and positive under any degree of market power — it directly measures the gap from this benchmark. In Cournot with $N \to \infty$ firms, the equilibrium converges to the competitive outcome.