Perfect Competition

The market demand curve for the industry's output slopes downward in the ordinary way, but the demand curve facing any single firm does not. Because output is homogeneous and each producer supplies a trivial share of total volume, a firm that raises its asking price loses all its customers to identical rivals, while cutting price is pointless when it can already sell every unit at the going rate. Its demand curve is therefore a horizontal line at the market price.

That geometry has a direct consequence for revenue. Total revenue rises linearly with quantity, so average revenue and marginal revenue both equal price and both equal the horizontal demand line. Marginal revenue is constant, which is why the profit-maximising condition collapses from the general rule of setting marginal revenue equal to marginal cost into the special case of setting marginal cost equal to price.

In the short run, capacity is fixed and fixed costs are unavoidable. The firm chooses the quantity at which marginal cost, measured on the upward-sloping portion of the marginal cost curve, equals the market price. Comparing that price with average total cost determines whether the firm earns an economic profit, breaks even, or takes a loss.

A loss does not automatically justify closing. If price covers average variable cost but not average total cost, continuing to operate generates a contribution toward fixed costs, so the loss is smaller than the loss from shutting down. The shutdown point is the minimum of average variable cost; below it, each unit sold fails to cover its own variable cost and production should cease. The breakeven point is the minimum of average total cost, where price just covers all economic costs. The firm's short-run supply curve is the segment of marginal cost lying above minimum average variable cost, and market supply is the horizontal sum of those curves.

Free entry and exit drive the long-run result. Positive economic profit signals attract new capacity; industry supply shifts outward and price falls. Losses cause exit, supply contracts, and price rises. The process ends only when price equals minimum average total cost, so each surviving firm earns exactly its opportunity cost of capital and economic profit is zero, even though accounting profit is positive.

At that point price equals marginal cost equals minimum average total cost. Because price equals marginal cost, the marginal buyer's valuation matches the resource cost of the last unit, which is the condition for allocative efficiency. Because output occurs at the lowest point of average total cost, production is also technically efficient. Consumer surplus is maximised and there is no deadweight loss, which is why perfect competition serves as the welfare benchmark against which monopoly, oligopoly and monopolistic competition are judged.

Be able to classify a described market by the number of sellers, degree of product differentiation, pricing power and entry conditions, and to distinguish perfect competition from monopolistic competition, where differentiation gives each firm a slightly downward-sloping demand curve. Expect to read a cost schedule or curve diagram and identify the profit-maximising quantity, the shutdown price, the breakeven price and the area representing short-run profit or loss.

You should also be able to explain why long-run economic profit is zero without confusing that with zero accounting profit, and why a price-taking firm's supply response is governed entirely by marginal cost. Concentration measures such as the N-firm concentration ratio and the Herfindahl-Hirschman Index approach their minimum values in this structure, but both ignore barriers to entry, which is the defining feature that keeps profits at zero.