Perfect Competition

The Invisible Hand at Work

Perfect competition is the benchmark market structure:

• Many small firms, each a price taker ($P$ is given by the market).
• Homogeneous product (one firm's output is identical to another's).
• Free entry and exit in the long run.
• Perfect information about prices and technology.

The firm's problem: choose output $Q$ to maximise profit $\pi = TR - TC = PQ - TC(Q)$.

First-order condition: $\frac{d\pi}{dQ} = P - MC = 0$, so:

$$P = MC$$

The competitive firm produces where price equals marginal cost. Since the firm is a price taker, its demand curve is a horizontal line at the market price.

Short-run supply: the MC curve above the minimum AVC (the shutdown point). If $P < AVC_{\min}$, the firm shuts down (loses only FC rather than FC + variable losses).

Long-run equilibrium: free entry/exit drives economic profit to zero:

$$P = MC = AC_{\min}$$

Firms produce at minimum average cost — the most efficient scale. Any profit attracts entry; any loss triggers exit. The long-run supply curve is perfectly elastic at $P = AC_{\min}$ (constant-cost industry).

Welfare: perfect competition maximises total surplus (consumer + producer). There is no deadweight loss — every unit where the consumer's willingness to pay exceeds the marginal cost is produced. This is the first theorem of welfare economics: competitive equilibrium is Pareto efficient.

The competitive firm's decision card

Common pitfall: Shutting down as soon as profit turns negative. Between $AVC$ and $AC$, operating loses less than closing: revenue covers all variable costs plus a slice of the fixed costs you owe anyway.