Perfect Competition and Profit Maximization

Perfect competition is the benchmark model of AP Microeconomics and the core of the unit on Production, Cost, and the Perfect Competition Model. Every other market structure is judged against it, so the graphs here are worth drawing until they are automatic.

The price taker

A perfectly competitive firm is too small to affect the market price. The market sets the price where market supply meets market demand, and the firm's demand curve is a horizontal line at that price.

P = MR = demand for the firm

The profit-maximizing rule

Every firm, in every market structure, produces where

MR = MC

For the competitive firm this means producing where P = MC, on the rising part of the marginal cost curve.

Measuring profit on the graph

profit = (P − ATC) × Q

• P > ATC: economic profit, the rectangle between price and ATC.
• P = ATC: zero economic profit; the firm still earns a normal return.
• P < ATC: a loss.

The shutdown rule

In the short run the firm must pay fixed costs even at zero output. It keeps producing if price covers average variable cost.

• P ≥ min AVC: produce, even at a loss, because revenue covers variable costs and part of fixed costs.
• P < min AVC: shut down; the loss is limited to fixed cost.

Worked example

The market price is $10. At the output where MC = 10 the firm produces 100 units, with ATC = $12 and AVC = $8.

1. Profit: (10 − 12) × 100 = −$200, a loss.
2. Shutting down would lose the whole fixed cost: (ATC − AVC) × Q = 4 × 100 = $400.
3. Price is above AVC, so the firm keeps producing and loses $200 instead of $400.

The long run

Economic profit attracts entry. Market supply shifts right, the price falls, and profit is squeezed to zero. Losses cause exit, and the price rises back. In long-run equilibrium:

P = MR = MC = min ATC

The market is then allocatively efficient (P = MC) and productively efficient (P = min ATC).

The FRQ approach

Draw the market and the firm side by side with the same price line, label MR = MC at the chosen quantity, and shade the profit or loss rectangle with its corners labeled.