Perfect Competition

You’ll be able to

Describe the characteristics of a perfectly competitive market
Explain why price equals marginal revenue for a price-taking firm
Analyze the short-run and long-run equilibria of a competitive firm

Characteristics of perfect competition

Perfect competition is defined by four features: many buyers and sellers, a standardized (identical) product, free entry and exit, and perfect information. Because each firm is tiny relative to the market and sells an identical product, no firm can influence the price — every firm is a price taker. The market price is set by industry supply and demand, and each firm simply decides how much to produce at that given price. Agriculture is the classic real-world approximation.

Price equals marginal revenue

For a price-taking firm, the demand curve it faces is perfectly elastic (horizontal) at the market price — it can sell any quantity at that price but nothing above it. Because each additional unit sells for the same market price, marginal revenue equals price (MR = P). Combined with the universal MR = MC rule, this means a competitive firm produces where P = MC. The rising portion of the firm’s marginal cost curve (above AVC) is therefore its short-run supply curve.

Short run vs. long run

In the short run, a competitive firm can earn positive economic profit (P > ATC), break even (P = ATC), or take a loss (P < ATC), depending on where the market price sits relative to its ATC. But free entry and exit drive the long run to zero economic profit: profits attract new firms, increasing supply and pushing price down; losses drive firms out, decreasing supply and pushing price up. In long-run equilibrium, price settles at minimum ATC, where P = MR = MC = minimum ATC and every firm earns exactly a normal profit.

The competitive firm’s conditions

Short run: P = MR = MC · Long-run equilibrium: P = MR = MC = minimum ATC (zero economic profit)

A price taker faces a horizontal demand curve, so MR = P. Free entry/exit competes economic profit away to zero in the long run.

Worked example

A perfectly competitive industry is currently in short-run equilibrium with firms earning positive economic profit. Explain what happens to the number of firms, market supply, price, and individual firm profit as the industry moves to long-run equilibrium.

1.Positive economic profit attracts new firms because entry is free.
2.As new firms enter, market (industry) supply increases, shifting the market supply curve right.
3.Greater supply lowers the market price, which lowers each price-taking firm’s horizontal demand/MR line.
4.Entry continues until price falls to minimum ATC, where economic profit is zero (normal profit) and firms no longer have an incentive to enter.

Answer: New firms enter, market supply rises, and the market price falls until each firm earns zero economic profit at minimum ATC. In long-run equilibrium, P = MR = MC = minimum ATC and entry stops — the hallmark of perfect competition.

Checkpoint

A perfectly competitive firm faces a market price of $8. What is the marginal revenue from selling one more unit?

Watch out

For a perfectly competitive firm, P = MR because the firm faces a horizontal demand curve. This is what distinguishes it from a monopoly or monopolistic competitor, whose downward-sloping demand makes MR < P. Do not use MR < P for a price taker.

Checkpoint

In long-run equilibrium, a perfectly competitive firm produces where:

On the exam

Draw the competitive firm with a horizontal demand = MR = price line and the U-shaped ATC/AVC/MC curves. In the long run, that price line is tangent to minimum ATC. Free-response prompts reward showing entry/exit driving profit to zero.

Answer the 2 checkpoints as you read.