AP Microeconomics / Production, Cost, and the Perfect Competition Model
Lesson
Perfect competition and the profit test
A price-taking firm rides the market price, then entry erases extra profit.
Learning goals
- Find a competitive firm’s output using P = MC.
- Explain long-run entry and exit using economic profit.
Explanation
In perfect competition many firms sell a matching product, and no single seller can set the price. Demand for one firm is a horizontal line at the market price, so price equals marginal revenue. The firm produces where P = MC, as long as price covers average variable cost.
If price is above average total cost, economic profit is positive and new firms enter. Market supply shifts right and price falls. If price is below ATC, firms leave. In a typical long-run picture, remaining firms earn zero economic profit—enough to stay, not a windfall.
Key terms
- Economic profit. Revenue minus all opportunity costs, including the next-best use of the owner’s time and capital.
- Shutdown rule. In the short run, produce only if price covers average variable cost.
Common mistakes
- Producing where ATC is minimized even if that is not where MR = MC.
- Calling zero economic profit a failure to cover accounting costs.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A firm in a perfectly competitive market faces a market price of $16. Its marginal cost is MC = 4 + 2q. (a) Find the profit-maximizing output. (b) If ATC at that output is $12, find economic profit. (c) Explain why this outcome is not a long-run equilibrium if entry is free.
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Why does a perfectly competitive firm not raise price to increase profit?
Answer. Buyers can buy the same good from other firms at the market price, so the firm would sell nothing.
Price taking is the definition of this market structure.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Microeconomics course page
Official six-unit framework for markets, costs, and market failure.
College Board · All rights reserved · accessed 2026-10-01
Open sourceCC BY — attribution required
Principles of Microeconomics: Scarcity and Social Provisioning
Open microeconomics background for scarcity, markets, and elasticity.
Open Textbook Library listing · CC BY · accessed 2026-10-01
Open source