Market supply and demand set a price (Market Equilibrium). Now zoom in on one seller among thousands. With so many sellers of the same thing, any one seller's buyers have perfect substitutes, the most elastic case of Elastic, Inelastic and What Decides It.
Key idea
Perfect competition: many small firms, an identical product, easy entry and exit, and buyers and sellers who know the prices. Each firm is a price taker: it can sell as much as it wants at the market price and nothing above it. Its own demand curve is horizontal at the market price, so for the firm $P = MR = AR$.
Why it works
A wheat farm grows a tiny share of the world's wheat. If it asks a penny above the market price, buyers simply buy identical wheat from someone else, so it sells nothing. If it charges less, it gives money away, because it can already sell all it grows at the market price. So every extra bushel brings in exactly the market price: marginal revenue equals price. The market's demand curve still slopes down; it's the single firm's slice of it that looks flat.
Worked example: Revenue for a price taker
Wheat sells for \$6 a bushel. A farm sells 2,000 bushels. Find its total revenue, average revenue and the marginal revenue of one more bushel.
- $TR = \$6 \times 2{,}000 = \$12{,}000$.
- $AR = TR \div Q = \$6$.
- One more bushel sells for \$6, without lowering the price on the others: $MR = \$6$.
Answer: \$12,000; \$6; \$6.
Worked example: Which markets come close?
Which comes closer to perfect competition: farms selling a grade of corn, or the market for smartphones?
- Corn of one grade is identical whoever grows it; there are many farms, and none can affect the price.
- Smartphones differ by brand and features, and a few large firms set their own prices.
Answer: Corn. No real market is perfectly competitive, but some come close.
Common mistake
Giving the single firm a downward-sloping demand curve because the market demand curve slopes down. The market's curve does slope down, but one small firm's output is too little to move the market price, so the firm faces a flat line. Check: ask "if this firm doubled its output, would the market price change?" For a price taker, no.