Price-taking firms, zero long-run profit, and why entry and exit discipline the market.
Warming up the engine…
Many identical firms each too small to affect price: every firm takes the market price as given and produces where price equals marginal cost, with free entry driving profit to zero in the long run.
P = MR = MC ; long run: P = min ATC, profit = 0
If firms in the industry earn profit, entrants flood in, supply shifts right, and price falls until the profit is gone; losses trigger exit and the reverse. The long run therefore pins price to the bottom of average cost: consumers get the good at the cheapest sustainable price, and 'zero economic profit' still includes a normal return.
Zero ECONOMIC profit is not zero accounting profit; it means covering all opportunity costs. Shutdown in the short run happens when P falls below AVC, not ATC.
Now prove you have it
Move the curve to where you think it lands, and get told exactly which part you got right.
Real-world scenarios on this model
Equilibrium at q 50, P 55.
Equilibrium: q* = 50.0, P* = 55.0
Current equations
One firm in a huge market: it can't move the price, so its demand curve is a flat line at P. It produces where that line meets rising marginal costmarginal costThe cost of producing one more unit., and the shaded box shows profit or loss against average cost.
Source: Varian, Intermediate Microeconomics, ch. 23-24