Difficulty: Intermediate | Prerequisites: Understanding of cost curves (fixed, variable, average, marginal) from Ch. 13.
This chapter explains how individual firms behave in perfectly competitive markets, where no single buyer or seller can influence the price. It connects the cost curves you studied in Ch. 13 to a firm's supply decisions, shutdown choices, and long-run entry/exit dynamics. If you missed the cost-curve chapter, go back and nail that first, because everything here builds on the relationship between marginal cost, average total cost, and average variable cost. By the end, you will understand why a firm's marginal-cost curve is its supply curve and why long-run profits in competitive markets are driven to zero.
A competitive firm is a price taker: it maximises profit by producing the quantity where price equals marginal cost. In the short run, it shuts down if price falls below average variable cost; in the long run, it exits if price falls below average total cost. Free entry and exit push long-run economic profit to zero, so surviving firms produce at their efficient scale.
Competitive market (perfectly competitive market)
A market with many buyers and sellers trading largely identical goods, so that no single participant can influence the market price.
In simple terms, everyone in this market is too small to matter on their own, so they all accept whatever price the market sets.
Price taker
A buyer or seller who accepts the market price as given and cannot influence it.
Think of it as being a single grain of sand on a beach: removing or adding one grain changes nothing about the beach.
Average revenue (AR)
Total revenue divided by quantity sold (TR / Q). For all firms, average revenue equals the price of the good.
Marginal revenue (MR)
The change in total revenue from selling one additional unit. For competitive firms, marginal revenue equals the price.
Sunk cost
A cost that has already been incurred and cannot be recovered, regardless of future decisions.
Think of it as money already spent: it should not factor into your decision about what to do next.
Shutdown (short-run)
A temporary decision not to produce anything during a specific period because current market conditions make revenue too low to cover variable costs.
Exit (long-run)
A permanent decision to leave the market entirely, eliminating both fixed and variable costs.
Efficient scale
The quantity of output that minimises average total cost. In long-run competitive equilibrium, every surviving firm produces at this level.
Marginal firm
The firm that would exit the market if the price dropped any lower. It earns zero economic profit; firms with lower costs earn positive profit.
Many buyers and many sellers, each small relative to the market
Goods offered by sellers are largely the same (homogeneous)
No single participant can influence the market price
A third condition, often assumed: firms can freely enter or exit the market
Free entry and exit is not required for firms to be price takers, but it is the force that shapes long-run equilibrium
Total revenue = P x Q (price times quantity)
Because the firm is a price taker, total revenue is proportional to output
Average revenue (AR) = TR / Q = P. This holds for all firms.
Marginal revenue (MR) = change in TR from one more unit sold = P. This holds only for competitive firms, because the price does not change when they sell more.
If MR > MC: increase output (each extra unit adds more revenue than cost)
If MC > MR: decrease output (each extra unit costs more than it brings in)
At the profit-maximising quantity: MR = MC, which for a competitive firm means P = MC
The firm chooses Q where P = MC for any given price
When the price rises, the firm moves along its MC curve to a higher quantity
Therefore the firm's MC curve (above the relevant cost threshold) is its supply curve
Profit = (P - ATC) x Q
If P > ATC: the firm earns positive profit (the rectangle between the price line and the ATC curve, with width Q)
If P < ATC: the firm makes a loss (the rectangle between ATC and price, with width Q)
Shutdown = temporarily produce nothing, but the firm still pays its fixed costs
The firm shuts down if total revenue < variable cost, which simplifies to P < AVC
If the firm does produce, it picks Q where P = MC
The short-run supply curve is the portion of the MC curve above the AVC curve
Sunk costs are irrelevant to the shutdown decision. Fixed costs are sunk in the short run: you pay them whether you produce or not.
Exit = leave the market permanently, saving both fixed and variable costs
The firm exits if TR < TC, which simplifies to P < ATC
The firm enters if P > ATC
The long-run supply curve is the portion of the MC curve above the ATC curve
If P is between AVC and ATC, the firm keeps producing in the short run (covers variable costs) but will exit in the long run (cannot cover total costs)
With a fixed number of identical firms, market supply = number of firms x each firm's supply
Each firm supplies along its MC curve (above AVC), so market supply is the horizontal sum
Profits attract new entrants, which increases supply and drives down the price
Losses cause firms to exit, which decreases supply and drives up the price
This process continues until P = minimum ATC and economic profit = zero
At long-run equilibrium, every firm produces at its efficient scale (the quantity that minimises ATC)
The long-run supply curve is perfectly elastic (horizontal) at the minimum of ATC, if all firms have identical costs
Some resources used in production may be available only in limited quantities (scarce inputs push costs up as the industry expands)
Firms may have different costs; new entrants are less efficient, so a higher price is needed to bring them in
In either case the long-run supply curve slopes upward rather than being flat
The long-run supply curve is typically more elastic than the short-run supply curve, because entry and exit are easier in the long run
Zero economic profit means total revenue covers all costs, including opportunity costs (the return the owner could earn elsewhere)
Accounting profit is still positive at zero economic profit, because accountants do not count opportunity costs
The farmer who earns zero economic profit is still covering the forgone interest on invested capital and the salary of the next-best job
\text{Total Revenue (TR)} = P \times Q\text{Average Revenue (AR)} = \frac{TR}{Q} = P\text{Marginal Revenue (MR)} = \frac{\Delta TR}{\Delta Q} = P \quad \text{(for competitive firms)}\text{Marginal Cost (MC)} = \frac{\Delta TC}{\Delta Q}\text{Profit} = TR - TC = (P - ATC) \times QDecision rules summary:
Decision | Condition | Action |
|---|---|---|
Profit-maximising output | P = MC | Produce this quantity |
Short-run shutdown | P < AVC | Produce nothing (still pay FC) |
Long-run exit | P < ATC | Leave the market |
Long-run entry | P > ATC | Enter the market |
Long-run equilibrium | P = min ATC | Zero economic profit |
Students often confuse shutdown with exit. Shutdown is short-run and temporary (the firm still pays fixed costs). Exit is long-run and permanent (the firm avoids all costs). The cost threshold differs: shutdown compares P to AVC, exit compares P to ATC.
Students often think zero economic profit means the firm is earning nothing. It does not. Zero economic profit means the firm covers all its opportunity costs, including the owner's forgone salary and investment returns. Accounting profit is still positive.
Students sometimes think the entire MC curve is the supply curve. Only the portion above AVC (short run) or above ATC (long run) counts, because the firm shuts down or exits below those thresholds.
Students sometimes confuse the firm's supply curve with the market supply curve. The market supply is the horizontal sum of all individual firms' supply curves.
The profit-maximising rule P = MC is tested constantly. Know how to find the optimal Q from a table or a graph.
Be ready to identify whether a firm should shut down, keep producing, or exit, given a price and its cost curves. The AVC vs ATC distinction is a favourite exam question.
Demand shifts and their short-run vs long-run effects are a common multi-part question. Short run: price changes, firms earn profit or loss. Long run: entry/exit returns price to minimum ATC and profit to zero.
Know the difference between economic profit and accounting profit, and why zero economic profit still means the owner is doing as well as their next-best alternative.
Real-world application: This model explains why commodity industries (farming, basic manufacturing) tend to have thin margins. Free entry means that whenever profits appear, new firms flood in until profits vanish. It is also why restaurants, another roughly competitive market, have notoriously high failure rates: many enter when they see profits, driving the market back toward zero.
True or false: A competitive firm's marginal revenue equals the market price. (True)
Fill in the blank: A firm should shut down in the short run if P < ____. (AVC)
True or false: In long-run competitive equilibrium, economic profit is zero but accounting profit is positive. (True)
Fill in the blank: A firm's short-run supply curve is its MC curve above its ____ curve. (AVC)
True or false: If a firm's price is between AVC and ATC, it will keep producing in the short run but exit in the long run. (True)
Q: A competitive firm has a market price of $6, marginal cost of $6, and average total cost of $5. Should the firm produce, and is it earning a profit?
A: Yes, it should produce because P = MC (profit-maximising condition is met). It earns a profit because P ($6) > ATC ($5). Profit per unit = $1.
Q: Suppose the market price drops to $3 and the firm's AVC at its current output is $2.50 while ATC is $4. What should the firm do in the short run? In the long run?
A: In the short run, the firm should keep producing because P ($3) > AVC ($2.50), so it covers its variable costs and contributes toward fixed costs. In the long run, it should exit because P ($3) < ATC ($4).
Q: Why does free entry and exit drive long-run economic profit to zero in a competitive market?
A: If firms earn positive profit, new firms enter the market, increasing supply and pushing the price down until profit disappears. If firms make losses, some exit, reducing supply and pushing the price up. Entry and exit stop only when P = min ATC and profit = zero.
Q: A perfectly competitive market is in long-run equilibrium. Demand suddenly increases. Describe the short-run and long-run effects.
A: Short run: demand shifts right, price rises above min ATC, existing firms increase output along their MC curves and earn positive profit. Long run: profits attract new entrants, supply shifts right, price falls back to min ATC, and economic profit returns to zero. The market now has more firms, a higher total quantity, but the same price as before.
Q: Why is sunk cost irrelevant to a firm's shutdown decision?
A: Sunk costs have already been incurred and cannot be recovered regardless of whether the firm produces. The shutdown decision depends only on whether revenue covers variable costs. Fixed costs are sunk in the short run and must be paid either way.
This chapter connects directly to Ch. 13 (costs of production), where you learnt the shapes of the MC, ATC, and AVC curves. The profit-maximisation rule (MR = MC) reappears in Ch. 15 (monopoly) and Ch. 16 (monopolistic competition), but the key difference is that competitive firms have MR = P while monopolists do not. The long-run zero-profit result also contrasts sharply with monopoly, where barriers to entry allow persistent profits.
Perfect competition, competitive market, price taker, marginal revenue equals price, MR = MC, profit maximisation, shutdown rule, P < AVC, exit rule, P < ATC, efficient scale, zero economic profit, accounting profit vs economic profit, sunk cost, short-run supply curve, long-run supply curve, free entry and exit, market supply, Mankiw Ch. 14