Difficulty: Intermediate | Prerequisites: Basic supply and demand, cost curves (ATC, AVC, MC)
This unit is the backbone of intermediate micro. It explains how firms in competitive markets choose output levels, when they should keep producing and when they should shut down, and how the market adjusts in the short run versus the long run. If you are coming in cold, make sure you are comfortable with marginal cost, average cost curves, and the idea that firms are price takers. Everything in welfare economics and tax incidence builds on this.
Firms in perfect competition are price takers: they maximise profit by producing where marginal revenue equals marginal cost (MR = MC). In the short run a firm keeps operating as long as price covers average variable cost, even if it is losing money overall. In the long run, economic profit is driven to zero by entry and exit, and in a constant-cost industry the long-run price returns to its original level after a demand shift.
Perfect competition
A market structure in which many firms sell an identical product, no single firm can influence the market price, and there are no barriers to entry or exit.
In simple terms, this means every seller is a price taker: you sell at the going price or you do not sell at all.
Marginal profit
The change in total profit from producing one additional unit of output. At the profit-maximising quantity, marginal profit equals zero.
Think of it as the "extra profit per extra unit." When it hits zero, squeezing out one more unit no longer helps you.
MR = MC (profit-maximisation rule)
The condition that a firm maximises profit by producing the quantity at which marginal revenue equals marginal cost.
In simple terms, keep producing as long as the next unit brings in more money than it costs to make. Stop the moment those two figures meet.
Barrier to entry
Anything that prevents new firms from entering an industry. Patent rights are a common example.
Think of it as a gate that keeps competitors out, letting existing firms earn above-normal profits for longer.
Average variable cost (AVC) curve
The curve showing variable cost per unit at each output level. A firm never operates on the downward-sloping portion of its AVC curve.
In simple terms, the rising part of the AVC curve is where the firm's supply decisions actually happen.
Short run vs. long run
In the short run, at least one input is fixed (typically capital), so a firm may continue operating at a loss if price covers variable costs. In the long run, all inputs are variable, firms can enter or exit, and economic profit tends to zero.
Constant-cost industry
An industry in which expansion or contraction does not change input prices. After a demand increase, entry drives the long-run price back to exactly where it started.
Think of it as an industry where growing bigger does not make inputs more expensive.
Market structure assumptions
Having only a few sellers does not automatically mean the market is uncompetitive. Markets with many sellers are not always perfectly competitive either. Both statements are false as blanket rules.
Even when some assumptions of perfect competition are violated, the model can still be useful if there is enough rivalry in the industry.
Profit maximisation rules
A firm maximises profit at the output where MR = MC.
At that quantity, marginal profit (the extra profit from one more unit) is zero. Producing beyond that point means each additional unit costs more than it brings in.
If current output is below the profit-maximising level, the next unit produced will increase revenue more than it increases cost, so the firm should expand.
Short-run operating decisions
A firm never operates on the downward-sloping portion of its AVC curve. The supply curve is the MC curve above AVC.
Rihanna's Diner example: price is $5.00, AVC is $3.95, AFC is $1.25. Total ATC = $5.20, so the firm is losing $0.20 per meal. Because price ($5.00) exceeds AVC ($3.95), Rihanna should keep producing in the short run but plan to exit in the long run.
Effects of ignoring implicit costs
If a manager underestimates marginal cost (perceived MC sits below actual MC), the firm produces more than the true optimum and earns lower profits. The output looks profitable on the understated cost curve, but the real costs eat into the gains.
Lump-sum taxes and firm behaviour
A lump-sum tax (e.g. $250/year on all restaurants) is a fixed cost. It does not shift the MC curve, so it does not change the profit-maximising output.
However, if the tax exceeds the firm's economic profit, the firm shuts down in the long run. Cicero's Pizza House: output stays at 80 pizzas per day if profits exceed $250/year; otherwise, exit.
Input price changes
When the price of a variable input rises (e.g. taco cheese), the MC curve shifts upward. The new intersection of MR and MC occurs at a lower quantity, so profit-maximising output falls.
Short-run vs. long-run demand shifts (competitive industry)
A demand increase raises the short-run market price because supply cannot fully adjust.
In the long run (especially in a constant-cost industry), new firms enter, supply expands, and price returns to the original level. So statement I (short-run price rises) is true and statement II (long-run price rises) is false.
Constant-cost industry adjustment
After a demand increase, the long-run supply curve is perfectly elastic. Entry brings price back to the pre-shift level, so only quantity changes permanently.
Factor substitution and effluent fees
The more easily a firm can substitute capital for waste water, the more effective an effluent fee is at reducing pollution. Greater substitutability also means the firm pays less in fees, because it shifts away from the taxed input more readily.
Profit-maximisation condition
MR = MC
In perfect competition, MR equals the market price (P), so the rule simplifies to P = MC.
Short-run shutdown rule
Continue producing if P >= AVC. Shut down if P < AVC.
If price covers variable costs, the firm loses less by staying open than by closing (it still contributes toward fixed costs).
Long-run exit rule
Exit the industry if P < ATC (economic profit is negative and all costs are variable in the long run).
Economic profit
Profit = TR - TC = (P - ATC) x Q
The Rihanna's Diner scenario mirrors what happens in real restaurant markets: many small eateries operate at a loss for months because their revenue still covers wages and ingredients (variable costs), even though it does not cover rent (a fixed cost). They hang on in the short run and close when the lease comes up.
Effluent fees work the same way in environmental policy. When firms can easily swap capital for polluting inputs, a pollution tax becomes a powerful tool because firms respond by substituting rather than just paying.
Students often think that a market with only a few sellers cannot be competitive. It can. The number of sellers alone does not determine the degree of competition.
Students often confuse "marginal profit = zero" with "the firm earns zero profit." Marginal profit being zero at the optimum simply means the last unit adds nothing extra. Total profit can still be positive.
Students sometimes believe that a lump-sum tax (a flat annual fee) changes the firm's output decision. It does not, because it is a fixed cost and does not shift the MC curve.
Students frequently assume that a demand increase raises the long-run price in a competitive industry. In a constant-cost industry, entry pushes the price back to its original level.
The MR = MC rule is tested in nearly every exam on this material. Know it cold.
Shutdown vs. exit: short-run shutdown compares P to AVC; long-run exit compares P to ATC. These are different decisions with different thresholds.
Rihanna's Diner is a classic exam setup: you will be given price, AVC, and AFC and asked whether the firm should produce, shut down, or exit. Work through the numbers, do not guess.
Lump-sum tax questions test whether you understand fixed vs. variable cost. The answer almost always involves "MC does not change, so output does not change."
Be ready to explain why the long-run price does not rise after a demand increase in a constant-cost industry. This trips up many students.
True or false: at the profit-maximising output, marginal profit is positive. (False. It is zero.)
Fill in the blank: a firm maximises profit where ______ = ______. (MR = MC.)
True or false: a firm should shut down in the short run whenever it is making an economic loss. (False. It should keep producing if P >= AVC.)
True or false: a lump-sum annual tax shifts the marginal cost curve. (False. It is a fixed cost.)
Fill in the blank: in a constant-cost industry, the long-run supply curve is perfectly ______. (Elastic.)
Q: A perfectly competitive firm sells its product for $5.00. AVC is $3.95 and AFC is $1.25. Should the firm produce in the short run? In the long run?
A: Yes in the short run, because P ($5.00) > AVC ($3.95). No in the long run, because P ($5.00) < ATC ($5.20), so the firm is making an economic loss.
Q: A plant manager underestimates marginal cost. What happens to output and profit?
A: The firm produces more than the true optimal quantity and earns lower profits, because the actual cost of those extra units exceeds the revenue they generate.
Q: A local government imposes a $250 per year lump-sum tax on restaurants. Does this change a firm's profit-maximising output?
A: No. The tax is a fixed cost and does not shift the MC curve. Output stays the same. However, if the firm's economic profit was less than $250, it will shut down in the long run.
Q: The price of a variable input (e.g. cheese) rises. What happens to a competitive firm's optimal output?
A: Output decreases. The MC curve shifts upward, so the new MR = MC intersection occurs at a lower quantity.
Q: In a constant-cost competitive industry, does a permanent increase in demand raise the long-run equilibrium price?
A: No. Entry by new firms increases supply until the price returns to the original level. Only the equilibrium quantity is permanently higher.
This connects to consumer and producer surplus because the competitive equilibrium is the benchmark against which welfare losses from taxes, price controls, and tariffs are measured. Understanding MR = MC here is also prerequisite for monopoly and oligopoly models, where MR diverges from price.
The shutdown rule reappears in labour economics (firms hiring workers up to the point where the marginal revenue product equals the wage) and in international trade (firms deciding whether to export based on covering variable costs of serving foreign markets).
Perfect competition, price taker, MR = MC, marginal revenue equals marginal cost, profit maximisation, marginal profit zero, barrier to entry, patent rights, AVC curve, shutdown rule, shutdown condition, short-run supply curve, long-run equilibrium, zero economic profit, constant-cost industry, long-run supply, lump-sum tax, fixed cost tax, input price increase, MC shift, factor substitution, effluent fee, competitive market assumptions, ECO 3101, intermediate microeconomics