Difficulty: Intermediate | Prerequisites: individual firm supply derivation (see companion notes on Short-Run Costs and Supply), basic algebra
Once you know how a single competitive firm decides how much to supply, the next step is to build the market. This material covers aggregating individual supply into market supply, finding the short-run equilibrium where supply meets demand, and testing whether that equilibrium is also a long-run equilibrium. These are the mechanics that explain how competitive markets settle on a price and quantity, and why economic profit tends toward zero over time.
Market supply is the horizontal sum of all individual firms' supply curves. Set market supply equal to market demand to find equilibrium price and quantity. If firms earn zero economic profit at the equilibrium price, the equilibrium also lies on the long-run supply curve and no firms will enter or exit.
Market supply, Q(p)
The total quantity supplied by all firms in the market at a given price. Found by multiplying individual firm supply q(p) by the number of firms N (when firms are identical): Q(p) = N × q(p).
Think of it as: "Stack everyone's output side by side at the same price."
Short-run market equilibrium
The price and quantity where market supply equals market demand: Q^s(p) = Q^d(p). At this point, every unit produced finds a buyer and no buyer is left unsatisfied at the going price.
In simple terms, this is where the supply and demand curves cross.
Zero-profit condition
The state where price equals minimum average cost, so each firm earns exactly zero economic profit. Revenue covers all costs, including the opportunity cost of the owner's time and capital, but nothing beyond that.
Think of it as: the firm is doing fine, just not making above-normal returns.
Long-run supply curve
The locus of price-quantity pairs at which the market can sustain itself with no incentive for entry or exit. In a constant-cost industry with identical firms, this is a horizontal line at the minimum of each firm's long-run average cost.
In simple terms, it is the price the market settles at once enough time passes for firms to come and go freely.
When the cost function is C(q) = 2 + 0.5q², the algebra is cleaner than the cubic case and worth understanding as a separate pattern.
MC(q) = dC/dq = q (the derivative of 0.5q² is q; the fixed cost 2 drops out)
In perfect competition, MR = P, so set MC = MR: q = P, meaning q(P) = P
This is a 45-degree line through the origin, but only above the shutdown price
Finding the shutdown price:
AC(q) = C(q)/q = 2/q + 0.5q
Set MC = AC: q = 2/q + 0.5q
Simplify: 0.5q = 2/q, so 0.5q² = 2, giving q² = 4, q = 2
Plug back into MC: MC(2) = 2, so the shutdown price is P = 2
Individual supply: q(P) = P for P ≥ 2, and q = 0 for P < 2.
With identical firms, market supply is simply the number of firms multiplied by individual firm supply.
There are 200 photographers, each with supply q(P) = P for P ≥ 2
Market supply: Q(P) = 200 × P for P ≥ 2
Below P = 2, no firm produces, so Q = 0
This is called horizontal summation: at each price, you add up all the individual quantities. When firms are identical, the multiplication shortcut works. When firms differ (different cost structures), you must add up their individual supply functions explicitly, which is trickier but follows the same logic.
Set market supply equal to market demand and solve for price.
Market demand: Q^d(P) = 1000 - 300P
Market supply: Q^s(P) = 200P
Equilibrium: 200P = 1000 - 300P, so 500P = 1000, giving P* = 2
Equilibrium quantity: Q* = 200(2) = 400
Is this a long-run equilibrium? Yes. At P* = 2, each firm earns zero economic profit because P = 2 is exactly the minimum of each firm's AC curve (the shutdown price we found earlier). When profit is zero, no new firms want to enter and no existing firms want to leave. The market is stable.
If the equilibrium price had been above 2, firms would be earning positive profit, which would attract new entrants in the long run, shifting supply right and pushing the price down. If the price had been below 2, firms would exit, shifting supply left and pushing the price up. Either way, the long-run equilibrium always gravitates back to the zero-profit price.
C(q) = 2 + 0.5q^2MC(q) = qAC(q) = \frac{2}{q} + 0.5qQ^s(P) = N \times q(P) = 200P \quad \text{for } P \geq 2Q^s(P) = Q^d(P) \implies 200P = 1000 - 300P \implies P^* = 2, \; Q^* = 400The wedding photography market is a good real-world illustration because it is reasonably competitive: many photographers, a broadly similar service, and relatively low barriers to entry and exit. When demand for weddings surges (as it did after pandemic restrictions lifted), prices rise above the zero-profit level, new photographers enter, supply increases, and prices drift back down. This entry-exit dynamic is exactly what the long-run equilibrium condition predicts.
Students often add supply curves vertically (adding prices at each quantity). Supply curves are summed horizontally: you add quantities at each price. Vertical summation is for public goods, not market supply.
A common mistake is thinking zero economic profit means the firm is failing. Zero economic profit means the firm covers all its costs, including the owner's opportunity cost. It is the normal state of a competitive market in the long run.
Students sometimes forget the shutdown condition when aggregating. If the individual supply is q(P) = P for P ≥ 2, then market supply Q(P) = 200P also applies only for P ≥ 2, not for all P.
Some students confuse short-run equilibrium with long-run equilibrium. A short-run equilibrium can exist at any price where supply meets demand. A long-run equilibrium requires the additional condition that P = min AC (zero profit), so there is no incentive for entry or exit.
⚠️ Aggregating individual supply to market supply and solving for equilibrium is a staple exam problem. Be comfortable with the algebra of setting Q^s = Q^d.
⚠️ Expect a follow-up question asking whether the short-run equilibrium is also a long-run equilibrium. The answer hinges on whether firms earn zero profit at the equilibrium price.
⚠️ Know the entry/exit logic cold: positive profit attracts entry, shifting supply right and lowering price; negative profit causes exit, shifting supply left and raising price. The long-run outcome is P = min AC.
True or false: Market supply is found by adding individual supply curves vertically. False. Horizontal summation (add quantities at each price).
Fill in the blank: If there are 200 identical firms each supplying q(P) = P, market supply is Q(P) = ______. 200P.
True or false: Zero economic profit means the firm should exit the market. False. Zero economic profit means the firm is covering all costs including opportunity costs.
Fill in the blank: A short-run equilibrium is also a long-run equilibrium when firms earn ______ economic profit. Zero.
True or false: If firms in a competitive market are making positive economic profit, new firms will enter in the long run. True.
Q: If there are 500 identical firms, each with supply q(P) = 2P for P ≥ 5, what is market supply?
A: Q(P) = 500 × 2P = 1000P for P ≥ 5, and Q = 0 for P < 5.
Q: Given market supply Q^s = 200P and market demand Q^d = 1000 - 300P, find the equilibrium price and quantity.
A: Set 200P = 1000 - 300P. Solve: 500P = 1000, P* = 2. Plug back in: Q* = 200(2) = 400.
Q: At the equilibrium above, are firms earning positive, negative, or zero economic profit? How do you know?
A: Zero economic profit. P* = 2 equals the minimum of each firm's AC curve (the shutdown price), so revenue per unit exactly equals cost per unit.
Q: What would happen in the long run if demand shifted so that equilibrium price rose to P = 4?
A: At P = 4, each firm earns positive economic profit (P > min AC = 2). New photographers would enter the market, increasing market supply, which shifts the supply curve right. Entry continues until price falls back to P = 2 and profit returns to zero.
This material builds directly on individual firm supply (the companion study notes). It also sets the stage for welfare analysis (consumer and producer surplus at equilibrium), the effects of taxes and subsidies on competitive markets, and the comparison with monopoly and imperfect competition, where the equilibrium logic changes because firms have market power.
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