A single Kansas wheat farmer harvests 5,000 bushels in a market that trades billions. Can she charge $1 above the market price? Buyers vanish — identical wheat is everywhere. Charge less? Pointless — she can already sell every bushel at the market price. Her only real decision is how much to produce. This is perfect competition, the benchmark model of the entire course, and the side-by-side graph you learn today is the highest-value drawing on the AP exam — FRQ 1 asks for some version of it more years than not.
Result: each firm is a price taker. The market sets the price; the firm can sell any quantity at that price.
The market demand curve slopes down as always — but the individual firm faces a horizontal (perfectly elastic) demand curve at the market price:
P = MR = AR = D(firm)
Why MR = P: each extra unit sells at the market price without lowering the price on earlier units. (Contrast with monopoly next lesson, where MR < P.)
Produce every unit for which MR ≥ MC; stop where MR = MC (with MC rising). Producing less leaves profitable units on the table; producing more adds units costing more than they bring in. In perfect competition MR = P, so the rule becomes P = MC.
MR = MC picks the QUANTITY. It says nothing about whether profit is positive. Profit sign comes from comparing P to ATC at that quantity:
Profit = (P − ATC) × Q
[GRAPH: Two panels. LEFT (Market): downward D, upward S, equilibrium P = $10, Q_market. RIGHT (Firm): horizontal line at $10 labeled MR = D = AR = P; J-shaped MC crossing it at q = 200; U-shaped ATC with minimum at $8 (below price) at q ≈ 190; profit rectangle between $10 and ATC(200) ≈ $8.20 from 0 to 200 shaded, labeled "economic profit".]
Case 1 — Economic profit: P > ATC at q. Profit rectangle: height (P − ATC), width q.
Case 2 — Loss but keep operating: AVC < P < ATC. Loss rectangle: height (ATC − P), width q. The firm covers all variable costs and part of fixed costs — shutting down would mean losing all* fixed costs.
Case 3 — Shutdown: P < minimum AVC. Operating doesn't even cover variable costs; every unit deepens the loss. Produce zero; loss = TFC.
P ≥ min AVC → produce where MR = MC
P < min AVC → shut down (Q = 0, lose TFC)
Fixed costs are irrelevant to the operate/shutdown decision — they're paid either way in the short run. Consequently the firm's short-run supply curve is its MC curve above minimum AVC.
(Long-run exit rule: leave if P < ATC persistently — in the long run all costs must be covered.)
Free entry/exit is a profit-seeking missile:
Long-run equilibrium: P = MR = MC = minimum ATC. Every firm earns exactly normal profit (zero economic profit — Lesson 8's punchline), and:
P = MC — society's value of the last unit equals its marginal cost. The right amount is produced.Perfect competition delivers both efficiencies in the long run — the benchmark every other market structure is judged against.
Market price $12. Firm's MC: q=1→$4, 2→$6, 3→$8, 4→$10, 5→$12, 6→$15. ATC at q=5 is $9. Find output and profit.
Solution: Produce every unit with MC ≤ MR ($12) → q = 5 (MC of 5th = $12 = MR). Profit = (12 − 9) × 5 = $15*.
Interpretation: The 6th unit costs $15 but brings $12 — producing it would burn $3.
Price falls to $6. At the best quantity q = 3: ATC = $9, AVC = $5, and min AVC = $4.80. Should the firm operate? Compute the loss either way (TFC = $12).
Solution: P ($6) > min AVC ($4.80) → operate. Operating loss = (9 − 6) × 3 = $9. Shutdown loss = TFC = $12. Operating saves $3 — exactly the $1/unit surplus over AVC × 3 units that chips away at fixed costs.
Interpretation: "Minimize loss" is still MR = MC. Compare the two losses explicitly on FRQs — that comparison is the point.
The competitive market for oat milk is in long-run equilibrium. A durable shift in tastes raises demand. Trace: (i) short-run effects on market and firm; (ii) the long-run adjustment; (iii) final resting point for the typical firm.
Solution: - (i) Demand right → market P ↑ → each firm's horizontal MR line jumps up → firms slide up MC to higher q* → P > ATC: short-run economic profits (market Q ↑ via both price and output). - (ii) Profits attract entry → market supply shifts right → price falls back toward minimum ATC. - (iii) Entry stops at zero economic profit: P = min ATC again; each firm back near original output, but the market has more firms and higher total quantity.
Interpretation: The two-panel story — market moves first, firm reacts, entry resets — is the classic FRQ 1 narrative. Practice narrating it in exactly these three beats.
At its MR = MC output of 40 units, a firm has TR = $480 and TC = $560; its TVC = $400. Short run: operate or shut down?
Solution: P = 480/40 = $12; AVC = 400/40 = $10; ATC = 560/40 = $14. Loss = $80 operating. P ($12) > AVC ($10) → operate; shutdown would lose TFC = 560 − 400 = $160. Operating cuts the loss in half.
Interpretation: Any totals-based question converts to per-unit in one division. Do that first.
1. (C) Price takers can sell any amount at P: horizontal (perfectly elastic) firm demand. (A) confuses the market and the firm.
2. (A) Each additional unit sells at the unchanged market price → MR = P.
3. (B) MR = MC plus the shutdown proviso. (A) is productive-efficiency confusion; (C) maximizes revenue, not profit.
4. (D) Loss operating = (10 − 8) × 100 = $200. TFC = (10 − 7) × 100 = $300. P > AVC → operate; losing $200 beats losing $300. (C) impossible for a price taker.
5. (C) Below min AVC the firm supplies zero; above it, MC gives quantity at each price.
6. (A) Profit → entry → supply right → price down → zero economic profit. The full chain, stated to its endpoint.
7. (B) The long-run triple equality: P = MR = MC = min ATC. Normal profit only.
8. (E) Allocative: value of last unit (P) = cost of last unit (MC). (A) is productive efficiency.
9. (D) TC = 1,000 > TR = 900 → $100 loss operating. Shutdown loses TFC = $400. TR − TVC = $300 of fixed costs covered → operate. (B) ignores the shutdown comparison.
10. (C) Losses → exit → supply left → price rises → survivors return to normal profit.
11. (FRQ rubric, 10 points) - (a) 3 pts: Market panel: D and S crossing at P₁, Q₁ (1). Firm panel: horizontal MR₁ = D = AR = P₁ at the same height, MC crossing MR at q₁, ATC tangent at that point — minimum ATC = P₁ (1). Zero economic profit indicated (P = ATC) (1). - (b) 3 pts: Market D shifts right → P₂ > P₁, Q₂ > Q₁ (1). Firm's MR line rises to P₂ (1). q₂ > q₁ where MC = MR₂ (1). - (c) 1 pt: Economic profit — at q₂, P₂ > ATC; rectangle between P₂ and ATC(q₂). - (d) 2 pts: Profits attract entry → market supply shifts right → price falls back to minimum ATC (1); entry ceases when the typical firm again earns zero economic profit (1). - (e) 1 pt: Yes — long-run equilibrium output is at minimum ATC, the definition of productive efficiency.
11. (FRQ-style) The perfectly competitive market for cranberries is in long-run equilibrium. (a) Draw correctly labeled side-by-side graphs for the market and a typical firm. Show market price P₁, market quantity Q₁, firm output q₁, and indicate the firm's economic profit. (b) A medical study announces cranberries fight inflammation, permanently increasing demand. Show on your graphs the short-run effects: new price P₂, market quantity Q₂, firm output q₂. (c) At q₂, is the firm earning economic profit, a loss, or zero profit? Explain. (d) Explain the long-run adjustment process and the effect on: market supply, price, and the typical firm's economic profit. (e) Is the firm productively efficient in the new long-run equilibrium? Explain.
1. (C) Price takers can sell any amount at P: horizontal (perfectly elastic) firm demand. (A) confuses the market and the firm.
2. (A) Each additional unit sells at the unchanged market price → MR = P.
3. (B) MR = MC plus the shutdown proviso. (A) is productive-efficiency confusion; (C) maximizes revenue, not profit.
4. (D) Loss operating = (10 − 8) × 100 = $200. TFC = (10 − 7) × 100 = $300. P > AVC → operate; losing $200 beats losing $300. (C) impossible for a price taker.
5. (C) Below min AVC the firm supplies zero; above it, MC gives quantity at each price.
6. (A) Profit → entry → supply right → price down → zero economic profit. The full chain, stated to its endpoint.
7. (B) The long-run triple equality: P = MR = MC = min ATC. Normal profit only.
8. (E) Allocative: value of last unit (P) = cost of last unit (MC). (A) is productive efficiency.
9. (D) TC = 1,000 > TR = 900 → $100 loss operating. Shutdown loses TFC = $400. TR − TVC = $300 of fixed costs covered → operate. (B) ignores the shutdown comparison.
10. (C) Losses → exit → supply left → price rises → survivors return to normal profit.
11. (FRQ rubric, 10 points) - (a) 3 pts: Market panel: D and S crossing at P₁, Q₁ (1). Firm panel: horizontal MR₁ = D = AR = P₁ at the same height, MC crossing MR at q₁, ATC tangent at that point — minimum ATC = P₁ (1). Zero economic profit indicated (P = ATC) (1). - (b) 3 pts: Market D shifts right → P₂ > P₁, Q₂ > Q₁ (1). Firm's MR line rises to P₂ (1). q₂ > q₁ where MC = MR₂ (1). - (c) 1 pt: Economic profit — at q₂, P₂ > ATC; rectangle between P₂ and ATC(q₂). - (d) 2 pts: Profits attract entry → market supply shifts right → price falls back to minimum ATC (1); entry ceases when the typical firm again earns zero economic profit (1). - (e) 1 pt: Yes — long-run equilibrium output is at minimum ATC, the definition of productive efficiency.
Exam tip: Build the side-by-side graph in a fixed order every time: (1) market axes, D, S, P, Q; (2) carry P across as the firm's horizontal MR line; (3) MC crossing MR → drop to q; (4) place ATC to show the profit case the question wants. Thirty seconds of ritual = the biggest point block on the exam. Next lesson we tilt the firm's demand curve down and everything changes.