No firm can move market supply by itself
Each firm supplies such a small share of industry output that changing its own output has no noticeable effect on the market price.
7.6 · Interactive notes and practice
Perfect competition
Connect the assumptions to price-taking behaviour, build the firm and industry diagrams, test shutdown and supply decisions, then compare the model with real evidence and alternative structures.
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01
Syllabus 7.6.1, 7.6.2 and 7.6.4 · Worksheet 1–18
Build price taking and the revenue curves
Buyers see every unit as identical
A firm charging above the market price loses buyers because an identical substitute is available from another seller.
Buyers know the available prices
A firm cannot quietly sustain a higher price when consumers can immediately identify the cheaper identical product.
Profit attracts firms and loss causes exit
The number of firms can change in the long run, altering industry supply and removing persistent supernormal profit or loss.
No buyer controls the market price
Demand is spread across buyers, so neither side of the market has the power to dictate price.
The firm is a price taker
The industry determines price. The individual firm accepts that price and chooses only how much to produce.
The model assumes no effective legal, market, cost or physical barrier prevents firms entering or leaving. If one of these barriers becomes substantial, the market moves away from the perfectly competitive benchmark.
A firm cannot sustain a higher price, while homogeneous products leave no meaningful product feature to promote. Competitive pressure is expressed mainly through cost control and the entry or exit of firms.
Do not stop at a list
Turn each characteristic into a behavioural explanation.
Identical products→Easy switching→No demand above market price→Price taker
The same logic connects small market share and perfect information to the horizontal demand curve facing the firm.
TR = P × q
At a fixed market price, each extra unit adds the same amount to total revenue, so the TR curve is a straight line from the origin.
AR = TR ÷ q = P
Because every unit is sold at the market price, revenue per unit equals price.
MR = ΔTR ÷ Δq = P
One more unit adds exactly the market price to revenue. Therefore the firm faces one horizontal P = AR = MR line.
Industry → firm
Move market demand and follow the price
The market has downward-sloping demand. Only the individual firm faces a horizontal P = AR = MR line.
IndustryDemand and supply determine P
Individual firmThe market price becomes P = AR = MR
At the industry equilibrium, the firm accepts P. It produces where the horizontal MR line meets rising MC.
Quick checkCan the firm sell unlimited output at the market price?
02
Syllabus 7.6.4 · Worksheet 19–38
Costs output profit and operating loss
1
→
The industry fixes the market price through demand and supply.
2
→
The firm produces q* where P = MR = MC on rising MC.
3
At q*, compare P with AC and AVC to identify profit, loss or shutdown.
Move the market price
Create four short-run outcomes
The firm cannot move its own AR = MR curve. A change in the market price moves that line.
Current resultPrice exceeds AC at q*: supernormal profit.
Choose an outcome, then move the price line to build it.
Worksheet question 23
Price $12 · output 200 · AC $9
Calculate total revenue, total cost, profit per unit and total economic profit before revealing the working.
TR = $12 × 200 = $2,400TC = $9 × 200 = $1,800Profit per unit = $12 − $9 = $3Profit = $3 × 200 = $600
Choose the last unit for which MR is at least MC. In Worksheet 24, profit rises to its maximum at q = 5 because the fifth unit adds $12 of revenue but only $9 of cost; the sixth adds $12 but costs $13.
MR = MC identifies the best operating output. The AVC test then decides whether operating is better than temporary shutdown.
03
Syllabus 7.6.4 · Worksheet 39–43 and 49–62
Shutdown exit and long-run adjustment
Shutdown price = minimum AVC
If P is below minimum AVC, revenue cannot cover variable cost. Producing would add to the fixed-cost loss, so zero output minimises the short-run loss.
Long-run exit price = minimum LRAC
All costs are variable in the long run. A price below minimum long-run average cost cannot cover all economic costs, so firms leave if the loss persists.
P = minimum AC = MC
Entry or exit stops when the representative firm earns normal profit. There is then no excess return attracting entry and no loss causing exit.
Long-run adjustment
Add firms and watch profit disappear
Entry changes industry supply, not the cost curves of the representative firm in this standard model.
IndustryMore firms shift supply to the right
Representative firmIts market price and output fall
With relatively few firms, price can lie above AC and existing firms earn supernormal profit. That profit attracts entry.
Supernormal profit→Entry→Industry supply rises→Price falls→Normal profit
Reverse the chain for loss: exit reduces industry supply, price rises and surviving firms move back towards normal profit. The adjustment assumes free entry and exit, unchanged market demand and unchanged cost curves for remaining firms.
04
Syllabus 7.6.4 · Worksheet 44–48
Derive the firm’s short-run supply curve
The derivation
Every possible market price selects one output on rising MC.
Choose P→Set P = MR→Find MC = MR→Read the firm’s q supplied
Joining those price-output points gives the rising section of MC above minimum AVC. Below the shutdown price, the quantity supplied is zero.
Trace the curve
Move price from shutdown to positive supply
The highlighted green section is the firm’s short-run supply curve. The lower MC section is not supplied because P would be below AVC.
Firm responseP is above minimum AVC, so the firm supplies the output where P = MC.
The firm’s supply curve is the rising MC section above minimum AVC. At prices below minimum AVC, short-run quantity supplied is zero.
All costs are avoidable. A firm that cannot cover minimum LRAC exits. Industry long-run supply also depends on entry, exit and whether input costs change as the industry expands.
05
Syllabus 7.6.3 and 7.6.4 · Worksheet 63–84
Efficiency innovation barriers and contestability
Change the time period
Do not award every efficiency in every period
For a producing price-taking firm, P = MR = MC. With no externalities, this supports allocative efficiency even when the firm is not at minimum AC.
Not guaranteed in the short run: the market price can place q* away from minimum AC.
Supernormal profit, normal profit or loss is possible in the short run.
Pressure to control avoidable cost
A price-taking firm cannot pass organisational waste on through a higher price. Waste reduces profit and can threaten survival.
X-efficiency is not minimum AC
X-efficiency means avoiding waste at a given output and technology. Productive efficiency means choosing the output or scale at minimum attainable AC.
Market structure is not the whole welfare story
If pollution creates external cost, P = private MC does not imply MSB = MSC. The competitive output may still be socially excessive.
Legal: licences or patents. Market: loyalty or established distribution. Cost: scale or sunk expenditure. Physical: control of a scarce site or resource. Perfect competition assumes none is effective.
A contestable market is disciplined by potential entry, even if it has few current firms. Perfect competition additionally requires many small firms and homogeneous output.
Quick checkIs perfect competition always productively efficient?
06
Syllabus 7.6.4 and 7.6.5 · Worksheet 85–119
Compare calculate use evidence and judge
Change the comparison
Perfect competition is a benchmark, not a universal winner
| Dimension | Perfect competition | Monopoly | Evaluation condition |
|---|---|---|---|
| Price and output | P = MC in the standard model | Normally P > MC for a single-price profit maximiser | The standard comparison assumes comparable demand and cost conditions. |
| Costs | Small firms may miss economies of scale | One large firm may achieve lower AC | Lower monopoly cost does not guarantee that savings reach consumers. |
| Innovation | Rivalry rewards cost-saving innovation | Retained profit may fund risky research | Neither competition nor profit guarantees successful innovation. |
| Dimension | Perfect competition | Monopolistic competition | Evaluation condition |
|---|---|---|---|
| Product | Homogeneous | Differentiated | Variety can raise welfare when differences match real preferences. |
| Firm demand | Horizontal at market price | Downward sloping | Differentiation creates some price-setting power. |
| Long-run outcome | Normal profit at minimum AC | Normal profit with excess capacity in the standard model | Consumers may accept a higher cost for location, service or features. |
Price and non-price competition: a perfectly competitive firm cannot profitably charge above the market price, and a homogeneous product leaves little scope for branding or product features. Competition is expressed mainly through cost control and market entry or exit.
Worksheet 91–95
Calculate CR1 and CR4
An n-firm concentration ratio is the combined market share of the largest n firms. A low ratio supports, but does not prove, perfect competition.
CR1 —
CR4 —
Enter the market shares, then calculate. Always define the product and geographical market before interpreting the result.
Farm numbers fell from 2,042,220 in 2017 to 1,900,487 in 2022, while average farm size rose from 441 to 463 acres. These national totals do not reveal sales shares within one defined crop market.
The monthly average fell from $7.67 per bushel in June 2023 to $6.19 in May 2024. A changing market price does not contradict price taking: an individual small firm can still lack control over that price.
20-mark essay practice
“Perfect competition always produces the best outcome for consumers.”
Build the case
Use P = MC, minimum AC and entry. Connect every mechanism to price, access or consumer welfare.
Challenge it
Use economies of scale, product variety, research and externalities. State when each counterargument applies.
Keep assumptions visible
Distinguish firm from industry, short run from long run, and private from social cost.
Judge “always”
Decide which conditions dominate in the industry rather than ending with a list of advantages and disadvantages.
Printable practice
Perfect competition worksheet
Complete 119 activities across 26 sections. The PDF includes short checks, diagrams, calculations, concentration ratios, real USDA evidence and evaluative essays.
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Worked-answer support
6 focused video slots
The videos will explain reasoning and diagrams by question group instead of reading out isolated answers.
Model price taking and revenue
Questions 1–18
Costs output profit and loss
Questions 19–38
Shutdown supply entry and exit
Questions 39–62
Efficiency innovation and barriers
Questions 63–84
Comparisons concentration and data
Questions 85–108
Arguments essays and self assessment
Questions 109–119
Suggested-answer guide
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