Key takeaways
- In perfect competition firms sell identical goods and are price takers, so AR = MR = p.
- Profit is maximised where p = MC, MC is rising, and p ≥ AVC (short run) or p ≥ AC (long run).
- The firm's short-run supply curve is the rising part of SMC above minimum AVC; below that price it produces nothing.
- Technological progress shifts supply right; higher input prices or a unit tax shift it left.
- Market supply is the horizontal sum of firms' supplies, and eS = %Δq ÷ %Δp.
This page is part of the Introductory Microeconomics chapter list for Class 12 Economics. It uses the cost curves from Chapter 3.
4.1Perfect Competition: Defining Features
A perfectly competitive market has two defining features: all firms sell an identical product, and every buyer and seller is a price taker. A price-taking firm cannot sell anything above the market price and can sell all it wants at or below it. This is plausible when there are many firms and buyers know the price.
- All firms produce a homogeneous (identical) good.
- Each buyer and seller is a price taker: its own actions cannot change the market price.
For the firm, price-taking means that if it charges more than the market price it loses all its buyers, and if it charges the market price or less it can sell as much as it wants. There is therefore no reason to charge less. Price-taking is reasonable when the market has many firms and buyers have perfect information. A firm that raised its price would lose every customer to its many rivals, who can easily serve them. A wheat farmer in a mandi is a good everyday example. Remember too that the chapter studies the firm's side only.
4.2Revenue
For a price-taking firm, total revenue is market price times output, so the revenue curve is a straight line through the origin with slope equal to the price. Average revenue and marginal revenue both equal the price. The demand curve facing such a firm is a horizontal line at the market price: the price line.
If a box of candles sells for ₹10, TR is ₹10 for one box, ₹20 for two and so on. TR is zero at zero output, rises with output, and is a straight line whose slope is the price. Because the firm can sell any quantity at p, the demand curve it faces is a horizontal price line at height p, which is perfectly elastic. Each extra unit adds exactly p to revenue, so marginal revenue equals price.
4.3Profit Maximisation
A firm's profit is total revenue minus total cost. If a profit-maximising firm produces a positive output, three conditions must hold: price equals marginal cost, marginal cost must not be falling, and price must be at least average variable cost in the short run, or average cost in the long run.
Condition 1: price equals marginal cost
If p > MC, one more unit adds more to revenue (p) than to cost (MC), so output should rise. If p < MC, the last unit adds more to cost than to revenue, so output should fall. Only when p = MC is no gain left from changing output.
Condition 2: marginal cost must be rising
A point where p = MC on a falling part of the MC curve cannot be a maximum. Slightly lower output would have price below marginal cost, and cutting output there would raise profit.
Condition 3: price must cover average variable cost (short run) or average cost (long run)
In the short run the fixed cost is paid whatever the firm does. If the price is below AVC, revenue does not even cover variable cost, so the firm does better by producing nothing and losing only its fixed cost. In the long run there is no fixed cost, so the firm needs price at least equal to average cost; otherwise it shuts down with zero profit.
The graphical picture
Take TC = q³ − 6q² + 14q + 20, so MC = 3q² − 12q + 14 and AVC = q² − 6q + 14. At p = ₹14, set p = MC: 3q² − 12q = 0, so q = 4 on the rising part. At q = 4, AVC = 6 and AC = 11, both below the price. Profit is (14 − 11) × 4 = ₹12, shown as the shaded rectangle.
4.4Supply Curve of a Firm
A firm's supply curve shows how much it produces at each market price. In the short run it is the rising part of marginal cost from the minimum of average variable cost upwards, with zero output below that price. In the long run it is the rising part of long-run marginal cost above minimum long-run average cost.
Short-run supply curve
For any price at or above the minimum of AVC, set p equal to SMC on the rising part to find output. For any price below minimum AVC, no positive output meets condition 3, so output is zero. The short-run supply curve is therefore the rising part of SMC from the minimum AVC upwards, together with zero output at lower prices.
Long-run supply curve
The same argument applies with long-run costs: the long-run supply curve is the rising part of LRMC from the minimum of LRAC upwards, with zero output at lower prices.
The shut-down point
Moving down the supply curve, the last price at which the firm still produces is minimum AVC, where SMC cuts AVC. This is the short-run shut-down point. In the long run the shut-down point is the minimum of LRAC.
Normal profit and the break-even point
Some inputs belong to the firm itself and need no payment, but they have an opportunity cost: the return they would have earned elsewhere. Normal profit is just enough to cover both explicit costs and these opportunity costs. If both are counted in total cost, normal profit is the case where TR = TC. Profit above this is super-normal profit. The firm's break-even point, where it earns normal profit, is the minimum of average cost, where the supply curve cuts the AC curve. In the example it is at about q = 3.72 and p = ₹10.9.
4.5Determinants of a Firm's Supply Curve
Anything that changes a firm's marginal cost curve shifts its supply curve, since supply is a part of marginal cost. Technological progress lowers costs and shifts supply right. A rise in input prices raises costs and shifts supply left. A unit tax adds to cost at every output level and also shifts it left.
| Change | Effect on marginal cost | Effect on supply curve |
|---|---|---|
| Technological progress | Falls at each output | Shifts right: more at every price |
| Rise in input prices, such as wages | Rises at each output | Shifts left: less at every price |
| Unit tax of ₹t per unit | Rises by ₹t at each output (LRMC and LRAC both rise by t) | Shifts left: less at every price |
| Fall in input prices or removal of a unit tax | Falls | Shifts right |
For example, if a tax of ₹2 is charged on each unit sold, a firm selling 10 units pays ₹20 in total. Its average and marginal costs rise by ₹2 at every level, so the price needed to make it supply a given quantity rises by ₹2.
4.6Market Supply Curve
The market supply at any price is the total of what all firms supply at that price. The market supply curve is therefore the horizontal sum of the firms' supply curves. It is drawn for a fixed number of firms; if more firms enter, the curve shifts right, and if some leave, it shifts left.
Suppose firm 1 supplies S₁(p) = p − 10 for p ≥ 10 and zero below, and firm 2 supplies S₂(p) = p − 15 for p ≥ 15 and zero below. Then market supply is zero below ₹10, equals p − 10 between ₹10 and ₹15, and equals 2p − 25 from ₹15 upwards.
| Price (₹) | Firm 1 | Firm 2 | Market supply |
|---|---|---|---|
| 8 | 0 | 0 | 0 |
| 12 | 2 | 0 | 2 |
| 15 | 5 | 0 | 5 |
| 20 | 10 | 5 | 15 |
| 25 | 15 | 10 | 25 |
4.7Price Elasticity of Supply
Price elasticity of supply is the percentage change in quantity supplied divided by the percentage change in price. For an upward-sloping supply curve it is positive. A vertical curve has zero elasticity. A straight-line curve through the origin has an elasticity of exactly one at every point.
Like demand elasticity, it is a pure number independent of units. If a market supplies 200 balls at ₹10 and 1,000 at ₹30, quantity rises 400% and price 200%, so eS = 2.
The geometric method for a straight-line supply curve
| If the line meets the… | Elasticity at every point is | Reason |
|---|---|---|
| Price axis above the origin | Greater than 1 | Its extension cuts the quantity axis in the negative range |
| Origin | Exactly 1 | The extension passes through the origin |
| Quantity axis to the right of the origin | Less than 1 | Its extension cuts the quantity axis in the positive range |
Worked Examples
Example 1: Profit maximisation
- Given
- TC = q³ − 6q² + 14q + 20 and market price p = ₹14.
- Formula
- MC = 3q² − 12q + 14; set p = MC with MC rising; π = TR − TC
- Steps
- 3q² − 12q + 14 = 14 gives q = 0 or q = 4; MC is rising at q = 4. TR = 14 × 4 = 56. TC = 64 − 96 + 56 + 20 = 44. Profit = 56 − 44 = 12. AVC at 4 = 6, below the price.
- Answer
- Output = 4 units; profit = ₹12.
Example 2: Should the firm shut down?
- Given
- Same cost function; the price falls to ₹4. Minimum AVC is ₹5 at 3 units.
- Formula
- Shut down if p < min AVC; loss from shutting down = TFC
- Steps
- Price 4 is below 5, so condition 3 fails. If the firm produced where MC = 4 (q ≈ 2.82), its loss would be TC − TR ≈ 22.9. If it shuts down, its loss is only TFC = 20.
- Answer
- The firm should shut down: it loses ₹20 by stopping, but about ₹22.9 by producing.
Example 3: Market supply and elasticity
- Given
- S₁(p) = p − 10 for p ≥ 10; S₂(p) = p − 15 for p ≥ 15. The price rises from ₹20 to ₹30.
- Formula
- Sm = S₁ + S₂; eS = %Δq ÷ %Δp
- Steps
- At ₹20: 10 + 5 = 15. At ₹30: 20 + 15 = 35. %Δq = 20 ÷ 15 × 100 = 133.3. %Δp = 10 ÷ 20 × 100 = 50. eS = 133.3 ÷ 50 = 2.67.
- Answer
- Market supply rises from 15 to 35 units; eS ≈ 2.67 (elastic).
Common Mistakes
- Equating price with MC on the falling part of the MC curve. Condition 2 needs MC to be rising.
- Using average cost instead of average variable cost as the short-run shut-down test.
- Saying MR and AR differ for a perfectly competitive firm. Both equal the market price.
- Forgetting that the supply curve is only the part of MC above minimum AVC, with zero output below it.
- Treating normal profit as if it were a loss. It is the minimum return, covering opportunity costs, that keeps the firm in business.
10 Important Questions for Class 12 Board and School Examinations
Questions are written in original words. Marks are shown beside each question.
Q1For a firm in a perfectly competitive market, marginal revenue is equal to: (A) zero (B) total revenue (C) market price (D) average cost 1 mark · MCQ
(C) Market price.
Each extra unit is sold at the market price, so it adds exactly that to revenue.
Q2Assertion (A): A firm's short-run supply curve is the rising part of SMC above the minimum of AVC. Reason (R): At prices below minimum AVC, producing makes a loss greater than the fixed cost. Choose: (a) both true, R explains A; (b) both true, R does not explain A; (c) A true, R false; (d) A false, R true. 1 mark · Assertion-reason
(a) Both are true and R correctly explains A.
Because producing below minimum AVC loses more than shutting down, output there is zero.
Q3State three characteristics of a perfectly competitive market. 3 marks
A perfectly competitive market has a homogeneous product, price-taking buyers and sellers, and many firms with free entry and exit.
- All firms sell an identical product.
- Every buyer and seller is a price taker.
- There are many firms and buyers, with perfect information.
Example: the wheat market in a mandi.
Together these conditions make each firm a price taker, so it can sell any quantity at the market price but nothing at a higher price.
Q4Why is the total revenue curve of a price-taking firm a straight line through the origin? 3 marks
Because TR = p × q with p fixed, so TR is proportional to output.
- At q = 0, TR is zero, so the line passes through the origin.
- Each extra unit adds the same amount, p.
- The slope of the line is therefore the market price.
Example: at p = ₹10, TR is 10, 20, 30 for 1, 2, 3 units.
Q5What is the shut-down point? What is the break-even point? 3 marks
The shut-down point is minimum AVC; the break-even point is minimum average cost.
- Below the shut-down price the firm produces nothing in the short run.
- At the break-even point price equals average cost, so the firm earns normal profit.
- Break-even is above shut-down because AC exceeds AVC.
Example: in the chapter's example, shut-down is at ₹5 and break-even near ₹10.9.
Q6Explain the conditions that must hold at the profit-maximising output of a firm. 4 marks
At a positive profit-maximising output, price equals marginal cost, MC does not fall, and price covers average variable cost.
- p = MC: if p > MC, more output adds profit; if p < MC, less output does.
- MC rising: a falling MC means a slightly lower output does better.
- p ≥ AVC: otherwise the firm loses more than its fixed cost.
- Long run: p ≥ AC.
Example: with p = 14 and MC = 3q² − 12q + 14, output is 4 units.
Q7How do technological progress, a rise in input prices and a unit tax affect a firm's supply curve? 4 marks
Anything that changes marginal cost shifts the supply curve, because supply is part of the MC curve.
- Technological progress: lowers MC, so supply shifts right.
- Higher input prices: raise MC, so supply shifts left.
- Unit tax: adds the tax to MC and AC at every output, so supply shifts left.
- At a given price, the firm now supplies more or less accordingly.
Example: a ₹2 unit tax raises the supply price by ₹2.
A lower input price, such as cheaper electricity, has the opposite effect to a rise, shifting supply right.
Q8Derive the short-run supply curve of a competitive firm. 6 marks
The short-run supply curve is the rising part of SMC from the minimum of AVC upwards, with zero output below that price.
- Profit is maximised where p = SMC on its rising part, if p ≥ AVC.
- For a price at or above minimum AVC, equating p with SMC gives positive output.
- For a price below minimum AVC, no positive output satisfies p ≥ AVC.
- So output is zero at those prices.
- The minimum AVC point, where SMC cuts AVC, is the shut-down point.
- Combining the two cases gives the bold curve in Figure 4.2.
Thus the supply curve is upward sloping above the shut-down price.
For example, if SMC is 2q and minimum AVC is 8, supply is q = p ÷ 2 for p ≥ 8 and zero at lower prices.
Q9Explain how the market supply curve is obtained from the supply curves of firms. If S₁ = p − 10 (p ≥ 10) and S₂ = p − 15 (p ≥ 15), find market supply. 6 marks
Market supply is the horizontal sum of the individual firms' supply curves.
- At each price, add the quantities all firms supply.
- Below ₹10 neither firm supplies, so market supply is zero.
- Between ₹10 and ₹15 only firm 1 supplies, giving p − 10.
- From ₹15 upwards both supply: (p − 10) + (p − 15) = 2p − 25.
- More firms shift the curve right; fewer shift it left.
At p = ₹20, market supply = 10 + 5 = 15 units.
The same idea works with many firms: the market curve is flatter than any firm’s, since adding firms increases quantity at each price, so market supply is more responsive to price. Supply elasticity of the market is therefore usually higher than that of a single firm. When firms are identical, market supply is simply the number of firms times one firm’s supply.
Q10When the price of a good rises from ₹10 to ₹15, the quantity supplied rises from 40 to 70 units. Calculate the price elasticity of supply. 4 marks · Numerical
The price elasticity of supply is 1.5, so supply is elastic.
Given: p₀ = 10, p₁ = 15; q₀ = 40, q₁ = 70.
Formula: eS = %Δq ÷ %Δp.
Steps: %Δq = 30÷40 × 100 = 75. %Δp = 5÷10 × 100 = 50. eS = 75÷50 = 1.5.
Final answer: eS = 1.5.
10 Points for State PSC and UPSC Aspirants
- e-NAM, the National Agriculture Market, was launched on 14 April 2016 as an online trading platform linking mandis, to move farm markets closer to competition.Why it matters: a policy attempt to widen the market and improve price discovery.Source: e-NAM portal, Government of India
- The Centre announces MSP for 22 mandated crops on the recommendation of the CACP; sugarcane has a Fair and Remunerative Price (FRP).Why it matters: MSP is a price floor over a competitive-looking market.Source: CACP; Department of Agriculture
- Budget 2018–19 announced that MSP would be at least 1.5 times the cost of production (A2+FL).Why it matters: links cost concepts to price policy.Source: Union Budget 2018–19
- The CACP began in 1965 as the Agricultural Prices Commission and was renamed in 1985.Why it matters: static fact on the price-advice body.Source: CACP
- The Price Stabilisation Fund (2014–15) supports market interventions to moderate prices of perishables such as onion and potato.Why it matters: applies supply shocks to policy.Source: Department of Consumer Affairs
- Agricultural supply is often inelastic in the short run because of biological lags between sowing and harvest.Why it matters: explains price swings in farm goods.Source: NCERT Introductory Microeconomics
- The Goods and Services Tax (1 July 2017) works as an ad valorem tax with input tax credit; a unit tax is a specific tax per unit. Their supply effects differ.Why it matters: distinguishes tax forms in the chapter's analysis.Source: GST Council, Government of India
- Normal profit includes the opportunity cost of the owner's own capital and time, so accounting profit overstates economic profit.Why it matters: a classic distinction in economics essays.Source: NCERT Introductory Microeconomics
- The Insolvency and Bankruptcy Code, 2016 provides an exit route for firms that cannot cover costs.Why it matters: a legal counterpart to free exit.Source: India Code
- Few real markets are perfectly competitive; agriculture, small retail and commodity trading come closest.Why it matters: helps frame real-world examples in mains answers.Source: Economic Survey, Ministry of Finance
These are revision notes, not official predictions of exam content.
Quick Revision
- Perfect competition: identical goods, price takers.
- TR = pq; AR = MR = p; the price line is horizontal.
- Profit max: p = MC, MC rising, p ≥ AVC (SR) or AC (LR).
- Short-run supply: rising SMC above min AVC.
- Shut-down at min AVC; break-even at min AC (normal profit).
- Supply shifts right with technology; left with higher input prices or a unit tax.
- Market supply: horizontal sum of firms' supply.
- eS = %Δq÷%Δp; line through origin has eS = 1.
Related Glossary Terms and Chapters
Glossary:
Perfect competitionMarginal costOpportunity costEquilibrium priceShut-down point
Related chapters: Production and Costs, whose cost curves are used here; Market Equilibrium, which combines this supply with demand; Non-Competitive Markets, where the firm is no longer a price taker. From Class 11: Rural Development.
Sources
- NCERT textbooks: Introductory Microeconomics, Class 12, National Council of Educational Research and Training. Syllabus reference. Accessed 23 October 2026.
- National Agriculture Market (e-NAM), Government of India. Accessed 23 October 2026.
- Commission for Agricultural Costs and Prices, Government of India. MSP policy reports. Accessed 23 October 2026.
- Union Budget, Ministry of Finance, Government of India. Accessed 23 October 2026.
- Economic Survey, Ministry of Finance, Government of India. Accessed 23 October 2026.
About this page
Drafted with AI assistance from the NCERT chapter structure, in original words. Content is checked and approved by Dr. Anamika Shrivastava, PhD Economics, before publication. Read how this site works or send a correction.
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