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The Theory of the Firm under Perfect Competition – Class 12 Economics Chapter 4 Notes & NCERT PDF

Class 12EconomicsChapter 4NCERT book: Introductory Microeconomics - Economics Class 12

Notes and a simple summary of Chapter 4 of the NCERT Class 12 Economics book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 4: The Theory of the Firm under Perfect Competition

Introduction

Chapter 4 asks how a firm decides how much to produce. The key assumption is that the firm is a profit maximiser, and that it sells whatever it produces. The chapter defines the market environment of perfect competition, derives the firm's revenue concepts, finds the conditions for profit maximisation, derives the firm's supply curve in the short run and the long run, examines what shifts it, adds firms' supplies to get market supply, and measures the price elasticity of supply.

Key Concepts

4.1 Perfect Competition: Defining Features

  • A large number of buyers and sellers; each is too small to influence the market.
  • Each firm sells a homogeneous product; one firm's output cannot be told apart from another's.
  • Free entry and exit of firms.
  • Perfect information about price, quality and the market.

    • Together these produce price-taking behaviour: a firm that charges more than the market price sells nothing, and it can sell as much as it wants at the market price. Similarly, a buyer cannot buy below the market price.

    4.2 Revenue

  • Total revenue: TR = p × q. With candles at ₹10 per box, TR = 0, 10, 20, 30, 40, 50 for 0 to 5 boxes. The TR curve is an upward-sloping straight line through the origin with slope p.
  • Average revenue: AR = TR/q = p. The horizontal line at p is the price line; it is also the firm's AR curve and the demand curve facing the firm, which is perfectly elastic.
  • Marginal revenue: MR = ΔTR/Δq. From 2 to 3 boxes, MR = (30 − 20)/(3 − 2) = 10 = p. In general, MR = [p(q2 − q1)]/(q2 − q1) = p. So for a competitive firm, MR = AR = p.

    • 4.3 Profit Maximisation

    Profit π = TR − TC. If profit is maximised at a positive output q0, three conditions must hold:

  • p = MC (since MR = p). As long as MR > MC, extra output adds to profit; when MR < MC, it reduces profit.
  • MC must be non-decreasing at q0. If MC were falling where p = MC (point q1 in Fig. 4.3), producing slightly less would raise profit.
  • Short run: p ≥ AVC. If p < AVC, TR does not even cover TVC, so the loss exceeds TFC; producing nothing (loss = TFC) is better. Long run: p ≥ AC, since a firm that shuts down in the long run earns zero profit.

    • At q0, total revenue is the rectangle OpAq0, total cost is OEBq0, and profit is the rectangle EpAB.

    4.4 Supply Curve of a Firm

  • Short run supply curve: the rising part of the SMC curve from and above minimum AVC, together with zero output for prices below minimum AVC.
  • Long run supply curve: the rising part of the LRMC curve from and above minimum LRAC, with zero output below minimum LRAC.
  • Shut-down point: in the short run, minimum AVC (where SMC cuts AVC); in the long run, minimum LRAC.
  • Normal profit: the minimum profit needed to keep a firm in business; it is part of total cost and can be seen as the opportunity cost of entrepreneurship. Profit above it is super-normal profit.
  • Break-even point: the point where the supply curve cuts the LRAC (short run: SAC) curve at its minimum; the firm earns only normal profit.
  • Opportunity cost (box): if ₹1,000 is invested in the family business instead of a bank paying 10% (the best alternative), the opportunity cost is the interest given up from that bank.

    • 4.5 Determinants of a Firm's Supply Curve

  • Technological progress lowers MC, so the supply curve shifts right.
  • Rise in input prices (such as wages) raises MC, so the supply curve shifts left.
  • Unit tax of ₹t raises LRAC and LRMC by t at every output; the supply curve shifts left (the minimum price for supply rises from p0 to p0 + t).

    • 4.6 Market Supply Curve

    Market supply is the horizontal sum of the firms' supply curves. With S1 = p − 10 for p ≥ 10 and S2 = p − 15 for p ≥ 15, market supply is 0 for p < 10, p − 10 for 10 ≤ p < 15, and 2p − 25 for p ≥ 15. An increase in the number of firms shifts market supply to the right.

    4.7 Price Elasticity of Supply

    eS = (% change in quantity supplied) / (% change in price) = (ΔQ/Q) ÷ (ΔP/P). A vertical supply curve has eS = 0.

    Geometric method (straight-line supply through point S, extended to meet the quantity axis at M): eS = Mq0/Oq0. If the line meets the quantity axis in the negative range (cuts the price axis), eS > 1; if it passes through the origin, eS = 1; if it meets the quantity axis in the positive range, eS < 1.

    Worked Numericals

  • Cricket balls. Price rises from ₹10 to ₹30, supply from 200 to 1,000. %ΔQ = 800/200 × 100 = 400%; %ΔP = 20/10 × 100 = 200%; eS = 2.
  • Exercise 21 (profit maximisation). Price ₹10; TC = 5, 15, 22, 27, 31, 38, 49, 63, 81, 101, 123 for q = 0 to 10. Profit = 10q − TC = −5, −5, −2, 3, 9, 12, 11, 7, −1, −11, −23. The maximum profit of ₹12 is at q = 5, where MC has risen to 7 and the next unit's MC (11) exceeds the price.
  • Exercise 20. TR = 0, 5, 10, ... so price = ₹5. Profit = −5, −2, 0, 3, 5, 2, −3, −5; the maximum is ₹5 at q = 4.
  • Exercise 25. Revenue ₹50 at ₹10 means q = 5; revenue ₹150 at ₹15 means q = 10. %ΔQ = 100%, %ΔP = 50%, so eS = 2.
  • Exercise 26. Price rises from ₹5 to ₹20 (300%). With eS = 0.5, %ΔQ = 150%. Since ΔQ = 15, 15/Q = 1.5, so Q = 10. Output rises from 10 to 25.
  • Exercise 27. Price rises from ₹10 to ₹30 (200%); eS = 1.25 gives %ΔQ = 250%. New quantity = 4 × 3.5 = 14 units.

    • Formula Sheet

  • TR = p × q; AR = TR/q = p; MR = ΔTR/Δq = p
  • π = TR − TC
  • Conditions: p = MC; MC non-decreasing; p ≥ AVC (short run), p ≥ AC (long run)
  • Market supply = sum of firms' supplies at each price
  • eS = (ΔQ/Q) ÷ (ΔP/P) = (ΔQ/ΔP) × (P/Q)
  • Geometric: eS = Mq0/Oq0

    • Board Exam Focus

  • Features of perfect competition and why the firm is a price taker (Exercise 1).
  • Why MR = AR = p for a competitive firm; the price line (Exercises 3 to 6).
  • Three conditions for profit maximisation with diagrams (Exercises 7 to 11).
  • Short and long run supply curves; effects of technology, input prices, a unit tax and the number of firms (Exercises 12 to 17).
  • Numericals on profit, market supply schedules and eS (Exercises 19 to 27).

    • Key Terms

  • Perfect competition (पूर्ण प्रतिस्पर्धा)
  • Price taker (कीमत स्वीकारक)
  • Total revenue (कुल संप्राप्ति)
  • Average revenue (औसत संप्राप्ति)
  • Marginal revenue (सीमांत संप्राप्ति)
  • Profit maximisation (लाभ अधिकतमीकरण)
  • Supply curve (पूर्ति वक्र)
  • Shut-down point (उत्पादन बंद बिंदु)
  • Normal profit (सामान्य लाभ)
  • Break-even point (लाभ-अलाभ बिंदु)
  • Unit tax (इकाई कर)
  • Market supply (बाज़ार पूर्ति)
  • Price elasticity of supply (पूर्ति की कीमत लोच)

    • Common Mistakes

  • Drawing the competitive firm's demand curve downward sloping. It is a horizontal price line (perfectly elastic).
  • Choosing the output where p = MC on the falling part of MC. Profit is maximised only where MC is non-decreasing.
  • Saying a firm shuts down whenever it makes a loss. In the short run it continues as long as p ≥ minimum AVC.
  • Confusing the shut-down point (minimum AVC) with the break-even point (minimum AC).
  • Summing supply curves vertically. Market supply is a horizontal summation of quantities.

    • 💡 Key Learning Points

    • ✓Perfect competition (many firms, homogeneous product, free entry and exit, perfect information) makes every firm a price taker facing a perfectly elastic demand curve.
    • ✓For a price-taking firm, TR = p × q is a straight line from the origin and MR = AR = p.
    • ✓Profit is maximised where p = MC with MC non-decreasing, and p ≥ AVC in the short run or p ≥ AC in the long run.
    • ✓The short run supply curve is the rising SMC above minimum AVC; the long run supply curve is the rising LRMC above minimum LRAC; technology shifts it right, input prices and unit taxes shift it left.
    • ✓Market supply is the horizontal sum of firms' supplies, and price elasticity of supply eS = %ΔQ/%ΔP.

    👨‍🏫 Teaching Tips

    • →Use a vegetable market with many identical tomato sellers to show why no single seller can charge more.
    • →Build the candle TR table (₹10 per box) and let students find AR and MR to discover that both equal p.
    • →Run a profit table (Exercise 21) and ask students to find the best output before stating the p = MC rule.
    • →Draw Fig. 4.3 and explain why the point on the falling part of MC is a trap for students.
    • →Contrast shut-down point and break-even point with a table showing losses at different prices.
    • →Show the unit-tax effect by shifting LRMC and LRAC up by t on transparent overlays.
    • →Practise horizontal summation with the S1 = p − 10 and S2 = p − 15 example and plot the kink at p = 15.

    📋 Assessment Questions

    1. Can the student list the features of perfect competition and explain price-taking behaviour?
    2. Can the student show that MR = AR = p for a competitive firm using a table and algebra?
    3. Can the student state and justify the three conditions for profit maximisation?
    4. Can the student derive short run and long run supply curves and explain shifts due to technology, input prices and taxes?
    5. Can the student compute market supply schedules and price elasticity of supply?

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    Keep learning

    All chapters in Introductory Microeconomics - Economics Class 12

    1. 1. Introduction
    2. 2. Theory of Consumer Behaviour
    3. 3. Production and Costs
    4. 4. The Theory of the Firm under Perfect Competition
    5. 5. Market Equilibrium