The Theory of the Firm under Perfect Competition

4.1 Perfect Competition: Defining Features

• Large number of buyers and sellers: Each is small compared to the market size.
• Homogenous product: Products from different firms are identical.
• Free entry and exit: Firms can easily enter or leave the market.
• Perfect information: All buyers and sellers are fully informed about prices and products.

4.2 Revenue

• Total Revenue (TR): TR = Market Price (p) × Quantity Sold (q)
• Average Revenue (AR): For price-taking firms, AR = Market Price.
• Marginal Revenue (MR): For price-taking firms, MR = Market Price.

4.3 Profit Maximisation

• Profit: Profit = Total Revenue - Total Cost
• Conditions for profit maximisation in the short run:
  1. p = Short-run Marginal Cost (SMC)
  2. SMC is non-decreasing
  3. p ≥ Average Variable Cost (AVC)
• Conditions for profit maximisation in the long run:
  1. p = Long-run Marginal Cost (LRMC)
  2. LRMC is non-decreasing
  3. p ≥ Long-run Average Cost (LRAC)

4.4 Supply Curves

• Short-run Supply Curve: Rising part of SMC curve above minimum AVC.
• Long-run Supply Curve: Rising part of LRMC curve above minimum LRAC.

4.5 Determinants of a Firm's Supply Curve

• Technological Progress: Shifts supply curve to the right (lower costs).
• Input Prices: Increase in input prices shifts supply curve to the left (higher costs).
• Unit Tax: Imposes additional cost per unit, shifting supply curve to the left.

4.6 Price Elasticity of Supply

• Definition: Price elasticity of supply (es) = Percentage change in quantity supplied / Percentage change in price.
• Example: If price rises from Rs 10 to Rs 30, and quantity supplied increases from 200 to 1000, then:
  • Percentage change in quantity supplied = (1000 - 200) / 200 × 100 = 400%
  • Percentage change in price = (30 - 10) / 10 × 100 = 200%
  • Therefore, es = 400% / 200% = 2.

4.7 The Shut Down Point

• Short Run Shut Down Point: Minimum AVC where SMC curve intersects AVC curve.
• Long Run Shut Down Point: Minimum LRAC curve.

4.8 Normal Profit and Break-even Point

• Normal Profit: Minimum profit needed to keep a firm in business, considered an opportunity cost.
• Break-even Point: Point where a firm earns only normal profit, at minimum average cost.

Common Mistakes and Exam Tips

Common Pitfalls

• Misunderstanding Profit Maximization Conditions: Students often confuse the conditions for profit maximization. Remember, for a firm to maximize profit, the following must hold:
  • Price (p) must equal Marginal Cost (MC)
  • Marginal Cost must be non-decreasing at the profit-maximizing output level
  • In the short run, price must be greater than Average Variable Cost (AVC); in the long run, price must be greater than Average Cost (AC).
• Ignoring Market Structure Characteristics: When discussing perfect competition, students sometimes forget key characteristics:
  • Large number of buyers and sellers
  • Homogeneous products
  • Free entry and exit from the market
  • Perfect information
• Confusing Short Run and Long Run Supply Curves: Students may mix up the definitions of short run and long run supply curves. Remember:
  • Short run supply curve is based on the rising part of the Short Run Marginal Cost (SMC) curve above minimum AVC.
  • Long run supply curve is based on the rising part of the Long Run Marginal Cost (LRMC) curve above minimum LRAC.

Exam Tips

• Always Relate Revenue to Price: In a perfectly competitive market, remember that Total Revenue (TR) is calculated as TR = Price (p) × Quantity (q). This relationship is crucial for solving problems related to revenue.
• Understand Elasticity: Be prepared to calculate the price elasticity of supply using the formula:
  $e_s = \frac{\text{Percentage change in quantity supplied}}{\text{Percentage change in price}}$
• Use Graphs Effectively: When asked to illustrate concepts like supply curves or profit maximization, use clear graphs to show the relationships between price, quantity, and costs.
• Practice Calculation Problems: Familiarize yourself with calculating profit, total revenue, and marginal revenue from given data, as these are common exam questions.
• Review Key Definitions: Make sure you can define and explain key terms such as normal profit, super-normal profit, and break-even point, as these are often tested.