Questions: Individual Firm Supply Curve in Competition

Question 1

A competitive firm's input costs rise, shifting both its marginal cost and average variable cost curves upward. What happens to the firm's supply curve?

• A. It shifts rightward — lower profit motivates the firm to produce more to remain viable
• B. It shifts leftward — at every price, the firm is now willing to supply less
• C. It does not shift — supply curves only shift when the market price changes
• D. Only the shutdown point changes; the firm produces the same quantity at prices above AVC

Answer: (B)

The firm's supply curve is its MC curve above AVC minimum. When input costs rise, MC shifts upward — for any given price P, the P = MC condition is satisfied at a lower quantity, so the firm supplies less at every price. The supply curve shifts leftward (a decrease in supply). This is a direct consequence of supply being derived from the cost structure: any change in marginal cost shifts supply in the same direction.

Question 2

A competitive firm's rent (a fixed cost) increases significantly. How does this affect its short-run supply curve?

• A. The supply curve shifts leftward — higher costs always reduce supply
• B. The supply curve shifts rightward — the firm must produce more to cover the higher cost
• C. The supply curve does not shift — fixed costs do not affect marginal cost
• D. The firm shuts down immediately since fixed costs now make production unprofitable

Answer: (C)

This is the most common misconception. Fixed costs (rent, licensing fees, equipment already purchased) do not affect marginal cost — they do not change with output. Since the supply curve is the MC curve above AVC, and neither MC nor AVC is changed by a fixed cost increase, the supply curve doesn't move. Fixed costs affect average total cost and the long-run exit decision, but not the short-run quantity decision at any given price.

Question 3

A competitive firm's supply curve is the portion of its marginal cost curve that lies above the average variable cost curve minimum, because below that price the firm shuts down rather than produce.

• T. True
• F. False

Answer: True

This is the precise definition of the individual firm's supply curve. The shutdown rule says a firm produces zero when P < AVC_min — it loses less by covering only fixed costs than by also losing money on each unit produced. For any price above AVC_min, the profit-maximizing output is found at P = MC (an upward-sloping portion of MC), so the supply curve traces the MC curve from the shutdown point upward.

Question 4

A technological improvement that reduces marginal cost shifts the firm's supply curve leftward, because lower costs reduce the firm's incentive to produce.

• T. True
• F. False

Answer: False

Lower marginal cost shifts supply rightward, not leftward. When MC falls, the P = MC condition is satisfied at a higher quantity for every price — the firm is willing and able to supply more at any given price. Intuitively, cheaper production means more units can be profitably produced. The supply curve is the MC curve, so moving the MC curve down/rightward moves the supply curve rightward (an increase in supply).

Question 5

Explain why the competitive firm's supply curve is derived from its marginal cost curve, and why a change in fixed costs does not shift it.

Model answer: A price-taking firm maximizes profit by producing where P = MC: the price received for one more unit exactly equals the cost of producing it. Sweeping P from zero upward maps out successively higher quantities along the upward-sloping MC curve (due to diminishing returns), giving the supply curve as the set of (P, q*) pairs that satisfy the profit-maximizing condition. Fixed costs are excluded from this logic because marginal cost is the derivative of variable cost with respect to quantity — fixed costs, by definition, don't change with output and therefore contribute nothing to MC. A fixed cost change shifts average total cost but leaves MC and AVC unchanged, so neither the P = MC quantity nor the shutdown threshold is affected. The supply curve stays put.

This derivation matters because it shows that supply is not an assumption — it is a consequence of rational profit-maximizing behavior under price-taking competition. It also means supply shifts only when the firm's cost of producing one more unit changes, which occurs with input price changes, technology changes, or anything else that moves the MC curve.