Questions: Individual Supply Curves: Quantity Supplied vs. Price

Question 1

The price of flour doubles, raising the bakery's cost of producing each loaf. What happens to the bakery's supply curve?

• A. The bakery moves up along its existing supply curve, supplying fewer loaves at the higher cost
• B. The supply curve shifts leftward — higher input costs raise marginal cost at every output level, so the bakery is willing to supply less at every price
• C. The supply curve shifts rightward — the bakery will produce more to compensate for the higher costs
• D. Nothing changes — supply curves only shift when the price of the good itself changes

Answer: (B)

Input cost changes shift the supply curve because they change the marginal cost at every output level. Higher flour costs make each loaf more expensive to produce, so the bakery's MC curve rises — meaning the bakery is willing to supply less at every given price. This is a leftward shift of the supply curve (a decrease in supply). Note that option A describes movement along the curve, which only occurs when the price of the bakery's output changes, not its inputs.

Question 2

When the market price of a good rises from $5 to $8, a student says 'supply has increased because producers are now making more.' Which best evaluates this claim?

• A. Correct — producers making more at a higher price is the definition of supply increasing
• B. Wrong — supply refers to the entire price-quantity relationship (the curve); a price change causes movement along the existing curve, increasing quantity supplied, not supply itself
• C. Correct — supply and quantity supplied mean the same thing in practice
• D. Partially correct — supply increases only if a new producer entered the market, not if existing producers expand

Answer: (B)

This is the supply-side version of the most common misconception in supply-demand analysis. 'Supply' refers to the entire supply curve — the relationship between all prices and the quantities producers are willing to offer. 'Quantity supplied' is the specific amount offered at one particular price. A price change moves producers along their existing supply curve (quantity supplied changes), but the curve itself — supply — doesn't shift. Supply only shifts when a non-price factor changes: input costs, technology, number of producers, expectations.

Question 3

For a competitive firm, the supply curve is equivalent to the firm's marginal cost curve above the point where price covers variable costs.

• T. True
• F. False

Answer: True

This is the key microeconomic identity linking supply curves to cost theory. A competitive firm (a price-taker) will produce additional units whenever the market price at least covers the marginal cost of that unit. So the firm chooses quantity where P = MC. Mapping out 'what quantity does the firm choose at each possible price?' traces the MC curve directly. The supply curve IS the MC curve (above the shutdown point), which explains why supply shifts whenever input costs change — because input cost changes shift the MC curve.

Question 4

When the price of the good a firm sells rises, the firm's supply curve shifts rightward.

• T. True
• F. False

Answer: False

A price change for the firm's own output causes movement along the existing supply curve — quantity supplied increases, but the supply curve itself does not shift. This is the same distinction as on the demand side: price changes cause movement along the curve; non-price factors (input costs, technology, number of producers, regulatory changes) shift the curve. The supply curve shifts rightward when, for example, input prices fall or technology improves — not when the output price rises.

Question 5

Explain why the individual firm's supply curve slopes upward. What is the underlying economic mechanism?

Model answer: The supply curve slopes upward because of rising marginal cost. As a firm expands output, it must use increasingly scarce or costly inputs — overtime labor, less efficient equipment, higher-priced raw materials. Each additional unit produced costs more than the previous one (marginal cost rises). A profit-maximizing firm will only produce an additional unit if the market price at least covers that unit's marginal cost. So at a low price, only the cheapest units are worth producing; at a higher price, it's profitable to expand output until MC rises to meet the new price. More output is supplied at higher prices, producing the upward slope.

The upward slope is not arbitrary — it follows directly from the law of increasing marginal cost (a consequence of diminishing returns to variable inputs). This is why the supply curve and the marginal cost curve are equivalent for a competitive firm: both trace the same relationship between output and the minimum price required to make that output profitable. Understanding this connection links the market-level supply curve to the firm-level cost analysis studied in microeconomics.