Microeconomics Foundations (ECON 101) · Module 5 of 9
Producer Theory & Costs
How firms' costs behave as output changes, and what separates the short run from the long run.
Production in the Short Run
- In the short run, at least one input, often capital, is fixed while others, like labor, are variable; the marginal product of labor is the additional output from hiring one more worker.
- Diminishing marginal returns set in as more of a variable input is added to a fixed input, eventually causing marginal product to decline.
Why does the marginal product of labor eventually decline as more workers are added to a fixed amount of capital, like a fixed kitchen size?
Solution. As more workers share the same fixed resources (equipment, space), each additional worker has progressively less capital to work with, leading to diminishing marginal returns — each new worker adds less additional output than the one before.
Cost Curves
- Average total cost (ATC) is total cost divided by quantity; marginal cost (MC) is the additional cost of producing one more unit.
- The marginal cost curve intersects the average total cost curve at its minimum point, since if marginal cost is below average cost, average cost is falling, and if marginal cost is above average, average cost is rising.
Why does the marginal cost curve always cross the average total cost curve at its minimum point?
Solution. If the cost of producing one more unit (marginal cost) is less than the current average, adding that unit pulls the average down; if marginal cost is more than the average, it pulls the average up — so average cost is at its lowest exactly where marginal cost equals it.
Short Run vs. Long Run
- In the long run, all inputs are variable, allowing firms to adjust their scale of production entirely, including entering or exiting an industry.
- Economies of scale occur when long-run average cost falls as output increases; diseconomies of scale occur when it rises as output increases beyond some point.
What distinguishes the long run from the short run in producer theory?
Solution. In the long run, all inputs — including capital, factory size, and other fixed resources — become variable, giving firms full flexibility to adjust their scale of production, unlike the short run where at least one input is fixed.
Producer Surplus
- Producer surplus is the difference between the price a producer receives and the minimum price (marginal cost) they would have been willing to accept, summed across all units sold — represented as the area above the supply curve and below the market price.
How is producer surplus represented graphically on a supply-and-demand diagram?
Solution. As the area above the supply curve and below the equilibrium price, up to the equilibrium quantity — representing the total extra value producers received beyond the minimum they would have accepted.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the marginal product of labor?
2. Why does the MC curve intersect the ATC curve at its minimum?