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2. KRUGMAN’S
Economics
for AP®
S E C O N D E D I T I O N

3. Section 10
Module 55

4. What You Will Learn in this Module
Describe the various types of costs a firm faces, including fixed cost, variable cost, and total cost
Explain how a firm’s costs generate marginal cost curves and average cost curves
Section 10 | Module 55

5. From the Production Function to Cost Curves
A fixed cost is a cost that does not depend on the quantity of output produced.
A fixed cost is the cost of the fixed input.
A variable cost is a cost that depends on the quantity of output produced.
A variable cost is the cost of the variable input.
Section 10 | Module 55

6. Total Cost Curve TC = FC + VC
The total cost of producing a given quantity of output is the sum of the fixed cost and the variable cost of producing that quantity of output.
The total cost curve becomes steeper as more output is produced due to diminishing returns.
TC = FC + VC
Section 10 | Module 55

7. Total Cost Curve for George and Martha’s Farm
Section 10 | Module 55

8. Two Key Concepts: Marginal Cost and Average Cost
As in the case of marginal product, marginal cost is equal to “rise” (the increase in total cost) divided by “run” (the increase in the quantity of output).
Section 10 | Module 55

9. Costs at Selena’s Gourmet Salsas
Section 10 | Module 55

10. The Total and Marginal Cost Curves for Selena’s Gourmet Salsas
Section 10 | Module 55

11. Why is the Marginal Cost Curve Upward Sloping?
Because there are diminishing returns to inputs in this example. As output increases, the marginal product of the variable input declines.
This implies that more and more of the variable input must be used to produce each additional unit of output as the amount of output already produced rises.
And since each unit of the variable input must be paid for, the cost per additional unit of output also rises.
Section 10 | Module 55

12. Average Cost
Average total cost, often referred to simply as average cost, is total cost divided by quantity of output produced.
A U-shaped average total cost curve falls at low levels of output, then rises at higher levels.
Average fixed cost is the fixed cost per unit of output.
ATC = TC/Q = (Total Cost) / (Quantity of Output)
AFC = FC/Q = (Fixed Cost) / (Quantity of Output)
Section 10 | Module 55

13. AVC = VC/Q = (Variable Cost) / (Quantity of Output)
Average Cost
Average variable cost is the variable cost per unit of output.
AVC = VC/Q = (Variable Cost) / (Quantity of Output)
Section 10 | Module 55

14. Average Total Cost Curve
Increasing output has two opposing effects on average total cost:
The spreading effect: the larger the output, the greater the quantity of output over which fixed cost is spread, leading to lower the average fixed cost.
The diminishing returns effect: the larger the output, the greater the amount of variable input required to produce additional units leading to higher average variable cost.
Section 10 | Module 55

15. Average Costs for Selena’s Gourmet Salsas
Section 10 | Module 55

16. Average Total Cost Curve for Selena’s Gourmet Salsas
Section 10 | Module 55

17. Two Key Concepts: Marginal Cost and Average Cost
Marginal cost is upward sloping due to diminishing returns.
Average variable cost also is upward sloping but is flatter than the marginal cost curve.
Average fixed cost is downward sloping because of the spreading effect.
The marginal cost curve intersects the average total cost curve from below, crossing it at its lowest point. This last feature is our next subject of study.
Section 10 | Module 55

18. Marginal Cost and Average Cost Curves for Selena’s Gourmet Salsas
Section 10 | Module 55

19. Minimum Average Total Cost
The minimum-cost output is the quantity of output at which average total cost is lowest—the bottom of the U-shaped average total cost curve.
At the minimum-cost output, average total cost is equal to marginal cost.
At output less than the minimum-cost output, marginal cost is less than average total cost and average total cost is falling.
And at output greater than the minimum-cost output, marginal cost is greater than average total cost and average total cost is rising.
Section 10 | Module 55

20. The Relationship Between the Average Total Cost and the Marginal Cost Curves
Section 10 | Module 55

21. Does the Marginal Cost Curve Always Slope Upward?
In practice, marginal cost curves often slope downward as a firm increases its production from zero up to some low level.
This initial downward slope occurs because a firm that employs only a few workers often cannot reap the benefits of specialization of labor.
This specialization can lead to increasing returns at first, and so to a downward-sloping marginal cost curve.
Once there are enough workers to permit specialization, however, diminishing returns set in.
Section 10 | Module 55

22. More Realistic Cost Curves
Section 10 | Module 55

23. Links Between Productivity and Cost
Section 10 | Module 55

24. Summary
Total cost is equal to the sum of fixed cost, which does not depend on output, and variable cost, which does depend on output.
Average total cost, total cost divided by quantity of output, is the cost of the average unit of output, and marginal cost is the cost of one more unit produced.
U-shaped average total cost curves are typical, because average total cost consists of two parts: average fixed cost, which falls when output increases (the spreading effect), and average variable cost, which rises with output (the diminishing returns effect).
Section 10 | Module 55

25. Summary
When average total cost is U-shaped, the bottom of the U is the level of output at which average total cost is minimized, the point of minimum-cost output. This is also the point at which the marginal cost curve crosses the average total cost curve from below.
Section 10 | Module 55