Production, Costs, and Perfect Competition — AP Microeconomics
AP Microeconomics · AP Microeconomics Unit 3 · 18 min read
1. Short-Run vs Long-Run Production★☆☆☆☆⏱ 3 min
The distinction between the short run and long run depends entirely on the flexibility of a firm's inputs, not a fixed length of calendar time.
2. Diminishing Marginal Returns★★☆☆☆⏱ 4 min
Diminishing marginal returns (DMR) is a short-run only phenomenon that describes how adding more variable input to fixed inputs impacts total output. We first define marginal product (MP), the additional output from adding one more unit of variable input (usually labor).
MP = \frac{\Delta \text{Total Product (TP)}}{\Delta \text{Quantity of Labor (L)}}
3. Firm Cost Curves★★★☆☆⏱ 5 min
All firm cost curves are derived from production relationships, and their shapes are driven by the law of diminishing marginal returns. We first distinguish between core cost types:
**Fixed Costs (FC)**: Costs that do not change with output, even if output is 0 (e.g., rent, insurance, long-term leases).
**Variable Costs (VC)**: Costs that change directly with output (e.g., wages, raw materials, production electricity).
**Total Cost (TC)**: Sum of fixed and variable costs: $TC = FC + VC$.
Average costs measure cost per unit of output, while marginal cost is the additional cost of producing one more unit of output. Since fixed costs do not change with output, marginal cost only depends on changes in variable cost:
\begin{aligned} AFC &= \frac{FC}{Q} \\ AVC &= \frac{VC}{Q} \\ ATC &= \frac{TC}{Q} = AFC + AVC \\ MC &= \frac{\Delta TC}{\Delta Q} = \frac{\Delta VC}{\Delta Q} \end{aligned}
Key properties of cost curves tested on every AP exam:
AFC is always downward-sloping, as fixed costs are spread over more units of output.
AVC and ATC are U-shaped: they fall due to increasing marginal returns, then rise as diminishing marginal returns set in.
MC crosses AVC and ATC *exactly at their minimum points*. If MC < ATC, ATC falls; if MC > ATC, ATC rises.
Short-run shut-down rule: If $P < \text{minimum } AVC$, the firm should shut down immediately, as it cannot cover variable costs, and loses less money producing 0 (only losing FC).
4. Profit Maximization Under Perfect Competition★★★☆☆⏱ 3 min
Perfect competition is a theoretical baseline market structure that forms the foundation for analyzing all other market structures. It has 5 core characteristics:
Many small firms, each with no market power
Homogeneous (identical) products, no product differentiation
No barriers to entry or exit
Perfect information for all buyers and sellers
Firms are price takers: they cannot influence the market price, so their individual demand is perfectly elastic at the market price.
For perfectly competitive firms, marginal revenue (MR, additional revenue from one more unit sold) equals the market price, since every unit sells for the same fixed price. Total revenue is $TR = P \times Q$.
The universal profit maximization rule for *all* firms (not just perfect competition) is to produce the quantity where $MR = MC$. For perfect competition, this simplifies to $P = MC$.
5. Long-Run Equilibrium in Perfect Competition★★★★☆⏱ 3 min
A key distinction tested on the AP exam is between accounting profit and economic profit:
**Economic profit**: $TR - \text{explicit costs} - \text{implicit costs}$ (opportunity costs of owner's time, capital, next-best alternative uses of resources)
In the long run, free entry and exit drive all perfectly competitive firms to zero economic profit:
If firms earn positive economic profit in the short run, new firms enter, market supply increases, market price falls until $P = \text{minimum } ATC$ (zero profit).
If firms earn negative economic profit (losses) in the short run, existing firms exit, market supply decreases, market price rises until $P = \text{minimum } ATC$ (zero profit).
Long-run equilibrium in perfect competition is both allocatively efficient ($P = MC$, no deadweight loss) and productively efficient ($P = \text{minimum } ATC$, produced at the lowest possible cost), a common FRQ testing point.
Common Pitfalls
Why: Both involve falling output per unit of input, so they are easy to mix up.
Why: Students mix up the shut-down rule (which uses AVC) with profit calculations.
Why: Students confuse economic profit with accounting profit.
Why: Students forget the relationship between marginal and average values.
Why: Students apply real-world firm logic to the theoretical perfect competition model.