How do a firm's costs, revenues and profits behave in the short run and the long run?
Short-run and long-run costs, the law of diminishing returns, economies and diseconomies of scale, revenue concepts, and profit maximisation.
A focused CCEA A-Level Economics answer on costs, revenue and profit, covering fixed and variable costs, marginal and average cost, the law of diminishing returns, economies and diseconomies of scale, total, average and marginal revenue, normal and supernormal profit, and the MC equals MR profit-maximising rule, with a worked calculation.
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What this dot point is asking
CCEA wants you to define and distinguish short-run and long-run costs, explain the law of diminishing returns and the U-shaped short-run average cost curve, explain economies and diseconomies of scale and the long-run average cost curve, define total, average and marginal revenue, distinguish normal from supernormal profit, and apply the profit-maximising rule that marginal cost equals marginal revenue.
Short-run costs and diminishing returns
The law of diminishing returns (diminishing marginal returns) states that, as more variable factor is added to a fixed factor, the marginal output of each extra unit eventually falls. Because each extra worker adds less output, the cost of each extra unit rises, so MC rises. This is why the short-run MC curve eventually slopes upward and the AC curve is U-shaped: falling at first as fixed costs are spread, then rising as diminishing returns take hold.
Long-run costs and economies of scale
Economies of scale give large firms a cost advantage and act as a barrier to entry, helping explain why some industries are dominated by a few large firms.
Revenue and profit
A firm maximises profit where marginal cost equals marginal revenue (MC = MR). If MR exceeds MC, the extra unit adds more to revenue than to cost, so it pays to expand; if MC exceeds MR, the firm should cut back. The MC = MR rule is the foundation of the market-structure analysis that follows.
Try this
Q1. Distinguish between a fixed cost and a variable cost, with an example of each. [3 marks]
- Cue. Fixed cost does not change with output (rent); variable cost rises with output (raw materials).
Q2. State two internal economies of scale. [2 marks]
- Cue. Technical, purchasing, financial, managerial or marketing economies.
Q3. Explain why a profit-maximising firm produces where marginal cost equals marginal revenue. [4 marks]
- Cue. If MR exceeds MC it pays to expand; if MC exceeds MR it pays to cut back; profit is greatest where the two are equal.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA A2 16 marksExplain the law of diminishing marginal returns and its effect on a firm's short-run costs.Show worked answer →
Worth 6 marks. Markers reward the definition tied to the short run, the fixed-factor condition, and the effect on marginal cost.
Short run: at least one factor of production is fixed, usually capital, while others such as labour are variable.
Law: as more units of the variable factor are added to the fixed factor, beyond a point the marginal (extra) output of each additional unit falls. This is diminishing marginal returns.
Effect on cost: because each extra worker adds less output, the cost of producing each extra unit rises, so marginal cost rises. This is why the short-run marginal cost curve eventually slopes upward and average cost is U-shaped.
Development: it applies only in the short run, because in the long run all factors are variable and the firm can change its scale.
CCEA A2 18 marksExamine the economies of scale a large firm might enjoy and the limits to growing larger.Show worked answer →
Worth 8 marks. A strong answer explains several internal economies, then the diseconomies that limit growth, with a judgement.
Economies of scale: as output grows, long-run average cost falls. Sources include technical economies (larger, more efficient machinery), purchasing economies (bulk-buying discounts), financial economies (cheaper borrowing), managerial economies (specialist staff) and marketing economies (spreading advertising over more units).
Diseconomies of scale: beyond the minimum efficient scale, average cost can rise. Causes include communication and coordination problems, slower decision-making, and weaker worker motivation in a large, impersonal firm.
Judgement: economies of scale give large firms a strong cost advantage and a barrier to entry, but diseconomies set a limit, so the most efficient size varies by industry and the long-run average cost curve is typically U-shaped or L-shaped.
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Sources & how we know this
- CCEA GCE Economics specification — CCEA (2016)