ECON10004 · Introductory Microeconomics
Production and Costs
Production and costs is where supply curves come from. Before a firm can decide how much to make, ECON10004 teaches you to measure cost the way an economist does — counting the opportunity cost of every resource, not just the cheques written — and to sort costs by whether they should influence a decision at all.
From a firm's production function and diminishing marginal product, the topic builds the familiar short-run cost family (TC, ATC, AVC, AFC, MC) and the long-run returns-to-scale story behind the U-shaped LRAC. The exam treats this procedurally: hand you a total-cost function, ask you to derive the averages and the marginal, and read the geometry. It sits right after supply and demand and feeds directly into the next topic, perfect competition, where these cost curves drive the firm's output, shut-down and supply decisions.
What this chapter covers
- 01Economic cost vs accounting cost (opportunity cost, normal and economic profit)
- 02Sunk vs fixed vs variable cost — which costs affect decisions
- 03The production function and diminishing marginal product
- 04The short-run cost family: TC, ATC, AVC, AFC, MC
- 05The geometry the exam tests (MC cuts ATC/AVC at their minimums)
- 06Worked derivation from TC = 32 + q²/16
- 07The long run: returns to scale and the U-shaped LRAC
- 08Perfect competition link: P = MC, shut-down and break-even (preview)
Deriving the cost family from TC = 32 + q²/16
- +1Average total cost. ATC = TC/q = (32 + q²/16)/q = 32/q + q/16. The first term is AFC, the second is AVC.
- +1Marginal cost. MC = dTC/dq = d(32 + q²/16)/dq = 2q/16 = q/8. The constant 32 differentiates to 0 — fixed cost never appears in MC.
- +1At q = 16: ATC = 32/16 + 16/16 = 2 + 1 = $3.00; MC = 16/8 = $2.00. Here MC < ATC, so ATC is still falling.
- +1At q = 32: ATC = 32/32 + 32/16 = 1 + 2 = $3.00; MC = 32/8 = $4.00. Now MC > ATC, so ATC is rising.
- +1Set MC = ATC for min ATC: q/8 = 32/q + q/16 ⇒ q/16 = 32/q ⇒ q² = 512.
- +1Solve: q* = √512 ≈ 22.6, where ATC = MC = 22.6/8 ≈ $2.83 — the bottom of the U.
Key terms
- Economic cost
- Accounting cost plus the opportunity cost of the owner's own resources — their time, capital and premises. It captures what every input could have earned in its next-best use, so it always equals or exceeds the accountant's recorded cost.
- Sunk cost
- A cost already spent that cannot be recovered, such as specialised R&D. Its opportunity cost is zero, so it is independent of output and should be ignored in any forward-looking decision — not to be confused with a fixed cost.
- Diminishing marginal product
- With at least one input fixed, each extra unit of a variable input (e.g. labour) adds less to output than the unit before, because it has less of the fixed input to work with. This is what makes variable cost convex and marginal cost eventually rise.
- Marginal cost (MC)
- The cost of producing one more unit, MC = ΔTC/Δq = dTC/dq. Fixed cost drops out when differentiating, so MC depends only on variable cost. MC cuts both AVC and ATC at their minimum points.
- Returns to scale
- How output responds when every input is scaled up by the same proportion in the long run. Increasing returns lower long-run average cost (economies of scale), constant returns leave it flat, and decreasing returns raise it (diseconomies), tracing the U-shaped LRAC.
Production and Costs FAQ
Is a sunk cost the same as a fixed cost?
No — this is the classic trap. Both are independent of output, so students lump them together, but the difference is recoverability. A fixed cost (insurance, plant maintenance) has a positive opportunity cost and belongs in your decision; a sunk cost (already spent, unrecoverable) has zero opportunity cost and must be ignored. A pharma firm's R&D is fixed AND sunk; its marketing is fixed but NOT sunk. 'We've spent so much, we can't stop now' is the sunk-cost fallacy the exam loves to bait.
Does zero economic profit mean the firm is failing?
No. Because normal profit — the owner's opportunity cost — is already counted inside economic cost, a firm with zero economic profit still earns a perfectly healthy normal accounting profit. It is doing exactly as well as its next-best alternative, so it has no reason to leave. This is the resting point competitive markets are pulled toward in the perfect-competition topic, where long-run price settles at min ATC.
Why does MC cross ATC and AVC exactly at their minimum points?
It is a general marginal-versus-average rule. When marginal cost is below the average, it pulls the average down; when it is above, it pushes the average up — so the average can only stop falling and start rising at the point where MC equals it. That crossing is therefore the minimum, and setting MC = ATC is your fastest way to find minimum ATC.
Why does min AVC sit to the left of min ATC?
MC hits AVC's minimum before ATC's because, after AVC has bottomed out, ATC is still being dragged down by falling AFC (FC spread over more units). So ATC keeps falling for a while after AVC turns up. Always draw min AVC at a smaller quantity than min ATC — markers check this.
Exam move
These marks are procedural and free if you stay disciplined: from TC, split out FC and VC, write ATC = TC/q and MC = dTC/dq (the constant FC differentiates to zero), and remember MC = ATC at the bottom of the U. The fastest route to minimum ATC is setting MC = ATC rather than differentiating. Marks are lost on the definitions — never treat a sunk cost as decision-relevant, never swap shut-down (min AVC) with exit/break-even (min ATC), and when drawing curves put min AVC to the left of min ATC. Recite the chain 'diminishing MP → VC convex → MC rises' and you can justify every curve shape without memorising it.
