What is the difference between a movement along a curve and a shift of a curve?

A movement along a demand or supply curve is caused only by a change in the good's own price, with all other factors held constant. A shift of the whole curve is caused by a change in a non-price determinant: income, the price of substitutes or complements, population, tastes or expectations for demand; input costs, technology, the number of firms, government policy or natural events for supply. QCAA markers reward responses that explicitly name which factor changed and which curve it moves.

How do you calculate price elasticity of demand?

Price elasticity of demand (PED) equals the percentage change in quantity demanded divided by the percentage change in price, conventionally reported as an absolute value. PED greater than 1 is elastic (quantity is highly responsive, total revenue falls when price rises); PED less than 1 is inelastic (quantity is unresponsive, total revenue rises when price rises); PED equal to 1 is unit elastic. Key determinants are the number of substitutes, whether the good is a necessity or luxury, the proportion of income spent, and the time horizon.

What are the four forms of market failure in QCE Economics?

Public goods (non-rival and non-excludable, so the market under-supplies them because of free-riding), externalities (costs or benefits falling on third parties, causing over-production of negative-externality goods and under-production of positive-externality goods), asymmetric information (one party knows more, producing adverse selection and moral hazard), and market power (monopoly or oligopoly raising price above the competitive level, creating deadweight loss).

What is the difference between a public good and a good provided by the public sector?

A public good is technically non-rival and non-excludable, such as defence or a lighthouse. Many goods provided by the public sector, such as school education or public hospital care, are not technically public goods because they are rival and excludable. Government provides them because they carry large positive externalities or for equity reasons, not because they meet the public good definition. Confusing the two is a common QCE trap.

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QLDEconomics

QCE Economics markets deep-dive: demand, supply, elasticity, market failure and intervention

Deep-dive on the QCE Economics microeconomic foundation across Units 1 and 2. The market mechanism, demand and supply, equilibrium, price elasticity, the four forms of market failure, and the government intervention tools, with a worked data-response and exam-style questions.

Generated by Claude Opus 4.716 min readQCAA-ECON-U1-2Updated 2026-05-24

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How the market foundation fits into QCE Economics
The market mechanism
Equilibrium
Price elasticity
Market failure
Government intervention
Worked example: a model data-response
Common QCE microeconomics traps
Check your knowledge

How the market foundation fits into QCE Economics

QCE General Economics is a four-unit course. Units 1 and 2 (Markets and models, Modified markets) are the Year 11 foundation that the assessed Year 12 sequence assumes you already understand. The microeconomic toolkit built here (demand and supply, equilibrium, elasticity, market failure and intervention) reappears throughout Unit 3 (the foreign exchange market is a demand and supply diagram) and Unit 4 (aggregate demand and supply, and the case for government policy rests on market failure).

QCAA assessment items reward students who can draw and interpret diagrams, distinguish movements along curves from shifts, and apply theory to current Australian examples rather than reciting definitions in isolation. This guide builds that capability.

The market mechanism

A market is any arrangement that brings buyers and sellers together. In a competitive market, the interaction of demand and supply determines price and quantity, and price acts as a signal that allocates scarce resources.

The law of demand

The law of demand states that, holding all other factors constant, the quantity demanded of a good rises as its price falls. The demand curve slopes downward. Three reasons explain this:

Substitution effect. A higher price for good X leads consumers to switch to relatively cheaper substitutes.
Income effect. A higher price for X reduces real purchasing power, so consumers buy less of normal goods.
Diminishing marginal utility. Each additional unit consumed gives less satisfaction, so consumers will only buy more units at a lower price.

Non-price determinants of demand

Demand shifts (the whole curve moves) when a non-price factor changes:

Income. Higher income shifts demand right for normal goods, left for inferior goods.
Price of substitutes. A higher substitute price shifts demand right.
Price of complements. A higher complement price shifts demand left.
Population. A larger or growing population shifts demand right.
Tastes and preferences. Cultural change and advertising.
Expectations. An expected future price rise shifts current demand right.

The law of supply

The law of supply states that, holding all other factors constant, the quantity supplied rises as price rises. The supply curve slopes upward, because of rising marginal cost as firms expand, the profit incentive of higher prices, and the entry of new firms attracted by high prices.

Non-price determinants of supply

Supply shifts when input costs change, technology improves, the number of firms changes, government policy changes (subsidies shift supply right; taxes and regulation shift it left), or natural events affect production.

Equilibrium

Equilibrium is the price and quantity at which quantity demanded equals quantity supplied, shown diagrammatically as the intersection of the two curves.

At a price above equilibrium, quantity supplied exceeds quantity demanded, producing a surplus; sellers cut prices.
At a price below equilibrium, quantity demanded exceeds quantity supplied, producing a shortage; buyers bid prices up.

Market forces push price back toward equilibrium. This self-correcting behaviour is the central idea of the market mechanism.

Price elasticity

Price elasticity of demand

Price elasticity of demand (PED) measures how responsive quantity demanded is to a change in price.

PED = \frac{\% \text{ change in quantity demanded}}{\% \text{ change in price}}

It is conventionally reported as an absolute value.

PED greater than 1: elastic. Quantity is highly responsive. Raising price lowers total revenue. Example: branded goods with close substitutes.
PED equal to 1: unit elastic. Total revenue is unchanged when price changes.
PED less than 1: inelastic. Quantity is unresponsive. Raising price increases total revenue. Example: petrol in the short run, prescription medicines.
PED equal to 0: perfectly inelastic. Vertical demand curve.
PED approaching infinity: perfectly elastic. Horizontal demand curve.

Determinants of PED are the number of substitutes (more substitutes means more elastic), whether the good is a necessity or a luxury (luxuries are more elastic), the proportion of income spent, the time horizon (demand becomes more elastic over time), and how narrowly the market is defined (Coca-Cola is more elastic than all soft drinks).

Price elasticity of supply

Price elasticity of supply (PES) measures how responsive quantity supplied is to a change in price.

PES = \frac{\% \text{ change in quantity supplied}}{\% \text{ change in price}}

Supply is more elastic over a longer time horizon, when factors of production are mobile, when firms have spare capacity, and when the good can be stored.

Applications of elasticity

Tax incidence: When demand is inelastic and supply is elastic, consumers bear most of an indirect tax (cigarettes). When demand is elastic and supply is inelastic, producers bear most (fresh produce).
Pricing strategy: A firm facing inelastic demand can raise price to lift total revenue; a firm facing elastic demand cannot.
Commodity markets: Australian iron ore has inelastic supply in the short run (mine capacity is fixed) but elastic supply in the long run (new mines can be developed). This is why a demand increase raises price sharply in the short run.

Market failure

Market failure occurs when a competitive market fails to allocate resources efficiently, so the market outcome differs from the socially optimal outcome. QCE recognises four forms.

1. Public goods

Public goods are non-rival (one person's consumption does not reduce another's) and non-excludable (non-payers cannot be prevented from consuming). Markets under-supply them because of free-riding. Government provides them, funded from tax. Australian examples include defence, public broadcasting (ABC and SBS), lighthouses, and Bureau of Meteorology weather services.

2. Externalities

Externalities are costs or benefits that fall on third parties not involved in a transaction.

Negative externalities (external costs such as pollution, congestion and smoking) mean private cost is below social cost, so the market over-produces.
Positive externalities (external benefits such as vaccination, education and research) mean private benefit is below social benefit, so the market under-produces.

The negative externality diagram shows the social marginal cost curve above the private marginal cost curve. The free-market equilibrium produces more than the socially optimal level, and the deadweight loss is the triangle between the two cost curves over the over-produced output.

3. Asymmetric information

When one party has more or better information than the other, two problems arise: adverse selection before a contract (a used-car buyer cannot verify quality) and moral hazard after a contract (insurance changes risk-taking behaviour). Australian responses include ASIC regulation of financial product disclosure and APRA prudential standards.

4. Market power

When few firms can raise prices above the competitive level, output is too low and a deadweight loss results. Australia's two major supermarkets hold around 65 percent of grocery sales, and the four major banks hold around 75 percent of mortgages. Market power produces higher prices, lower output and possible innovation losses.

Government intervention

Government can correct market failure using five major tools.

Indirect (Pigouvian) taxes on goods with negative externalities raise the price toward the socially optimal level. Tobacco excise both reduces smoking and raises revenue.
Subsidies encourage goods with positive externalities, such as the National Immunisation Program and Commonwealth Supported university places.
Regulation sets direct rules, enforced by the ACCC (Competition and Consumer Act 2010), APRA, ASIC, and environmental and safety agencies.
Public provision: government supplies the good itself, as with Medicare, public schools and public hospitals.
State-owned enterprises such as Australia Post, NBN Co and Snowy Hydro provide goods commercially.

Costs of intervention

Intervention is justified only when the cost of the market failure exceeds the cost of intervening. Costs include compliance costs for firms, administration costs for agencies, allocative distortion from taxes and subsidies, government failure (regulatory capture, slow or politicised decisions), and unintended consequences (rent control reducing rental supply; first-home buyer grants raising house prices).

Worked example: a model data-response

The following is a model short-response of the kind QCAA expects in IA1, IA2 or the EA, working from stimulus data.

Worked example

Stimulus: Iron ore traded at around USD 80 per tonne, with Australia exporting roughly 900 million tonnes per year. A Chinese construction stimulus then raises demand for steel.
Question (6 marks): Using a demand and supply diagram, explain the effect of the Chinese stimulus on the Australian iron ore market in the short run, and explain why the price effect is larger than the quantity effect.
Model response

The Chinese stimulus raises construction activity, which raises the derived demand for steel and therefore for iron ore, a key steel input. This is a change in a non-price determinant of demand (the price of a complement in production and overall demand conditions in the buying market), so the demand curve shifts right from D1 to D2. It is not a movement along the curve, because the iron ore price did not cause the change.

At the original price, the rightward demand shift creates a shortage: quantity demanded now exceeds quantity supplied. Buyers bid the price up, and the market moves to a new equilibrium at a higher price and a higher quantity, where D2 intersects the supply curve.

The price effect is large and the quantity effect is small because short-run supply of iron ore is highly inelastic (PES less than 1). Mine capacity in the Pilbara is fixed in the short run: producers cannot quickly open new mines, expand rail capacity or commission new port infrastructure. With a steep (inelastic) supply curve, a rightward demand shift is absorbed mostly by a price rise rather than a quantity rise. Over the long run, supply becomes more elastic as Rio Tinto, BHP and Fortescue expand capacity, so a sustained demand increase would eventually draw out more quantity and moderate the price rise.

(A full-mark response would include a labelled diagram showing D1, D2, a steep supply curve, the original and new equilibria, and the larger vertical price movement compared with the horizontal quantity movement.)

Common QCE microeconomics traps

Confusing a movement along a curve with a shift of the curve.
Drawing both curves shifting when only one determinant changed.
Calculating PED without taking the absolute value.
Treating elasticity as constant along a linear demand curve (it varies, high near the price intercept and low near the quantity intercept).
Confusing a public good with a good provided by the public sector.
Forgetting the deadweight loss triangle on the externality and market power diagrams.
Treating government intervention as costless.

Check your knowledge

A mix of definitional, diagram and exam-style questions covering this guide. Attempt all under timed conditions, then check the solutions block.

Define the law of demand and give the three reasons the demand curve slopes downward. (3 marks)
Distinguish between a movement along a supply curve and a shift of a supply curve, giving one cause of each. (4 marks)
The price of coffee (a complement to milk) rises sharply. Using a diagram, explain the effect on the market for milk, identifying the new equilibrium price and quantity. (4 marks)
(a) Calculate the price elasticity of demand if a 10 percent rise in the price of a good causes quantity demanded to fall by 25 percent. (b) State whether demand is elastic or inelastic. (c) State what happens to the firm's total revenue if it raises this price, and explain why. (5 marks)
Explain, with reference to the four determinants, why the demand for petrol is inelastic in the short run but more elastic in the long run. (4 marks)
Identify the four forms of market failure and give one Australian example of each. (4 marks)
Using a negative externality diagram, explain why a competitive market over-produces a good that generates pollution, and explain how a Pigouvian tax corrects the over-production. (6 marks)
Evaluate the use of government intervention to correct market failure, referring to at least two intervention tools and at least two costs of intervention. (8 marks)

Solutions

Q1: The law of demand states that, holding all other factors constant, the quantity demanded of a good rises as its price falls (and falls as price rises); the curve slopes downward. The three reasons are: the substitution effect (consumers switch to cheaper substitutes when price rises), the income effect (a higher price reduces real purchasing power, so less of the good is bought), and diminishing marginal utility (each extra unit yields less satisfaction, so consumers will only buy more at a lower price).
Q2: A movement along a supply curve is caused only by a change in the good's own price (for example, a higher market price for wheat raises the quantity of wheat supplied along the curve). A shift of the supply curve is caused by a non-price determinant (for example, a fall in fertiliser costs or a productivity-improving technology shifts the whole supply curve right, raising quantity supplied at every price).
Q3: Coffee and milk are complements, so a sharp rise in the coffee price reduces the quantity of coffee bought and therefore reduces demand for milk. This is a change in a non-price determinant (price of a complement), so the milk demand curve shifts left from D1 to D2. At the original price a surplus appears, sellers cut prices, and the market settles at a new equilibrium with a lower price and a lower quantity of milk. A correct diagram shows D1, D2 shifted left, an unchanged supply curve, and the new lower-price, lower-quantity equilibrium.
Q4: (a) $PED = |{-25\%} / {10\%}| = 2.5$ . (b) Because PED is greater than 1, demand is elastic. (c) Raising the price would reduce total revenue: because demand is elastic, the percentage fall in quantity (25 percent) exceeds the percentage rise in price (10 percent), so price times quantity falls. Firms facing elastic demand should be cautious about raising price.
Q5: Short-run petrol demand is inelastic because (i) there are few close substitutes for fuel for an existing petrol car, (ii) petrol is a necessity for commuting, (iii) although it can be a sizeable share of income, drivers cannot easily change consumption immediately, and (iv) over a short time horizon consumers cannot adjust. Over the long run demand becomes more elastic because the time horizon allows adjustment: consumers buy more fuel-efficient or electric vehicles, move closer to work, or switch to public transport, so quantity becomes more responsive to the petrol price.
Q6: (i) Public goods: defence, or public broadcasting (ABC, SBS), or lighthouses. (ii) Externalities: greenhouse gas emissions (negative) or vaccination (positive). (iii) Asymmetric information: used-car markets, or financial product disclosure regulated by ASIC. (iv) Market power: the supermarket duopoly (Coles and Woolworths, around 65 percent of sales) or the four major banks.
Q7: A good that generates pollution has a social marginal cost above its private marginal cost, because the producer does not pay for the pollution imposed on third parties. The competitive market equilibrium is set where demand meets private marginal cost, producing quantity Qm. The socially optimal quantity Q* is where demand meets social marginal cost, which is lower. The market therefore over-produces by (Qm minus Q*), and the deadweight loss is the triangle between the SMC and PMC curves over that range. A Pigouvian tax equal to the marginal external cost raises the producer's private cost up to the social cost, shifting the private supply curve left until it coincides with SMC. The new equilibrium occurs at Q*, the socially optimal level, internalising the externality and eliminating the deadweight loss (while also raising revenue).
Q8: A balanced evaluation. For intervention: indirect taxes on negative-externality goods (tobacco excise) reduce consumption toward the social optimum and raise revenue, while subsidies for positive-externality goods (the National Immunisation Program) raise consumption toward the social optimum and generate herd immunity. Regulation (ACCC enforcement of the Competition and Consumer Act) constrains market power directly. Against, or limits: intervention carries compliance and administration costs; taxes and subsidies create their own allocative distortions; government failure (regulatory capture, slow or politicised decisions) can mean the cure is worse than the disease; and unintended consequences are common (rent controls reduce rental supply; first-home buyer grants are partly capitalised into higher house prices). Judgement: intervention is justified where the market failure is significant and persistent, the right instrument can be identified, government has the information and capacity to design it, and the benefit of correction exceeds the total cost of intervening. Where market imperfections are minor or self-correcting, or where the instrument is poorly targeted, the costs may exceed the benefits and intervention is not warranted.