amazon.co.ukMarkets: Supply and Demand
A* revision for demand and supply, PED, YED, XED and PES, market equilibrium, consumer surplus, market structures and wage determination for AQA 7136, Edexcel A 9EC0 and OCR H460.
π― Learning Objectives
- Draw and shift demand and supply curves correctly, distinguishing movements along from shifts of the curve
- Calculate and interpret PED, YED, XED and PES, applying each to pricing, revenue, tax incidence and business strategy
- Explain the three functions of the price mechanism and the interrelationship between markets (joint, competitive, composite and derived demand)
- Analyse market equilibrium, shortage and surplus, and evaluate how elasticities determine the speed of adjustment
- Compare market structures using surplus analysis, showing the deadweight loss of monopoly against perfect competition
- Evaluate monopoly using dynamic efficiency, economies of scale and X-inefficiency arguments
- Analyse wage determination using MRP and the monopsony model, and evaluate the effect of a National Minimum Wage
π Key Definitions and Core Concepts
Price elasticity of demand (PED): The percentage change in quantity demanded divided by the percentage change in price; measures responsiveness of demand to price.
Income elasticity of demand (YED): The percentage change in quantity demanded divided by the percentage change in income; positive for normal, negative for inferior goods.
Cross elasticity of demand (XED): The percentage change in quantity demanded of good A divided by the percentage change in the price of good B; positive for substitutes, negative for complements.
Price elasticity of supply (PES): The percentage change in quantity supplied divided by the percentage change in price; determined mainly by the time period and spare capacity.
Price mechanism: The allocation of resources by prices acting as signals, incentives and rationing devices (Smith's invisible hand).
Consumer surplus: The difference between what consumers are willing to pay and what they actually pay; the area under demand above price.
Allocative efficiency: The output level where P = MC, so the value society places on the last unit equals its marginal cost; total surplus is maximised.
Derived demand: Demand for a factor of production (such as labour) that arises from demand for the good or service it produces.
Marginal revenue product (MRP): Marginal physical product multiplied by marginal revenue; the demand curve for labour.
Monopsony: A market with a single buyer of labour, whose marginal cost of labour lies above the supply curve, depressing wages and employment.
Demand, supply and the price mechanism
The law of demand: as price falls, quantity demanded rises (an inverse relationship), ceteris paribus. Three explanations earn AO1 marks: the income effect (a lower price raises real income, so more can be bought), the substitution effect (the good becomes relatively cheaper than alternatives) and diminishing marginal utility (each extra unit gives less satisfaction, so consumers only buy additional units at a lower price). Determinants of demand: income (normal goods vs inferior goods), tastes and preferences, the price of substitutes (Coke and Pepsi) and complements (printers and ink), expectations of future prices (expected rises bring purchases forward), and the number of buyers. Determinants of supply: costs of production, technology and productivity, taxes and subsidies, expectations, and the number of firms in the market.
Movements vs shifts β the single most common source of lost marks: a change in the good's own price causes a movement along the curve (extension or contraction); a change in any other determinant shifts the whole curve. Before drawing any diagram, ask which variable has changed. Diagram description: axes are price (vertical) against quantity (horizontal); a downward-sloping demand curve D and upward-sloping supply curve S intersect at equilibrium (P*, Q*). If demand shifts right from D1 to D2, at the old price P1 quantity demanded exceeds quantity supplied (excess demand); price is bid up along both curves to a new equilibrium P2, Q2 β a price rise (extension of supply) with higher quantity. A supply shift works symmetrically: higher costs shift S left, raising price and cutting quantity (stagflation-style outcome in micro form).
The price mechanism (Adam Smith's “invisible hand”) performs three functions: signalling β prices convey information about relative scarcity; incentive β higher prices encourage production and discourage consumption; rationing β scarce goods are allocated to those willing and able to pay. Together these allocate resources without central planning; Hayek's “knowledge problem” argument holds that no planner can aggregate dispersed price information. Interrelated markets (AQA 4.1.2.7): joint demand (complements bought together), competitive demand (substitutes), composite demand (one good with several uses, e.g. milk for drinking or cheese), and derived demand (labour is demanded because of the demand for its output). A price change in one market feeds into others β e.g. a wheat price rise raises bread prices and cuts demand for butter β a chain-of-reasoning staple in data-response questions.
Elasticity of demand: PED, YED and XED
PED = %ΞQd Γ· %ΞP (negative by the law of demand; quote the absolute value).
| |PED| | Label | Meaning |
|---|---|---|
| > 1 | Elastic | Quantity responds proportionally more than price |
| < 1 | Inelastic | Quantity responds proportionally less than price |
| = 1 | Unitary | Proportional response; total revenue constant |
| = 0 | Perfectly inelastic | Vertical demand curve |
| = β | Perfectly elastic | Horizontal demand curve (price taker) |
Determinants of PED: number and closeness of substitutes, necessity vs luxury, the proportion of income spent on the good, habit/addiction, the time period (demand is more elastic in the long run as consumers adjust), and the breadth of the market definition (a single brand has many substitutes; “food” has almost none). Uses of PED: revenue maximisation β raise price while |PED| < 1, and total revenue is maximised where |PED| = 1; tax incidence β with inelastic demand, consumers bear most of an indirect tax (why tobacco and fuel duties raise so much revenue); pricing strategy under different market structures; and the effectiveness of devaluation (MarshallβLerner condition). Real-world anchors: UK petrol demand is price inelastic (estimates around β0.1 to β0.3 in the short run); Ofgem treats household energy demand as inelastic, which is why energy price shocks feed strongly into the CPI basket.
YED = %ΞQd Γ· %ΞIncome. Normal goods have YED > 0 (necessities 0 < YED < 1; luxuries YED > 1); inferior goods have YED < 0 (value-brand groceries, bus travel). Recession application: falling incomes shift demand towards inferior goods and away from income-elastic luxuries β discounters Aldi and Lidl grew rapidly after 2008 and again in 2022β24, while restaurants, holidays and new cars (luxuries, YED > 1) suffer most in downturns. This makes demand in cyclical, income-sensitive industries volatile.
XED = %ΞQd of A Γ· %ΞP of B. Substitutes have XED > 0 (Coke and Pepsi); complements have XED < 0 (printers and ink). |XED| measures closeness: the higher the XED, the stronger the competitive threat and the tighter the relevant market definition β this feeds directly into CMA merger assessments (a merger between firms with high XED between their products raises competition concerns). Worked example (adapted from a real exam-style calculation): a coffee shop raises latte prices by 10% and quantity demanded falls 4% β PED = β0.4, inelastic; total revenue changes by (1.10 Γ 0.96) β 1 β +5.6%. But if all coffee shops raise prices together, market demand is more elastic (supermarket coffee is a substitute), so the revenue gain shrinks β the difference between a firm's demand curve and the market demand curve is a key evaluation point.
PES, market equilibrium and disequilibrium
PES = %ΞQs Γ· %ΞP; elastic when > 1. Determinants: spare capacity, stock levels, the time period (the crucial one β supply is more elastic in the long run because all factors can be varied and firms can enter or exit), the mobility and availability of factors of production, and the length of the production cycle. Inelastic-supply examples: housing (planning constraints, construction lags), agriculture (growing seasons), and oil (extraction capacity). Consequence: demand shocks translate into large price swings β the wheat price spike after Russia's 2022 invasion of Ukraine, and UK rent inflation, both reflect inelastic supply meeting shifting demand.
Equilibrium occurs where Qd = Qs at P* β the market-clearing price with no tendency to change. A shortage (excess demand) at any price below P* puts upward pressure on price; a surplus (excess supply) above P* puts downward pressure on it, as unsold stocks build. The market is self-correcting, but the speed and size of adjustment depend on both elasticities: the more inelastic the curves, the larger the price change needed to restore equilibrium. Diagram description: at price P1 below equilibrium, draw the horizontal gap between the demand and supply curves β the shortage; an arrow shows price rising to P* where the gap closes.
Buffer stock schemes (used historically for commodities such as coffee under the International Coffee Agreement): the agency buys in gluts and sells from stock in shortages to stabilise the price. Evaluation: storage is costly, and if supply shocks persist (repeated bad harvests) the agency exhausts its funds β so buffer stocks manage volatility but cannot abolish it. Exam technique: in data-response questions, always complete the chain β shock β curve shift β new equilibrium price and quantity β extension/contraction on the other curve β welfare implication. Missing the final link is where AO2 marks are dropped; quoting elasticities in the analysis is where AO3 is earned.
Surplus, efficiency and market structures
Consumer surplus is the area under the demand curve and above the price paid β what consumers were willing to pay minus what they actually pay. Producer surplus is the area above the supply curve and below the price. Total surplus is maximised where allocative efficiency holds: P = MC, so the value society places on the marginal unit equals its marginal cost of production. Any price above MC (monopoly power, indirect tax) or below it (maximum price, subsidy distortions) creates deadweight welfare loss β surplus destroyed with no offsetting gain to anyone.
Diagram description (monopoly vs perfect competition): axes price/quantity; a downward-sloping demand curve D with marginal revenue MR below it (MR has twice the slope of a linear demand curve); an upward-sloping MC and U-shaped AC. Under perfect competition the firm is a price taker facing a horizontal demand curve, producing where P = MC; long-run equilibrium is P = minimum AC (normal profit only). The monopolist sets MR = MC at Qm < Qpc and charges Pm (read off the demand curve) with Pm > MC. The deadweight loss is the triangle between demand and MC over the output gap Qm to Qpc; consumer surplus falls, part being transferred to the monopolist as supernormal profit, sustained in the long run by barriers to entry (legal barriers such as patents, economies of scale, brand loyalty, control of key inputs, predatory pricing).
| Structure | Firms | Product | Entry barriers | Long-run profit |
|---|---|---|---|---|
| Perfect competition | Many, price takers | Homogeneous | None | Normal only (P = min AC) |
| Monopolistic competition | Many | Differentiated | Low | Normal (excess capacity) |
| Oligopoly | Few, interdependent | Homogeneous or differentiated | High | Supernormal possible (collusion/non-price competition) |
| Monopoly | One (25%+ market share definition) | Unique/no close substitutes | Very high | Supernormal sustained |
Evaluation (the A* framing): static DWL is not the whole story. Dynamic efficiency: supernormal profit funds R&D and innovation (Schumpeter's creative-destruction argument), so monopoly may deliver faster product improvement. Economies of scale: a large monopolist's AC may be far below that of many small competitive firms, so Pm could be lower than the competitive price the model predicts. Price discrimination can raise output closer to the competitive level and share gains with consumers. Counting against monopoly: X-inefficiency (costs drift above minimum AC without competitive pressure) and weak incentives to innovate behind protected markets. Competition authorities therefore judge each case on its merits β market share alone is not evidence of consumer harm.
Labour markets: MRP, monopsony and the minimum wage
Derived demand: labour is demanded because of the demand for its output. The marginal revenue product (MRP = MPP Γ MR) curve is the demand for labour; a profit-maximising firm hires where MCL = MRP β in a competitive labour market the wage equals MRP. Wage differentials reflect differences in MRP, human capital and skills, compensating differentials for unpleasant or risky work, and barriers to entry such as professional qualifications. Labour supply to an occupation depends on wages, non-monetary benefits, qualifications required, and participation decisions; at very high wages the income effect can eventually outweigh the substitution effect (the backward-bending supply curve) as workers choose more leisure.
Monopsony: a market with a single buyer of labour (a large employer in a small town, or the NHS for certain specialists). Because hiring one more worker requires raising the wage for all workers, the marginal cost of labour (MCL) lies above the upward-sloping supply curve. Unregulated, the monopsonist hires where MCL = MRP, giving employment Q1 and wage W1 (read off the supply curve) β both below the competitive outcome (Wc, Qc where S = MRP): the monopsonist exploits its buyer power. Diagram description: axes wage/employment; upward-sloping labour supply S (= ACL) with MCL above it; downward-sloping MRP (demand). Mark Q1/W1 at MCL = MRP; mark Qc/Wc at S = MRP; shade the under-employment gap.
National Minimum/Living Wage intervention: a statutory wage set at Wc (above W1 but below the MCL at Q1) makes the supply of labour horizontal (perfectly elastic) up to Qc β the monopsonist can hire more workers at a constant wage. The wage rises and employment rises: the minimum wage corrects the monopsony distortion. UK data: the National Living Wage rose to Β£11.44 an hour in April 2024 and Β£12.21 in April 2025; Low Pay Commission evidence (and the famous CardβKrueger studies) suggests moderate rises have had little negative employment effect, consistent with monopsony power in low-paid labour markets. Trade unions use collective bargaining to push wages above MRP-equivalent levels; in a monopsony a union can offset employer power without costing jobs. Evaluation: the benign outcome holds only if monopsony power genuinely exists β in competitive labour markets a minimum wage above equilibrium causes unemployment (Qs > Qd), concentrated on young and low-skilled workers whose MRP is low; firms may instead cut hours, benefits or training; and enforcement matters where gig-economy misclassification hides true employment relationships. The exam-ready judgement: the effect of a minimum wage depends on the structure of the labour market, its level relative to the market wage, and the state of labour demand.
π‘ Worked Example
Exam-style: A monopolist faces the demand curve P = 100 β 2Q and has constant marginal cost MC = 20 with total cost TC = 20Q + 100. Calculate the profit-maximising output, price and profit, and explain two reasons why this outcome differs from a perfectly competitive market. (AQA 7136 Paper 1 style, 15 marks)
Mark-scheme logic: Mark logic: [AO1] state the profit-maximising condition and competitive benchmark; [AO2] calculate accurately with working; [AO3/AO4] draw out the welfare implications. Working: MR = 100 β 4Q (twice the slope of a linear demand curve). Set MR = MC: 100 β 4Q = 20, so Q = 20. Price from the demand curve: P = 100 β 2(20) = Β£60. ATC = TC/Q = 20 + 100/20 = Β£25, so profit = (P β ATC) Γ Q = (60 β 25) Γ 20 = Β£700. Difference 1 β allocative inefficiency: P (60) > MC (20), so society values the last unit far above its marginal cost; the competitive output is where P = MC (Q = 40 at P = 20), so the monopolist restricts output and creates a deadweight welfare loss on units between Q = 20 and Q = 40. Difference 2 β supernormal profit persists: the Β£700 profit survives in the long run only because barriers to entry stop rival firms entering and bidding price down to minimum AC, where only normal profit would be earned. Evaluation: the static loss is not the final word β the Β£700 profit could fund R&D (dynamic efficiency), and economies of scale may give the monopolist a lower AC than small competitive firms would achieve, so the observed price could be below the textbook competitive price. The welfare verdict depends on the strength of entry barriers and the use made of the profit.
β Practice Questions & Mark Schemes
A firm knows PED for its product is β0.5. It raises price by 10%. Calculate the change in quantity demanded and assess the effect on the firm's total revenue and on consumer surplus.
Answer: %ΞQd = PED Γ %ΞP = (β0.5) Γ (+10%) = β5%: quantity demanded falls 5%. Since |PED| = 0.5 < 1, demand is price inelastic. Total revenue = P Γ Q changes by (1.10 Γ 0.95) β 1 β +4.5% β a price rise raises revenue when demand is inelastic (and lowers it when elastic; revenue is maximised at |PED| = 1). Consumer surplus falls: continuing consumers pay more (loss = extra price Γ remaining quantity) and some consumers stop buying (loss of their surplus entirely). Evaluation: PED is a point estimate along a linear demand curve β a further rise may meet a more elastic response; if rivals do not follow the rise, measured inelasticity overstates the protection; ceteris paribus may fail if incomes or tastes shift.
Using a diagram, compare the price and output decisions of a profit-maximising monopolist with a perfectly competitive industry, and evaluate the welfare implications.
Answer: Perfect competition: price taker, horizontal demand, P = MC; long run P = min AC (normal profit); industry output where MC (supply) cuts demand. Monopoly: faces the whole market demand, MR below it; MR = MC at Qm < Qpc; price Pm off the demand curve with Pm > MC; supernormal profit sustained by barriers to entry. Welfare: allocative inefficiency (P > MC) creates a deadweight loss triangle between demand and MC over the QmβQpc gap; consumer surplus falls, part transferred to the monopolist. Evaluation: supernormal profit funds R&D so dynamic efficiency may be higher (Schumpeter); economies of scale can give the monopolist lower AC and price than many small firms; price discrimination can push output towards Qpc; but X-inefficiency and weak incentives count against it β authorities judge each case on merits.
Using the monopsony model, analyse how a National Minimum Wage set above the market wage could raise both wages and employment, and evaluate this outcome.
Answer: Monopsony: MCL above supply S; hires where MCL = MRP at Q1, wage W1 read off S β both below competitive Wc/Qc. An NMW at Wc makes supply horizontal (perfectly elastic) up to Qc, so the monopsonist can hire more at a constant wage; the horizontal line cuts MRP at the higher employment Qc: wages AND employment rise β the distortion is corrected. UK: NLW Β£11.44 (Apr 2024), Β£12.21 (Apr 2025); Low Pay Commission and CardβKrueger evidence suggests little employment loss. Evaluation: depends on monopsony power genuinely existing β in competitive markets an NMW above equilibrium causes unemployment (Qs > Qd), hitting young and low-skilled workers with low MRP; firms may cut hours, benefits or training instead of jobs; and if set too high above Wc even a monopsonist sheds labour. Judgement: effect depends on labour-market structure, the wage level chosen, and labour demand conditions.
For linear demand P = 100 β Q, calculate consumer surplus when P = 50 and explain what happens to it if price rises to P = 60.
Answer: At P = 50, quantity = 100 β 50 = 50. CS = Β½ Γ base Γ height = Β½ Γ 50 Γ 50 = Β£1,250. At P = 60, quantity = 40: CS = Β½ Γ 40 Γ 40 = Β£800. CS falls by Β£450: part is transferred to the seller as extra revenue (the Β£10 Γ 40 units still bought = Β£400) and part is destroyed (the triangle lost on the 10 units no longer bought, β Β£50 here). This shows the general result: price rises above the competitive level cut consumer surplus, transferring some to producers and destroying the rest as deadweight loss.
Explain why PES is more elastic in the long run, and use this to analyse why agricultural prices are volatile.
Answer: In the short run at least one factor (land, capital) is fixed, so output barely responds to price; in the long run all factors can be varied and firms can enter or exit, so supply responds fully β PES rises with the time period. Agriculture: supply is inelastic in the short run (growing seasons, fixed land) and demand for food is price inelastic (a necessity). When harvests fail (supply shifts left, e.g. the 2022 wheat shock after Russia's invasion of Ukraine), the small quantity fall requires a large price rise to restore equilibrium; when harvests are good, prices collapse. Additive factors: futures markets, buffer stocks and imports smooth some volatility, but inelastic PES Γ inelastic PED is the core mechanism examiners want.
Using XED, explain how a firm can assess competitive threats to its product. Illustrate with reference to the soft drinks market.
Answer: XED = %ΞQd of A Γ· %ΞP of B; positive XED means substitutes, and |XED| measures closeness. A firm estimates XED between its brand and rivals: the higher the XED, the stronger the competitive threat β a rival's price cut will take a large share of its customers. In soft drinks, branded colas have high XED against each other (fierce interdependence β hence heavy advertising and non-price competition) but much lower XED against water or own-label drinks. Firms use XED for pricing, bundling (complements, XED < 0, e.g. consoles and games) and market definition; the CMA uses the same logic in merger tests β high cross-price effects imply the firms constrain each other. Evaluation: XED estimates are unstable over time and across price ranges, so they inform rather than determine strategy.