Find equilibrium
Identify equilibrium price and quantity, and explain shortages and surpluses when a market is away from equilibrium.
Market equilibrium and the price system
Demand and supply become most useful when they are brought together. This chapter explains how markets reach equilibrium, how changes in demand and supply alter price and quantity, how markets are linked, how prices allocate scarce resources, and how consumer and producer surplus measure benefits from exchange.
Book-aligned notesClear worked examplesExplained figuresInterrelated marketsConsumer & producer surplus
What this chapter prepares you to do
Analyse shifts
Use demand and supply diagrams to predict how a change in market conditions affects equilibrium price and quantity.
Link markets
Explain alternative demand, joint demand, derived demand and joint supply using relevant examples.
Evaluate welfare
Interpret consumer and producer surplus and explain how price changes and entry or exit affect them.
Core exam skill: when a determinant changes, identify the curve that shifts, state its direction, then trace the effect on equilibrium price and equilibrium quantity.
Chapter sections
4.1
Market equilibrium
Market equilibriumA situation in which, at the prevailing price, quantity demanded is exactly equal to quantity supplied.
Equilibrium priceThe price at which consumers wish to buy exactly the quantity that firms wish to sell.
Market demand slopes downwards and market supply normally slopes upwards. Their intersection determines the equilibrium price (P*) and equilibrium quantity (Q*). At this point the plans of buyers and sellers are consistent, so there is no automatic pressure for the price to change.
Bringing demand and supply together
Market equilibrium occurs at P* and Q*, where quantity demanded equals quantity supplied. A price above P* creates excess supply and puts downward pressure on price; a price below P* creates excess demand and puts upward pressure on price.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
Book context: the market for rice. At the equilibrium price, rice buyers want to purchase exactly the quantity rice sellers want to supply. If price is too high, unsold stocks give sellers an incentive to cut price. If price is too low, a shortage gives sellers an incentive to raise price. The adjustment continues until quantity demanded equals quantity supplied.
Disequilibrium: excess supply
If price is above equilibrium, firms wish to sell more than consumers wish to buy. Unsold stocks build up. Sellers have an incentive to reduce price, causing a contraction of supply and an extension of demand until equilibrium is restored.
Disequilibrium: excess demand
If price is below equilibrium, consumers wish to buy more than firms are willing to sell. A shortage develops. Buyers compete for the limited output and firms have an incentive to raise price. There is an extension of supply and a contraction of demand until the market returns to equilibrium.
| Market price | Condition | Quantity relationship | Pressure on price |
|---|---|---|---|
| Above equilibrium | Surplus / excess supply | Qs > Qd | Downward |
| At equilibrium | Market clears | Qs = Qd | No automatic pressure |
| Below equilibrium | Shortage / excess demand | Qd > Qs | Upward |
Exam language: a shortage or surplus causes movements along the existing demand and supply curves as price adjusts. Do not say the curves shift unless a non-price determinant changes.
4.2
Effects of shifts in demand and supply
Comparative analysis starts from one equilibrium, changes a determinant of demand or supply, and then identifies the new equilibrium. The key is to distinguish the initial shift from the movements along the other curve that follow as price changes.
Why the book calls this static analysis: Economists first hold the other determinants of demand and supply constant to identify an equilibrium. They then change one background condition and compare the original equilibrium with the new one. This keeps the chain of cause and effect clear.
| Initial change | Curve movement | Equilibrium price | Equilibrium quantity |
|---|---|---|---|
| Increase in demand | D shifts right | Rises | Rises |
| Decrease in demand | D shifts left | Falls | Falls |
| Increase in supply | S shifts right | Falls | Rises |
| Decrease in supply | S shifts left | Rises | Falls |
A change in consumer preferences for noodles
A favourable change in preferences, such as publicity about the health benefits of noodles, increases demand from D₀ to D₁. With supply unchanged, the new equilibrium has a higher price and a higher quantity traded.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
A change in the price of a substitute for dried noodles
If the price of fresh noodles falls, some consumers switch away from dried noodles. Demand for dried noodles shifts left, so both the equilibrium price and equilibrium quantity of dried noodles fall.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
New noodles-making technology
A more efficient noodles-making technology lowers firms’ production costs. Supply shifts to the right, producing a lower equilibrium price and a higher equilibrium quantity.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
An increase in labour costs
Higher wages or other labour-related production costs reduce supply at each price. The supply curve shifts left, raising the equilibrium price and reducing the equilibrium quantity traded.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
Why the market moves to the new equilibrium
1. A determinant changesExample: improved technology lowers production costs.
→2. A curve shiftsSupply shifts right because more can be supplied at every price.
→3. Price adjustsThe lower equilibrium price creates movements along both curves until Qd = Qs again.
Shifts in both demand and supply
Demand and supply can shift at the same time. In this case, one outcome may be certain while the other is indeterminate unless the relative size of the shifts is known.
Shifts in both demand and supply curves
Here demand falls while supply rises. Both shifts put downward pressure on equilibrium price, so price definitely falls. The final effect on quantity depends on the relative size of the two shifts; in this drawing the increase in supply is larger, so quantity rises.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
| Demand | Supply | Price | Quantity |
|---|---|---|---|
| Increases | Increases | Uncertain | Rises |
| Decreases | Decreases | Uncertain | Falls |
| Increases | Decreases | Rises | Uncertain |
| Decreases | Increases | Falls | Uncertain |
Common error: do not guess the uncertain result. If both curves shift, explicitly state that the final effect depends on the relative magnitude of the shifts unless enough information is provided.
4.3
Relationships between different markets
Markets often influence each other. A change in one market can alter demand or supply in another, so demand-and-supply analysis should not always treat markets as isolated.
Alternative demand
Occurs when goods are substitutes. If the price of tea rises, consumers may switch to coffee, increasing demand for coffee.
Joint demand
Occurs when goods are complements and are consumed together. An increase in demand for cars may increase demand for fuel.
Joint-demand example: The book uses coffee and sugar. If coffee becomes more popular, more coffee is consumed and demand for sugar may rise as well because the goods are often consumed together.
Derived demand
A good or factor is demanded because of what it helps to produce or obtain. Firms demand labour because labour contributes to output. Foreign currency is demanded because it buys foreign goods, services or assets.
Joint supply
Two products are produced together. A sheep farmer can supply both wool and meat; greater production of one may also increase the supply of the other.
Derived demand in the labour market
Firms demand labour for the output workers produce. The wage rate acts as the price of labour. A simple labour-market model therefore uses a downward-sloping demand curve for labour and an upward-sloping supply curve, with the market wage and employment level determined where they intersect.
Derived demand in the foreign exchange market
People demand foreign currency because they wish to buy another country's goods, services or assets. The exchange rate is the price of one currency in terms of another, so a foreign exchange market can also be analysed using demand and supply.
Foreign-exchange example: The book considers the market for Malaysian ringgits against US dollars. People outside Malaysia demand ringgits because they want Malaysian goods, services or assets; Malaysians supply ringgits when they want dollars to buy from the USA. The currency is therefore demanded for what it can purchase, which is why foreign-exchange demand is a derived demand.
High-mark habit: when explaining interrelated markets, identify the relationship first, then trace the chain of causation. Example: “Tea and coffee are substitutes → tea becomes more expensive → demand for coffee increases → coffee's demand curve shifts right.”
4.4
The functions of prices in allocating resources
Scarcity means every economy must answer what to produce, how to produce and for whom to produce. In a market economy, the price mechanism helps coordinate millions of decisions without a central planner directing every transaction.
Signalling
Changes in prices communicate changes in consumer preferences and scarcity to producers.
Incentive
Higher prices can make production more profitable, encouraging firms to expand output and attract resources.
Rationing
Because resources are scarce, price limits consumption to buyers who are willing and able to pay.
Resource allocation
Resources move towards markets where demand and profitability are stronger and away from markets where demand weakens.
How the price mechanism responds to rising demand
Book example: smartphones. If consumer preferences shift towards smartphones, demand rises. The higher equilibrium price is both a signal that consumers value smartphones more highly and an incentive for firms to expand output. Over time, resources are drawn into smartphone production.
Consumer preferences changeDemand shifts to the right.
→Equilibrium price risesThe higher price signals stronger demand and offers a profit incentive.
→Firms expand supplyThere is an extension of supply and resources are drawn into production.
The reverse can happen when demand falls. Lower prices signal weaker consumer valuation, firms have less incentive to produce, and resources can shift towards alternative uses.
Important distinction: price performs several functions at once. In an exam, do not simply list “signal, incentive, rationing”. Explain how the change in price influences decisions by consumers and producers.
4.5
Consumer and producer surplus
Consumer surplus
Consumer surplusThe difference between what consumers are willing to pay for a good and what they actually pay, summed across the units purchased.
The demand curve can be interpreted as showing consumers' willingness to pay. For the final unit bought, the market price reflects the value placed on that unit by the marginal consumer. Earlier units are valued more highly than the market price, so consumers gain a surplus.
Marginal social benefit (MSB)The additional benefit society gains from consuming one extra unit of a good. In this introductory analysis, the demand curve can be interpreted as the marginal benefit from consumption.
Consumer surplus
The demand curve shows willingness to pay. At market price P*, consumers who would have been willing to pay more than P* receive a surplus. The area below the demand curve and above the market price, up to Q*, represents total consumer surplus.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
Easy consumer-surplus calculation from the book: Six consumers are willing to pay $20, $18, $16, $14, $12 and $10. If the market price is $14, the first four buy. Their individual surpluses are $6, $4, $2 and $0, giving total consumer surplus of $12. If price rises to $16, only the first three buy and total consumer surplus falls to $6. This is why a higher price normally reduces consumer surplus.
Producer surplus
Producer surplusThe difference between the market price received by producers and the minimum price at which they would have been willing to supply the units sold.
In a competitive market, the supply curve can be interpreted as reflecting marginal cost: the cost of producing an additional unit. If the market price is above the minimum required to supply earlier units, firms gain producer surplus.
Marginal costThe additional cost of producing one extra unit of output.
Producer surplus
The supply curve shows the minimum price at which firms are willing to supply each unit. At market price P*, producers receive more than this minimum on earlier units. The area above the supply curve and below P*, up to Q*, represents producer surplus.
Adapted from Cambridge International AS & A Level Economics, Second Edition, by Peter Smith (with Adam Wilby and Mila Zasheva).
How to interpret producer surplus: In a competitive market the supply curve can be viewed as the marginal cost of producing extra units. Producer surplus is the amount producers receive above the minimum needed to supply those units. The book describes it as the surplus earned above the minimum that would have kept firms in the market.
Consumer and producer surplus together
| Concept | Area on a standard diagram | What it represents |
|---|---|---|
| Consumer surplus | Below demand and above market price | Benefit to consumers beyond what they pay. |
| Producer surplus | Above supply and below market price | Benefit to producers beyond the minimum required to supply. |
Entry and exit of firms
A rise in demand can raise the equilibrium price and increase producer surplus for existing firms. If there are no strong barriers to entry, the prospect of surplus and profit attracts new firms. Market supply shifts right, pushing price down and raising total output. Conversely, a sustained fall in demand can reduce producer surplus and encourage firms to leave; market supply shifts left.
Demand risesPrice and producer surplus rise in the short run.
→Entry becomes attractiveNew firms add to market supply.
→Supply shifts rightPrice is pushed down as the market adjusts in the longer run.
Exam warning: consumer surplus is not the same as total spending, and producer surplus is not the same as total revenue or profit. Each measures a surplus relative to willingness to pay or willingness to supply.
Chapter 4 revision checklist
Define market equilibrium and equilibrium price.
Explain how a shortage creates upward pressure on price.
Explain how a surplus creates downward pressure on price.
Distinguish curve shifts from movements along curves during adjustment.
Show the effect of an increase and decrease in demand.
Show the effect of an increase and decrease in supply.
Analyse simultaneous shifts in demand and supply.
Recognise when a price or quantity outcome is indeterminate.
Define and apply alternative demand.
Define and apply joint demand.
Define and apply derived demand.
Explain derived demand in labour and foreign exchange markets.
Define and apply joint supply.
Explain the signalling function of price.
Explain the incentive function of price.
Explain the rationing function of price.
Explain how prices guide resource allocation.
Define and identify consumer surplus.
Define and identify producer surplus and marginal cost.
Explain how entry and exit of firms alter market supply in the longer run.
20 questions. Each answer is marked immediately with a short explanation of why it is correct or incorrect.