This is the canonical content source for Topic 2.4.

Official syllabus coverage

Students must understand:

• 2.4.1 the definition of market equilibrium and disequilibrium;
• 2.4.2 the effects of shifts in demand and supply curves on equilibrium

price and quantity;

• 2.4.3 relationships between different markets:
• joint demand (complements);
• alternative demand (substitutes);
• derived demand;
• joint supply;
• 2.4.4 the functions of price in resource allocation:
• rationing;
• signalling or transmission of preferences;
• incentivising.

Product mastery map

The official syllabus is divided into seven measurable portal skills:

1. define equilibrium and disequilibrium;
2. analyse shortages, surpluses and price adjustment;
3. analyse demand shifts and new equilibrium;
4. analyse supply shifts and new equilibrium;
5. analyse simultaneous shifts and ambiguous outcomes;
6. explain relationships between linked markets;
7. explain the rationing, signalling and incentive functions of price.

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The topic in one idea

Market equilibrium occurs where quantity demanded equals quantity supplied.<br>Changes in demand or supply create disequilibrium, and price changes help<br>coordinate buyers and sellers towards a new equilibrium.

Topic 2.4 brings together:

• demand;
• supply;
• elasticity;
• equilibrium;
• interdependent markets;
• resource allocation.

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1. Market equilibrium

Examination-ready definition

Market equilibrium occurs at the price where quantity demanded equals<br>quantity supplied.

At equilibrium:

• there is no excess demand;
• there is no excess supply;
• there is no inherent pressure for the market price to change;
• planned purchases equal planned sales.

Equilibrium price

The price at which quantity demanded equals quantity supplied.

Equilibrium quantity

The quantity bought and sold at the equilibrium price.

Diagram interpretation

On a standard demand and supply diagram:

• the downward-sloping demand curve intersects the upward-sloping supply curve;
• the intersection is labelled E;
• a horizontal guide identifies equilibrium price Pe;
• a vertical guide identifies equilibrium quantity Qe.

Important qualification

Equilibrium does not mean:

• fair;
• socially optimal;
• affordable to everyone;
• environmentally sustainable;
• permanently fixed.

It only means that planned quantity demanded equals planned quantity supplied at that price.

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2. Disequilibrium

Examination-ready definition

Disequilibrium occurs when quantity demanded does not equal quantity supplied<br>at the current market price.

There are two principal forms:

• excess demand or shortage;
• excess supply or surplus.

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3. Excess demand or shortage

Meaning

A shortage occurs when:

\[ Q_d > Q_s \]

This normally occurs when the current price is below equilibrium.

Diagram logic

At a price below equilibrium:

• consumers wish to buy a relatively large quantity;
• firms wish to sell a relatively small quantity;
• quantity demanded exceeds quantity supplied.

The shortage is measured horizontally:

\[ \text{shortage} = Q_d - Q_s \]

Price-adjustment chain

Price below equilibrium → quantity demanded exceeds quantity supplied →<br>consumers compete for scarce output → firms observe unsatisfied demand → price<br>rises → quantity demanded contracts and quantity supplied extends → shortage<br>narrows → equilibrium is restored.

Non-price rationing during a shortage

Before price adjusts fully, shortages may create:

• queues;
• waiting lists;
• informal rationing;
• favouritism;
• black markets;
• empty shelves.

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4. Excess supply or surplus

Meaning

A surplus occurs when:

\[ Q_s > Q_d \]

This normally occurs when the current price is above equilibrium.

Diagram logic

At a price above equilibrium:

• firms wish to sell a relatively large quantity;
• consumers wish to buy a relatively small quantity;
• unsold stock accumulates.

The surplus is measured horizontally:

\[ \text{surplus} = Q_s - Q_d \]

Price-adjustment chain

Price above equilibrium → quantity supplied exceeds quantity demanded → unsold<br>stock accumulates → firms reduce price → quantity demanded extends and<br>quantity supplied contracts → surplus narrows → equilibrium is restored.

Possible firm responses

Firms may also:

• reduce production;
• offer discounts;
• increase advertising;
• store goods;
• dispose of perishable output;
• leave the market in the long run.

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5. Why markets may not adjust immediately

The price mechanism is a model, and adjustment can take time.

Possible reasons include:

• menu costs;
• contracts;
• imperfect information;
• price controls;
• expectations;
• stock limitations;
• production delays;
• market power;
• government intervention.

A market can therefore remain in disequilibrium for a period.

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6. Increase in demand

An increase in demand shifts the demand curve right from D1 to D2.

At the original price:

• quantity demanded increases;
• quantity supplied has not yet changed;
• a shortage emerges.

Price then rises, causing an extension of supply.

Final result

Increase in demand → equilibrium price rises → equilibrium quantity rises.

Examples

• rising income for a normal good;
• increased popularity;
• population growth;
• higher price of a substitute;
• lower price of a complement.

Analytical chain

Demand shifts right → excess demand at the original price → upward pressure on<br>price → firms extend quantity supplied → consumers contract quantity demanded<br>along the new demand curve → new equilibrium at higher price and quantity.

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7. Decrease in demand

A decrease in demand shifts demand left from D1 to D2.

At the original price:

• quantity demanded falls;
• quantity supplied temporarily exceeds quantity demanded;
• a surplus emerges.

Price falls, causing a contraction of supply.

Final result

Decrease in demand → equilibrium price falls → equilibrium quantity falls.

Examples

• falling income for a normal good;
• changing tastes;
• population decline;
• lower price of a substitute;
• higher price of a complement.

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8. Increase in supply

An increase in supply shifts the supply curve right from S1 to S2.

At the original price:

• firms wish to supply more;
• quantity supplied exceeds quantity demanded;
• a surplus emerges.

Price falls, causing an extension of demand.

Final result

Increase in supply → equilibrium price falls → equilibrium quantity rises.

Examples

• lower production costs;
• improved technology;
• subsidy;
• favourable weather;
• more firms entering the market.

Analytical chain

Supply shifts right → excess supply at the original price → downward pressure<br>on price → consumers extend quantity demanded → firms contract quantity<br>supplied along the new supply curve → new equilibrium at lower price and higher<br>quantity.

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9. Decrease in supply

A decrease in supply shifts supply left from S1 to S2.

At the original price:

• firms supply less;
• quantity demanded exceeds quantity supplied;
• a shortage emerges.

Price rises, causing a contraction of demand.

Final result

Decrease in supply → equilibrium price rises → equilibrium quantity falls.

Examples

• higher production costs;
• indirect tax;
• adverse weather;
• fall in productivity;
• firms leaving the market.

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10. Summary of single-curve shifts

This table should be memorised, but exam answers must still explain the adjustment process.

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11. Role of elasticity in equilibrium changes

Elasticity affects the size of price and quantity changes.

Increase in demand

If supply is inelastic:

• price tends to rise substantially;
• quantity rises relatively little.

If supply is elastic:

• quantity tends to rise substantially;
• price rises relatively little.

Decrease in supply

If demand is inelastic:

• price tends to rise substantially;
• quantity falls relatively little.

If demand is elastic:

• quantity tends to fall substantially;
• price rises relatively little.

Important examination point

The direction of change comes from the shift. The magnitude of change depends partly on elasticity.

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12. Simultaneous shifts

Real markets often experience more than one change at once.

When demand and supply both shift, one outcome may be certain while the other is ambiguous.

Demand increases and supply increases

• equilibrium quantity definitely rises;
• equilibrium price is uncertain.

Price rises if demand increases by more than supply. Price falls if supply increases by more than demand. Price remains unchanged if the shifts offset exactly.

Demand decreases and supply decreases

• equilibrium quantity definitely falls;
• equilibrium price is uncertain.

Demand increases and supply decreases

• equilibrium price definitely rises;
• equilibrium quantity is uncertain.

Demand decreases and supply increases

• equilibrium price definitely falls;
• equilibrium quantity is uncertain.

Summary table

Strong exam language

Do not simply write "cannot tell."

Write:

The final outcome depends on the relative magnitude of the shifts.

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13. Joint demand: complements

Definition

Joint demand exists when two goods are consumed together, so an increase in<br>demand for one tends to increase demand for the other.

Examples:

• cars and petrol;
• printers and ink;
• games consoles and games;
• smartphones and mobile-data plans.

Market relationship

Suppose the price of cars falls.

• quantity demanded for cars rises;
• more cars are used;
• demand for petrol shifts right;
• petrol equilibrium price and quantity tend to rise, ceteris paribus.

Link to XED

Complements normally have a negative cross elasticity of demand.

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14. Alternative demand: substitutes

Definition

Alternative demand exists when goods can be used instead of one another, so a<br>rise in the price of one tends to increase demand for the other.

Examples:

• tea and coffee;
• bus and rail travel;
• butter and margarine;
• competing smartphone brands.

Market relationship

Suppose the price of coffee rises.

• quantity demanded for coffee contracts;
• consumers switch towards tea;
• demand for tea shifts right;
• tea equilibrium price and quantity tend to rise.

Link to XED

Substitutes normally have a positive cross elasticity of demand.

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15. Derived demand

Definition

Derived demand occurs when demand for a factor of production or intermediate<br>good arises from demand for the final product it helps produce.

Examples:

• demand for construction workers derives from demand for buildings;
• demand for steel derives partly from demand for cars and machinery;
• demand for airline pilots derives from demand for air travel;
• demand for cocoa derives from demand for chocolate.

Analytical chain

Demand for final product rises → firms plan greater output → demand for<br>relevant factors or inputs rises → input demand curve shifts right.

Important distinction

Derived demand is not the same as joint demand.

• joint demand concerns goods consumed together;
• derived demand concerns inputs required to produce output.

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16. Joint supply

Definition

Joint supply occurs when the production of one good creates another good or<br>by-product from the same production process.

Examples:

• beef and leather;
• wool and mutton;
• petrol and other petroleum products;
• wheat and straw.

Market relationship

Suppose demand for beef rises.

• beef price may rise;
• cattle production increases;
• leather supply also increases;
• leather supply curve shifts right;
• leather equilibrium price tends to fall and quantity tends to rise, ceteris

paribus.

Important distinction

Joint supply is a supply-side relationship. It is not the same as complementary demand.

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17. Linked-market comparison

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18. Price as a rationing mechanism

Meaning

When a good is scarce, price helps decide who receives it.

If demand exceeds supply:

• price rises;
• some consumers reduce demand or leave the market;
• scarce output is allocated to those willing and able to pay.

Critical evaluation

Price rationing reflects:

• willingness to pay;
• ability to pay.

It does not necessarily reflect:

• need;
• fairness;
• social importance.

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19. Price as a signal

Meaning

Prices transmit information about:

• consumer preferences;
• relative scarcity;
• changing costs;
• market opportunities.

Example

Demand for electric bicycles rises → price rises → firms receive a signal that<br>consumers value more electric bicycles.

A falling price may signal:

• weak demand;
• excess supply;
• reduced scarcity;
• lower profitability.

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20. Price as an incentive

Producer incentive

A higher price may increase potential profit and encourage firms to:

• expand output;
• enter the market;
• invest;
• move resources into production;
• innovate.

Consumer incentive

A higher price encourages consumers to:

• reduce quantity demanded;
• seek substitutes;
• conserve the product.

Integrated price-mechanism chain

Preference changes → demand changes → price transmits the signal → consumers<br>and firms receive incentives → quantities adjust → resources are reallocated.

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21. Price mechanism and resource allocation

Suppose demand for solar panels rises.

1. Demand shifts right.
2. A shortage appears at the original price.
3. Price rises.
4. Higher price signals stronger consumer preference.
5. Potential profit incentivises firms to expand.
6. Labour and capital move into solar-panel production.
7. Output rises.
8. Price rations the available panels among buyers.
9. A new equilibrium is reached.

This example integrates all three functions of price.

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22. Why equilibrium can change continuously

Markets are dynamic.

Equilibrium may change because of:

• income;
• tastes;
• technology;
• costs;
• expectations;
• taxes;
• weather;
• population;
• related markets;
• global events.

The market may be constantly moving towards a new equilibrium without remaining at one point for long.

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23. Primary-product application

Agricultural markets often combine:

• volatile demand;
• inelastic short-run supply;
• weather-driven supply shifts.

Example:

Poor harvest → supply shifts left → price rises strongly and quantity falls.

If demand is inelastic, farmer revenue may rise even though output falls. This links Topic 2.4 to elasticity.

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24. Transport-market application

A fall in the price of rail travel may:

• increase quantity demanded for rail travel;
• reduce demand for substitute bus travel;
• reduce demand for petrol if some car journeys are replaced;
• raise derived demand for train drivers if services expand.

One initial market change can therefore create several linked-market effects.

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25. Foreign-exchange application

Demand for a currency and supply of a currency can be analysed using the same market model.

• demand for exports may increase demand for the domestic currency;
• demand for imports may increase supply of the domestic currency;
• equilibrium determines the market exchange rate under a floating system.

Detailed exchange-rate analysis appears in Topic 6.4.

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26. Common exam traps

Trap 1: Equilibrium means socially desirable

Wrong. Equilibrium only means quantity demanded equals quantity supplied.

Trap 2: A shortage is shown by a vertical gap

Wrong. Shortages and surpluses are measured horizontally between quantities at a given price.

Trap 3: A price below equilibrium creates a surplus

Wrong. It creates a shortage.

Trap 4: A price above equilibrium creates a shortage

Wrong. It creates a surplus.

Trap 5: An increase in demand means movement along demand

Wrong. It is a rightward shift of the demand curve.

Trap 6: Increased supply raises equilibrium price

Wrong, ceteris paribus: price falls and quantity rises.

Trap 7: Simultaneous shifts always give a definite price and quantity result

Wrong. One result may be ambiguous.

Trap 8: Joint demand means substitutes

Wrong. Joint demand refers to complements.

Trap 9: Derived demand means consumers want two goods together

Wrong. Derived demand arises from demand for final output.

Trap 10: Joint supply means two goods have similar supply curves

Wrong. It means they emerge from the same production process.

Trap 11: Price signalling and price incentivising are identical

They are linked but distinct:

• signalling communicates information;
• incentives motivate action.

Trap 12: Price rationing allocates by need

It allocates according to willingness and ability to pay.

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27. Paper 1 technique

Common tasks include:

• identify equilibrium;
• calculate shortages and surpluses;
• identify new equilibrium after a shift;
• distinguish linked-market relationships;
• classify functions of price;
• identify certain and ambiguous outcomes from simultaneous shifts.

Shift-analysis method

1. Identify the market.
2. Identify whether demand or supply changes.
3. Identify direction of shift.
4. State the temporary disequilibrium at the original price.
5. Explain price adjustment.
6. State the new equilibrium price and quantity.

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28. Paper 2 technique

Model four-mark answer: increase in demand

An increase in demand shifts the demand curve to the right. At the original<br>price, quantity demanded exceeds quantity supplied, creating a shortage. This<br>places upward pressure on price. As price rises, quantity supplied extends and<br>quantity demanded contracts along the new demand curve until a new equilibrium<br>is reached at a higher price and quantity.

Model six-mark answer: joint supply

Joint supply occurs when two goods are produced from the same production<br>process. Beef and leather are jointly supplied because greater cattle<br>production produces more of both. If demand for beef rises, cattle production<br>may increase. This shifts the supply of leather to the right, causing the<br>equilibrium price of leather to fall and the equilibrium quantity to rise,<br>ceteris paribus.

Model eight-mark answer: simultaneous shifts

Question:

Explain the likely effect on the market for electric cars of an increase in<br>consumer income and a fall in battery-production costs.

Model structure:

• if electric cars are normal goods, demand shifts right;
• lower battery costs shift supply right;
• equilibrium quantity definitely rises;
• equilibrium price is uncertain;
• price depends on relative magnitude of shifts;
• elasticity affects size of changes.

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29. Active recall

1. Define market equilibrium.
2. Define disequilibrium.
3. What creates a shortage?
4. What creates a surplus?
5. How does price adjust during a shortage?
6. How does price adjust during a surplus?
7. What happens to equilibrium after demand increases?
8. What happens after demand decreases?
9. What happens after supply increases?
10. What happens after supply decreases?
11. How does elasticity affect the magnitude of equilibrium changes?
12. What happens if demand and supply both increase?
13. What happens if demand rises and supply falls?
14. Define joint demand.
15. Define alternative demand.
16. Define derived demand.
17. Define joint supply.
18. Give one example of each linked-market relationship.
19. Explain price rationing.
20. Explain price signalling.
21. Explain price incentivising.
22. Why can equilibrium be socially undesirable?
23. Why might market adjustment take time?
24. How is a shortage measured on a diagram?
25. How can a change in one market affect several other markets?

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30. One-minute revision

Equilibrium

Quantity demanded equals quantity supplied.

Below equilibrium price

Shortage → price rises.

Above equilibrium price

Surplus → price falls.

Single shifts

• demand right → price up, quantity up;
• demand left → price down, quantity down;
• supply right → price down, quantity up;
• supply left → price up, quantity down.

Linked markets

• joint demand → complements;
• alternative demand → substitutes;
• derived demand → input linked to final output;
• joint supply → products produced together.

Price functions

• rationing;
• signalling;
• incentivising.

Best exam chain

Shift → disequilibrium at original price → price pressure → movement along the<br>other curve → new equilibrium.