What you'll learn
This guide covers the fundamental economic forces that determine prices and quantities in markets. You'll learn how demand and supply interact to establish market equilibrium, what causes curves to shift, and how to apply these concepts to real-world scenarios. This topic forms the foundation for understanding how markets operate and is essential for success in your WJEC GCSE Economics exam.
Key terms and definitions
Demand — the quantity of a good or service that consumers are willing and able to purchase at different price levels during a given time period.
Supply — the quantity of a good or service that producers are willing and able to offer for sale at different price levels during a given time period.
Equilibrium price — the price at which the quantity demanded equals the quantity supplied, clearing the market with no excess demand or supply.
Ceteris paribus — a Latin phrase meaning "all other things being equal," used when analyzing the effect of one variable while assuming others remain constant.
Complement goods — products that are typically consumed together, such as smartphones and data plans, where demand for one affects demand for the other.
Substitute goods — products that can replace each other in consumption, such as butter and margarine, where an increase in price of one increases demand for the other.
Price elasticity of demand (PED) — a measure of how responsive the quantity demanded is to a change in price, calculated as the percentage change in quantity demanded divided by the percentage change in price.
Excess supply — occurs when quantity supplied exceeds quantity demanded at the current price, creating a surplus that puts downward pressure on prices.
Core concepts
The law of demand
The law of demand states that as the price of a good or service increases, the quantity demanded falls, ceteris paribus. This creates a downward-sloping demand curve when plotted on a graph with price on the vertical axis and quantity on the horizontal axis.
This inverse relationship exists for several reasons:
- Income effect: when prices rise, consumers' real purchasing power falls, so they can afford to buy less
- Substitution effect: higher prices make alternatives relatively cheaper, encouraging consumers to switch to substitutes
- Law of diminishing marginal utility: each additional unit consumed provides less satisfaction, so consumers only buy more at lower prices
Individual demand curves can be aggregated to create market demand curves by adding up quantities demanded by all consumers at each price level.
The law of supply
The law of supply states that as the price of a good or service increases, the quantity supplied rises, ceteris paribus. This creates an upward-sloping supply curve.
Producers are willing to supply more at higher prices because:
- Higher prices increase potential profits, making production more attractive
- Existing producers can afford to use more expensive production methods to increase output
- Higher prices attract new firms into the market
- Opportunity costs are more easily covered at higher price levels
Market supply represents the total quantity all producers are willing to offer at different prices.
Market equilibrium
Market equilibrium occurs where the demand and supply curves intersect. At the equilibrium price, the quantity buyers wish to purchase exactly matches the quantity sellers wish to sell. This is also called the market-clearing price.
At prices above equilibrium:
- Quantity supplied exceeds quantity demanded
- Excess supply (surplus) develops
- Competition among sellers drives prices down toward equilibrium
At prices below equilibrium:
- Quantity demanded exceeds quantity supplied
- Excess demand (shortage) develops
- Competition among buyers drives prices up toward equilibrium
Markets naturally tend toward equilibrium through these price adjustment mechanisms, though in reality prices may fluctuate around equilibrium rather than settling permanently.
Factors causing shifts in demand
A movement along the demand curve occurs when price changes. A shift of the entire demand curve occurs when non-price factors change:
Factors that shift demand rightward (increase demand):
- Rise in consumer income (for normal goods)
- Fall in consumer income (for inferior goods like value supermarket brands)
- Rise in price of substitutes (e.g., coffee price rises, increasing tea demand)
- Fall in price of complements (e.g., games console price falls, increasing games demand)
- Increase in population or target demographic
- Changing consumer tastes and preferences favoring the product
- Effective advertising campaigns
- Expected future price increases (consumers buy now)
- Seasonal factors (e.g., ice cream in summer)
Factors that shift demand leftward (decrease demand):
- Fall in consumer income (for normal goods)
- Rise in price of complements
- Fall in price of substitutes
- Negative publicity or health concerns
- Changing fashions and trends away from the product
- Expected future price decreases (consumers delay purchases)
Factors causing shifts in supply
A movement along the supply curve occurs when price changes. A shift of the entire supply curve occurs when non-price factors change:
Factors that shift supply rightward (increase supply):
- Improved production technology reducing costs
- Fall in costs of raw materials or components
- Fall in wages or other production costs
- Reduction in indirect taxes (VAT, excise duties)
- Introduction or increase of subsidies
- Favorable weather conditions (for agricultural products)
- Increase in number of suppliers in the market
- Expected future price decreases
Factors that shift supply leftward (decrease supply):
- Higher raw material costs (e.g., oil price rises affecting plastics)
- Wage increases raising labor costs
- Introduction or increase of indirect taxes
- Removal or reduction of subsidies
- Poor weather or natural disasters
- Firms leaving the market
- Stricter regulations increasing compliance costs
Price elasticity of demand
Price elasticity of demand measures the responsiveness of quantity demanded to price changes. The formula is:
PED = % change in quantity demanded ÷ % change in price
The value is typically negative (inverse relationship) but economists often refer to it in absolute terms.
Elastic demand (PED > 1): quantity demanded changes proportionately more than price. A price rise causes total revenue to fall.
Inelastic demand (PED < 1): quantity demanded changes proportionately less than price. A price rise causes total revenue to rise.
Unit elastic demand (PED = 1): quantity demanded changes proportionately equal to price. Total revenue stays constant.
Factors affecting PED:
- Availability of substitutes: more substitutes make demand more elastic (consumers can easily switch)
- Necessity vs luxury: necessities tend to be inelastic; luxuries more elastic
- Proportion of income: expensive items tend to be more elastic
- Time period: demand becomes more elastic over time as consumers find alternatives
- Brand loyalty: strong loyalty makes demand more inelastic
- Addiction: addictive products have highly inelastic demand
Understanding PED helps businesses make pricing decisions and helps governments predict the impact of taxation.
Worked examples
Example 1: Identifying equilibrium (2 marks)
Question: The market for strawberries has a demand schedule and supply schedule as shown:
| Price (£ per kg) | Quantity demanded (kg) | Quantity supplied (kg) |
|---|---|---|
| 5.00 | 200 | 800 |
| 4.00 | 400 | 600 |
| 3.00 | 600 | 600 |
| 2.00 | 800 | 400 |
State the equilibrium price and quantity.
Mark scheme answer:
- Equilibrium price = £3.00 per kg (1 mark)
- Equilibrium quantity = 600 kg (1 mark)
Explanation: Equilibrium occurs where quantity demanded equals quantity supplied, which is at £3.00 where both equal 600 kg.
Example 2: Analyzing a demand shift (4 marks)
Question: A government health campaign successfully convinces consumers that drinking orange juice daily improves immunity. Explain the likely effect on the market for orange juice. Use a demand and supply diagram in your answer.
Mark scheme answer:
- The health campaign increases consumer preference for orange juice (1 mark)
- This causes the demand curve to shift rightward/to the right (1 mark)
- At the original price, there is now excess demand/shortage (1 mark)
- This causes the equilibrium price to rise and equilibrium quantity to increase (1 mark)
Diagram should show: original demand and supply curves intersecting at equilibrium; new demand curve shifted right; new higher equilibrium price and quantity labeled.
Example 3: Price elasticity of demand calculation (6 marks)
Question: A cinema increases ticket prices from £8.00 to £10.00. Weekly ticket sales fall from 5,000 to 4,000.
(a) Calculate the price elasticity of demand. (4 marks) (b) Explain whether the cinema's total revenue increased or decreased. (2 marks)
Mark scheme answer:
(a)
- % change in quantity = [(4,000 - 5,000) ÷ 5,000] × 100 = -20% (1 mark)
- % change in price = [(10 - 8) ÷ 8] × 100 = +25% (1 mark)
- PED = -20% ÷ 25% = -0.8 (1 mark)
- Therefore demand is price inelastic (1 mark)
(b)
- Original revenue: £8 × 5,000 = £40,000
- New revenue: £10 × 4,000 = £40,000
- Total revenue stayed the same/very close to unit elastic (1 mark), because the percentage fall in quantity demanded was less than the percentage rise in price (1 mark)
Alternative calculation showing revenue unchanged also acceptable.
Common mistakes and how to avoid them
Confusing shifts with movements: A change in price causes a movement along a curve, not a shift. Only non-price factors shift curves. Always check whether the question specifies a price change or another factor.
Drawing curves incorrectly: Demand curves slope downward (top-left to bottom-right); supply curves slope upward. Always label axes clearly with "Price" on the vertical and "Quantity" on the horizontal. Mark equilibrium points with E or E₁, E₂.
Mixing up complements and substitutes: Complements are used together (increase in price of A decreases demand for B). Substitutes replace each other (increase in price of A increases demand for B). Use clear examples in your mind like bread/butter vs. Pepsi/Coca-Cola.
Forgetting ceteris paribus: When analyzing one factor's effect, you must assume all other factors remain constant. Explicitly state this assumption in longer answers to show economic thinking.
Incorrect PED interpretation: Remember that PED is usually negative, but we discuss it in absolute terms. Also, don't confuse elastic with inelastic — elastic means responsive (>1), inelastic means unresponsive (<1).
Not explaining the process: In explain/analyze questions, don't just state the outcome. Show the chain of reasoning: initial change → effect on demand or supply → excess demand/supply → price adjustment → new equilibrium.
Exam technique for "Demand and Supply"
Command words matter: "State" requires a simple answer (1 mark). "Explain" requires reasoning showing cause and effect (2-4 marks). "Analyze" requires examining how/why something happens with logical steps (4-6 marks). "Evaluate" requires weighing up arguments with a supported judgment (6-12 marks).
Use diagrams effectively: When a question says "use a diagram," you must draw one to access full marks. Label all curves (D, S, D₁, S₁), axes (Price, Quantity), and equilibrium points (E, E₁). Add arrows showing shifts. Brief annotations help examiners follow your reasoning.
Apply to context: Generic answers score lower marks. Use details from the question — if it's about chocolate bars, mention chocolate specifically. Reference real examples like supermarkets, petrol prices, or housing markets when appropriate to show application.
Structure PED answers: Always calculate first (show working), interpret the number (elastic/inelastic), then explain the revenue/business implications. This logical structure ensures you don't miss mark-worthy points.
Quick revision summary
Demand shows quantity consumers will buy at different prices (downward-sloping); supply shows quantity producers will sell (upward-sloping). Equilibrium occurs where they intersect. Price changes cause movements along curves. Non-price factors (income, tastes, costs, technology, taxes) shift entire curves. Excess demand raises prices; excess supply lowers them. Price elasticity of demand measures responsiveness to price changes: elastic (>1) means responsive, inelastic (<1) means unresponsive. Availability of substitutes, necessity, and time period affect elasticity. Understanding these mechanisms explains how markets allocate resources.