The interaction of demand and supply
The interaction of demand and supply: build the core model, apply it to evidence and qualify conclusions where outcomes depend on context.
Section 2.4 · The interaction of demand and supply
9708 section 2.4; AS Level. Checked against Cambridge's 2026–2028 Version 2 (latest version published December 2025) and 2029 Version 1 (published September 2026). Cambridge states that neither version has significant changes affecting teaching. Always follow the syllabus for the candidate's examination year.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What defines market equilibrium?
What you will learn
- Define equilibrium and disequilibrium
- Analyse curve shifts and equilibrium changes
- Explain linked markets and the allocation roles of price
Core model and terms
Market equilibrium occurs where planned quantity demanded equals planned quantity supplied. A shortage or surplus is disequilibrium at the current price; price changes can signal scarcity, ration demand and incentivise a supply response.
Apply and evaluate
Markets can be linked: goods may be complements or substitutes, factors may have derived demand, and joint products may share production. A change in one market can affect another, but trace each link and state the conditions before predicting the final outcome.
Pause and explain
Which statement best applies the interaction of demand and supply?
Worked example
A rise in the price of a complement reduces demand for another good. Explain the chain.
Show the worked solution
- The complement becomes less attractive to buy together
- Demand for the related good shifts left
- Its equilibrium price and quantity tend to fall
Answer The complement becomes less attractive to buy together → Demand for the related good shifts left → Its equilibrium price and quantity tend to fall
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A rise in the price of a complement reduces demand for another good. Explain the chain.
Give me a hint
Identify the relevant idea in the interaction of demand and supply, then trace the economic chain.
Compare my reasoning
- The complement becomes less attractive to buy together
- Demand for the related good shifts left
- Its equilibrium price and quantity tend to fall
The complement becomes less attractive to buy together Demand for the related good shifts left Its equilibrium price and quantity tend to fall
Look for these in your work
- The complement becomes less attractive to buy together
- Demand for the related good shifts left
- Its equilibrium price and quantity tend to fall
At the current price, buyers want 120 units and firms offer 90. Identify the imbalance.
Give me a hint
Compare planned demand and supply at that price.
Compare my reasoning
- Quantity demanded exceeds quantity supplied by 30
- There is a shortage
- Buyers may bid price upward
Quantity demanded exceeds quantity supplied by 30 There is a shortage Buyers may bid price upward
Look for these in your work
- Quantity demanded exceeds quantity supplied by 30
- There is a shortage
- Buyers may bid price upward
A higher wage increases costs for bakeries. Predict a likely effect on bread supply, other things equal.
Give me a hint
Trace a factor-market change into the product market.
Compare my reasoning
- Wages are a production cost
- Bread supply may shift left
- Equilibrium price tends to rise and quantity to fall
Wages are a production cost Bread supply may shift left Equilibrium price tends to rise and quantity to fall
Look for these in your work
- Wages are a production cost
- Bread supply may shift left
- Equilibrium price tends to rise and quantity to fall
A rise in demand for beef increases demand for cattle feed. Identify the relationship and mechanism.
Give me a hint
Ask why the input is demanded.
Compare my reasoning
- Feed is an input used to produce beef
- Its demand is derived from beef production
- More beef production may raise feed demand
Feed is an input used to produce beef Its demand is derived from beef production More beef production may raise feed demand
Look for these in your work
- Feed is an input used to produce beef
- Its demand is derived from beef production
- More beef production may raise feed demand
Common mistakes
- Predicting a final market outcome before identifying which curve shifts and why.
What can you explain now?
What defines market equilibrium?
Compare with the explanation
Planned quantity demanded equals planned quantity supplied
Equilibrium is the equality of planned quantities, not an equal distribution of output.
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Make it stick
Before moving on, explain the central idea in the interaction of demand and supply and state one condition that could change its effect.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.