What you'll learn
Demand and supply is the central model of microeconomics, and almost everything later in the syllabus is built on it. Demand describes how much buyers are willing and able to purchase at each price; supply describes how much sellers are willing and able to offer. Where the two are equal, the market is in equilibrium — the price at which the quantity buyers want exactly matches the quantity sellers will provide.
The crucial discipline in this topic is distinguishing a movement along a curve from a shift of it. A change in the price of the good itself moves you along the curve. A change in anything else — income, the price of a substitute, the cost of an input, the weather — shifts the whole curve. Candidates who blur the two produce answers that cannot be marked, because the diagram no longer says what they think it says.
The model also explains what happens when a price is not allowed to settle at equilibrium. A maximum price below equilibrium creates a shortage; a minimum price above it creates a surplus. Both are common policies in Caribbean economies and both have predictable consequences.
By the end you should be able to derive an equilibrium from demand and supply schedules, distinguish movements from shifts, analyse the effect of a change in any determinant, and evaluate price controls.
Key terms and definitions
Effective demand — the quantity buyers are willing and able to purchase at a given price. Desire without purchasing power is not demand.
Law of demand — as price rises, quantity demanded falls, ceteris paribus.
Law of supply — as price rises, quantity supplied rises, ceteris paribus.
Equilibrium — the price and quantity at which quantity demanded equals quantity supplied, so there is no tendency to change.
Excess demand (shortage) — quantity demanded exceeds quantity supplied, which happens below equilibrium price.
Excess supply (surplus) — quantity supplied exceeds quantity demanded, which happens above equilibrium price.
Substitute — a good bought instead of another; a rise in the price of one raises demand for the other.
Complement — a good bought alongside another; a rise in the price of one lowers demand for the other.
Maximum (ceiling) price — a legal limit above which price may not rise. Effective only if set below equilibrium.
Minimum (floor) price — a legal limit below which price may not fall. Effective only if set above equilibrium.
Core concepts
Why the curves slope as they do
Demand slopes downward for two reasons worth stating separately. The income effect: a lower price leaves the buyer with more real purchasing power, so more can be bought. The substitution effect: a lower price makes the good cheaper relative to alternatives, so buyers switch towards it.
Supply slopes upward because a higher price makes production more profitable, drawing existing firms to produce more and attracting new firms into the market. In the short run it also reflects rising marginal costs as output expands.
Movement along against shift — the distinction that decides the answer
Only a change in the price of the good itself causes a movement along its own curve. That movement is called a change in quantity demanded or quantity supplied, and the terminology matters in a mark scheme.
Everything else shifts the curve, and that is a change in demand or supply.
Demand shifts with: income, the price of substitutes and complements, tastes and fashion, population size and structure, expectations of future prices, and government policy such as a tax on the buyer.
Supply shifts with: the cost of inputs, technology, the number of firms, weather and natural events, taxes and subsidies on producers, and the price of goods the firm could produce instead.
A rightward shift of demand raises both equilibrium price and quantity. A rightward shift of supply raises quantity but lowers price. Being able to state those four outcomes without a diagram is a reliable way to check your own work.
Finding equilibrium
Given schedules or equations, equilibrium is where quantity demanded equals quantity supplied. Below that price, buyers want more than sellers offer, and the shortage bids the price up. Above it, sellers offer more than buyers want, and the surplus pushes the price down. The market therefore tends towards equilibrium without anyone directing it — which is the price mechanism at work.
Price controls
A maximum price set below equilibrium is intended to keep an essential good affordable. It creates a shortage, and because price can no longer ration the good, something else must: queues, waiting lists, rationing by sellers, or a black market where the good changes hands above the legal price. The policy helps those who obtain the good and does nothing for those who cannot.
A minimum price set above equilibrium is intended to protect producers' incomes, most often in agriculture, or workers' incomes through a minimum wage. It creates a surplus, which the government must buy, store or destroy if the policy is to hold.
Neither control is simply good or bad. The evaluation turns on who gains, who loses, what the surplus or shortage costs, and whether a different instrument — a subsidy, an income transfer — would achieve the aim at lower cost.
Consumer and producer surplus
Consumer surplus is the difference between what buyers were willing to pay and what they actually paid — the area below the demand curve and above the price. Producer surplus is the difference between the price received and the minimum sellers would have accepted — the area above the supply curve and below the price.
Together they measure the total gain from trade, and they are the tool used later to judge whether a market failure or a policy has made society better or worse off.
Worked examples
Example 1 — Finding equilibrium from schedules (6 marks)
In the market for bottled water in a Trinidad town, demand is Qd = 2,000 − 100P and supply is Qs = 100P + 200, with P in dollars.
At equilibrium, Qd = Qs: 2,000 − 100P = 100P + 200 1,800 = 200P P = $9.
Quantity: Qd = 2,000 − (100 × 9) = 1,100 units. Check with supply: Qs = (100 × 9) + 200 = 1,100. They agree, so the answer is right.
Total revenue at equilibrium = $9 × 1,100 = $9,900.
Example 2 — Disequilibrium and the adjustment (5 marks)
Using the same market, suppose price is $12.
Qd = 2,000 − 1,200 = 800. Qs = 1,200 + 200 = 1,400. Excess supply = 1,400 − 800 = 600 units.
Sellers cannot sell all they have produced, so they cut price. As price falls, quantity demanded rises and quantity supplied falls, closing the gap until $9 is reached.
Now suppose price is $8. Qd = 1,200 and Qs = 1,000, so there is excess demand of 200 units. Buyers bid the price up until equilibrium is restored. The market corrects itself from either direction.
Example 3 — A maximum price (6 marks)
The government sets a maximum price of $7 for bottled water, believing $9 is too high.
Qd = 2,000 − 700 = 1,300. Qs = 700 + 200 = 900. Shortage = 1,300 − 900 = 400 units.
Consequences: 900 units are sold instead of 1,100, so fewer people obtain the good than before the policy. The 900 who do buy pay $7 rather than $9 and gain $2 each. Those unable to buy gain nothing. Non-price rationing appears — queues, allocation by sellers to favoured customers, or a black market at above $7.
Evaluation: the policy transfers a benefit to some buyers and reduces the total quantity traded. If the aim is to make water affordable to poor households, a targeted subsidy or income transfer would achieve it without creating a shortage. Stating that alternative is what turns description into evaluation.
Example 4 — A minimum price (5 marks)
The government instead sets a minimum price of $11 to protect producers.
Qd = 2,000 − 1,100 = 900. Qs = 1,100 + 200 = 1,300. Surplus = 1,300 − 900 = 400 units.
Producers who sell receive $11 rather than $9, but only 900 units are sold rather than 1,100. The 400-unit surplus must be bought by the government, stored or destroyed if the price is to be maintained, and that is a cost to the taxpayer.
Note the symmetry with Example 3: a $2 departure from equilibrium in either direction produces a 400-unit imbalance and reduces the quantity traded by 200. Price controls reduce the volume of trade whichever way they push.
Example 5 — Shift or movement? (4 marks)
Classify each: (a) the price of bottled water falls; (b) a heatwave raises demand for cold drinks; (c) the cost of plastic bottles rises; (d) a cheaper substitute enters the market.
(a) Movement along the demand curve — a change in quantity demanded, caused by the good's own price. (b) Rightward shift of demand — a change in tastes and conditions, not price. (c) Leftward shift of supply — an input cost, raising equilibrium price and lowering quantity. (d) Leftward shift of demand — buyers switch away, lowering both equilibrium price and quantity.
Common mistakes and how to avoid them
Shifting a curve when the good's own price changes. Own price moves you along the curve, never shifts it.
Confusing the terms. A change in quantity demanded is a movement; a change in demand is a shift.
Saying a maximum price makes a good more available. It reduces the quantity traded and creates a shortage.
Setting a ceiling above equilibrium or a floor below it. Such a control has no effect at all, and a question sometimes tests exactly that.
Forgetting to check the equilibrium against both equations. Substituting into supply as well as demand catches most algebraic errors.
Treating desire as demand. Effective demand requires willingness and ability to pay.
Describing a price control without evaluating it. Say who gains, who loses, and what alternative policy exists.
How this links to your Internal Assessment
Demand and supply supplies the analytical framework for most workable Internal Assessment questions, because almost any market issue can be framed as a shift in one curve or the other.
If you study a price change in a real market — food, transport fares, construction materials — identify which curve moved and what moved it, then predict the effect on price and quantity and test that prediction against what actually happened. Where the outcome differs from the model, that discrepancy is the most interesting thing in your assessment, and explaining it earns more than a confirmation would.
If your topic is a price control, collect evidence on the quantity traded rather than only the price. The model predicts a fall in volume, and data on shortages, queues or informal sales is what tests it.
Draw your diagrams from your own data where you can, and label the axes with the actual good and the actual price range rather than generic P and Q.
Exam technique for demand, supply and market equilibrium
Diagrams carry substantial marks and they must be labelled: both axes, both curves, the original and new equilibrium price and quantity, and arrows showing the direction of any shift. An unlabelled diagram earns very little regardless of how correct the shape is.
Work through shifts in a fixed order: identify which curve moves, decide the direction, then read off the effect on price and quantity. Saying it in words before drawing prevents the common error of shifting the wrong curve.
Where equations are given, solve them algebraically and then check the answer in the other equation. It takes one line and catches most mistakes.
Command words follow the usual pattern. Explain why the demand curve slopes downward wants the income and substitution effects. Analyse the effect of a rise in input costs wants the supply shift and both resulting changes. Discuss a maximum price wants the shortage, the non-price rationing and the distributional effect. Evaluate wants all of that plus a comparison with an alternative policy and a stated conclusion.
Quick revision summary
- Effective demand requires willingness and ability to pay.
- Demand slopes down through the income and substitution effects; supply slopes up because higher prices make production more profitable.
- Own price → movement along the curve (change in quantity demanded or supplied).
- Anything else → shift of the curve (change in demand or supply).
- Demand shifters: income, substitutes and complements, tastes, population, expectations.
- Supply shifters: input costs, technology, number of firms, weather, taxes and subsidies.
- Demand shifts right: price and quantity both rise. Supply shifts right: quantity rises, price falls.
- Equilibrium is where Qd = Qs; check the answer in both equations.
- A maximum price below equilibrium creates a shortage and non-price rationing.
- A minimum price above equilibrium creates a surplus someone must buy, store or destroy.
- Both controls reduce the quantity traded; evaluate by naming who gains, who loses and what alternative exists.
- Consumer surplus sits below the demand curve above the price; producer surplus above the supply curve below the price.