Demand, supply and market equilibrium

The big picture

A market brings together the buyers of a good and its sellers, and the demand and supply model is the course’s way of predicting what they will agree on. Almost every question in Theme 1, and a surprising number elsewhere, comes down to deciding which curve moves, in which direction, and what that does to price and quantity. Learn to drive this model and you have a way into taxes, subsidies, labour markets, exchange rates and commodity shocks before you have studied any of them formally.

The theory

The demand curve shows the quantity buyers are willing and able to purchase at each price; it slopes downward because a lower price makes a good more affordable and better value relative to the alternatives. The supply curve shows the quantity firms are willing to offer at each price; it slopes upward because a higher price rewards production and covers rising marginal costs. Where the two curves cross, the market is in equilibrium: the quantity buyers want to buy equals the quantity firms want to sell, and there is no pressure on price to change.

Keep one distinction watertight, because examiners test it constantly. A change in the good’s own price causes a movement along a curve. A change in any other condition, income, tastes, the price of a related good, costs of production, shifts the whole curve to a new position. Confusing the two is the most common way a good answer quietly goes wrong.

Application

The model earns its keep when you point it at a real market. When wholesale gas became scarce across Europe, supply shifted left in a market where demand is inelastic in the short run, and the model predicts exactly what households experienced: a large rise in price and only a modest fall in quantity, because few could quickly stop heating their homes. Housing tells the opposite story about which curve matters: demand in growing cities keeps shifting right while planning rules hold supply close to fixed, so the adjustment falls almost entirely on price.

In an answer, the application is what turns a textbook diagram into an argument about the case in front of you. Name the market, say which curve shifts and why, and state which elasticity decides whether the burden lands on price or on quantity. That single habit, tying every step of the model to the actual market in the question, is what the application marks reward.

Building the paragraph

Analysis marks (AO3) are earned by the links between steps, not by the diagram alone. Open with a topic sentence that carries the whole argument, then set out the reasoning so each step causes the next, with nothing assumed:

Rising incomes should raise both the price of city-centre housing and the quantity traded, because housing is a normal good whose demand shifts right against a supply that planning constraints hold steep.

1. Incomes rise, and for a normal good this increases the quantity demanded at every price.
2. The demand curve therefore shifts to the right, from D₁ to D₂.
3. At the old price P₁ there is now excess demand: buyers want more than firms are supplying.
4. This shortage bids the price up; as price rises, firms extend supply (a movement along S) and some buyers are priced out (a movement along D₂).
5. The market settles at a new equilibrium E₂, with a higher price P₂ and a higher quantity Q₂.

Key terms

Term	Precise definition	Where it earns marks
Effective demand	The quantity buyers are willing and able to buy at a given price in a given period.	Defining demand precisely (AO1); "able" rules out mere desire.
Movement along vs shift	A change in own price moves along the curve; a change in another condition shifts the whole curve.	The distinction that keeps an analysis answer accurate.
Excess demand	A shortage: at the current price, quantity demanded exceeds quantity supplied.	The mechanism that drives price back to equilibrium (AO3).
Equilibrium	The price and quantity at which the market clears and there is no tendency to change.	The anchor for every "what happens if..." question.