The determination of equilibrium market prices, how excess demand and excess supply are eliminated, the functions of the price mechanism, and the effect of shifts in demand and supply.

What this dot point is asking

AQA wants you to explain how the equilibrium price and quantity are determined, how disequilibrium (surplus or shortage) is removed, the three functions of the price mechanism, and how shifts in demand or supply change the equilibrium. This is the engine of the whole microeconomics module.

Equilibrium

This is shown where the demand and supply curves intersect, giving the equilibrium price and equilibrium quantity. At this point the plans of buyers and sellers are mutually consistent.

Removing disequilibrium

This self-correcting adjustment means a free market tends back towards equilibrium without any outside direction. Surpluses signal firms to cut price and output; shortages signal them to raise both. The speed of adjustment depends on how quickly buyers and sellers respond.

Functions of the price mechanism

The price mechanism allocates resources through three linked functions, often summarised by Adam Smith's "invisible hand":

• Rationing. When a good is scarce, its price rises, discouraging some buyers so the limited quantity is allocated to those willing and able to pay.
• Signalling. Price changes carry information about changing market conditions to producers and consumers, indicating where resources are over- or under-supplied.
• Incentive. A higher price rewards producers for supplying more and reallocating resources towards that good, while a lower price does the reverse.

Shifts in demand and supply

When demand rises (shifts right) there is initially excess demand at the old price, so price and quantity both rise. When supply rises (shifts right) there is initially excess supply, so price falls and quantity rises. The reverse holds for leftward shifts. When both curves shift, one of price or quantity has an ambiguous outcome that depends on which shift dominates. The size of any change in price versus quantity depends on the elasticities of the curves: inelastic curves produce larger price changes.

Interrelated markets

Prices do not move in isolation. A change in one market ripples through related markets via substitutes, complements, joint supply and derived demand. A rise in the oil price raises the cost of petrol (a complement to cars), lowering car demand, while raising demand for substitutes such as public transport and electric vehicles. A poor wheat harvest raises bread prices and the price of livestock feed, feeding through to meat prices. Tracing these chains, identifying which curve shifts in which market and in which direction, is exactly the kind of multi-step analysis higher-tariff questions reward.

The price mechanism's great strength is that it coordinates millions of independent decisions without central direction, using only price signals. Its limitation is that it ignores externalities, under-provides public and merit goods, and distributes goods according to ability to pay rather than need. This is why even market economies intervene, and why price determination is the foundation on which the market failure and intervention topics build.