Competitive Market Equilibrium: IB Economics 2.3 notes

Equilibrium and disequilibrium

A competitive market is in equilibrium at the price where quantity demanded equals quantity supplied. This is the market-clearing price: everything offered for sale is bought, and every buyer willing to pay that price is satisfied. Graphically it is where the demand and supply curves intersect.

Any other price is a disequilibrium. Left to a free market, price adjusts automatically toward equilibrium, which is why economists call it a self-correcting or stable position.

Shortages and surpluses

If price is set below equilibrium, quantity demanded exceeds quantity supplied and there is a shortage (excess demand). Buyers compete for scarce goods, bidding the price up until the shortage disappears.

If price is set above equilibrium, quantity supplied exceeds quantity demanded and there is a surplus (excess supply). Unsold stock builds up, so sellers cut the price until the surplus disappears. In both cases the price signal drives the market back to equilibrium.

Worked numeric example

Suppose demand is Qd = 100 - 2P and supply is Qs = 20 + 2P, with P in dollars and Q in thousands of units. Equilibrium is where Qd = Qs, so 100 - 2P = 20 + 2P. Solving: 80 = 4P, so P = 20 dollars. Substituting back, Qd = 100 - 2(20) = 60, and Qs = 20 + 2(20) = 60, confirming equilibrium quantity is 60 thousand units.

Now test a price ceiling of 10 dollars. Qd = 100 - 2(10) = 80 and Qs = 20 + 2(10) = 40, so there is a shortage of 80 - 40 = 40 thousand units. At a price of 30 dollars, Qd = 40 and Qs = 80, a surplus of 40 thousand units. This shows numerically how prices away from 20 dollars fail to clear the market.

Functions of the price mechanism

Prices coordinate a market through three functions. The signalling function: prices convey information about where goods are wanted, a rising price signals that a good is scarce or newly popular. The incentive function: a higher price rewards producers for supplying more and encourages consumers to economise. The rationing function: when a good is scarce, a higher price rations it to those most willing and able to pay.

Together these functions allocate resources without any central planner, which Adam Smith described as the invisible hand. When demand for a good rises, its price rises, signalling and incentivising producers to shift resources into that good.

Consumer and producer surplus

Consumer surplus is the difference between the maximum price consumers are willing to pay and the price they actually pay. On the diagram it is the triangular area below the demand curve and above the market price. Producer surplus is the difference between the price producers receive and the minimum they were willing to accept, shown as the area above the supply curve and below the market price.

In the worked example, with equilibrium at P = 20 and Q = 60, consumer surplus is the triangle between the demand curve and the price line, and producer surplus the triangle between the price line and the supply curve. The sum of the two is total welfare or community surplus.

Allocative efficiency

A competitive market equilibrium is allocatively efficient: resources are allocated so that the marginal benefit to consumers equals the marginal cost to producers (where the demand and supply curves cross). At this point community surplus (consumer plus producer surplus) is maximised, and it is impossible to make one party better off without making another worse off.

This links to the key concept of efficiency, and its limits link to equity: an efficient market outcome is not necessarily fair, since it depends on the existing distribution of income. Market failures, covered later, are cases where the free-market equilibrium is not allocatively efficient.

Common Paper mistakes

Describing a shortage as a surplus or vice versa. A price below equilibrium causes a shortage (excess demand); a price above causes a surplus (excess supply).

Shading surplus areas incorrectly: consumer surplus is above the price and below demand; producer surplus is below the price and above supply. Mislabelled areas lose marks.

Forgetting to show the return to equilibrium: after a shift, always mark the new intersection and state the new equilibrium price and quantity.

How this is examined

• Equilibrium and surplus appear on Paper 1 (theory and diagrams) and Paper 2 (data response), and the numeric and area calculations are HL Paper 3 staples.
• When asked to evaluate the price mechanism, name and explain all three functions (signalling, incentive, rationing) with a linked example rather than just listing them.
• Always shade and label consumer and producer surplus correctly on the diagram; examiners check that consumer surplus sits above price and producer surplus below it.
• For HL calculations, show every step: set Qd = Qs, solve for P, substitute back for Q, then use area = 0.5 x base x height for each triangle.

Key terms

market equilibriumconsumer surplusproducer surplusallocative efficiencyinvisible hand