Demand & Supply — Short Model Essay

“Explain how demand and supply determine price and quantity, and discuss the model's limits.” (~1 to 1½ pages)

Demand and supply is the basic model economists use to explain how prices and quantities are set in a market. This essay explains how the model works and then looks at where it is useful and where it falls short.

A market is where buyers and sellers meet. In a free market, prices guide their choices automatically — what Adam Smith called the “invisible hand”. Prices do three jobs: they signal what is scarce or wanted, they ration limited goods to those willing to pay, and they give firms an incentive to produce.

The law of demand says that when a good's price rises, people buy less, so the demand curve slopes downward. The law of supply says the opposite for firms: when the price rises, they supply more, so the supply curve slopes upward. The market settles at equilibrium, where demand meets supply, at the price (P superscript *) and quantity (Q superscript *) where the two are equal. If the price is too high there is a surplus and it falls back; if it is too low there is a shortage and it rises. In this way the market corrects itself.

Image summary: This figure is a line graph depicting a market equilibrium model. It features two intersecting lines representing supply and demand, plotted against axes for price and quantity. The point where the upward-sloping supply curve and the downward-sloping demand curve meet is labeled as the equilibrium point, with corresponding values indicated on the price and quantity axes. The intersection demonstrates that market equilibrium occurs at the specific price and quantity where the amount of a good supplied equals the amount demanded.

It is important to separate a movement along a curve from a shift of the curve. A change in the good's own price moves us along the curve. A change in anything else — income, tastes, costs or technology — shifts the whole curve and gives a new equilibrium. For example, higher income shifts demand to the right, raising both price and quantity.

Image summary: This figure is a line graph illustrating an economic model of supply and demand. The graph features a constant upward-sloping supply curve and two downward-sloping demand curves, with an arrow indicating a rightward shift from the initial demand curve to a new one. The intersection points of the supply curve with each demand curve are marked as equilibrium points. The shift in the demand curve leads to a movement along the supply curve, resulting in a higher equilibrium price and a greater equilibrium quantity.

How large these changes are depends on elasticity, which measures how strongly quantity reacts to a change in price. This matters for taxes: an indirect tax shifts the supply curve up, so buyers pay more and sellers receive less, and the quantity falls. The side that reacts less to price — the inelastic side — ends up paying most of the tax.

The model works well, but only when markets are competitive and people are well informed. When they are not, we get market failure — for example pollution, monopoly, or public goods that the market under-provides — and the government may need to step in.

In short, demand and supply, working through the price mechanism, set prices and quantities and let us predict how a market reacts to any change. The model is powerful and largely self-correcting, but it is not perfect, so there remains a clear role for government.

Add This Paragraph If The Question Is About Taxes Q.1

Insert it after the elasticity/tax paragraph, and adjust your conclusion to mention the macro effects.

Taxes come in two types. A direct tax is paid on income or profit, such as income tax or corporation tax, and cannot be passed on to someone else. An indirect tax is paid on spending, such as V.A.T or excise duty, and the seller passes part of it to the buyer through a higher price.

An indirect tax shifts the supply curve up, so the price buyers pay rises, the quantity falls, and the government collects revenue, while some useful trades are lost — the deadweight loss. The wider effects matter too: indirect taxes raise the price level and add to inflation, higher taxes reduce spending and so lower aggregate demand, and although taxes improve the government's budget, indirect taxes tend to hit poorer households harder because they are regressive.

Add This Paragraph If The Question Is About The Shadow Economy Q.2

Insert it after the “movement vs shift” paragraph, and adjust your conclusion to mention South East Europe.

The shadow or “black” economy is activity that is hidden to avoid tax or regulation, such as cash-in-hand work or smuggling. Its existence does not mean the price system has failed — in fact it shows the opposite. Black markets appear where official prices are blocked by taxes, controls or bans, and they still clear at an unofficial price, so they obey the laws of demand and supply rather than breaking them. What a large shadow economy does reveal is weak institutions and heavy taxes. In South East Europe, where it is especially large, this means lost tax revenue, unfair competition for firms that follow the rules, and mismeasured G.D.P, all of which make it harder for the region to catch up with the rest of the E.U.

Begg, D. and Ward, D. Economics for Business, 6th ed. Maidenhead: McGraw-Hill.

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