Supply & Demand

What are supply and demand?

Supply and demand are the two forces that set prices in a competitive market. The demand curve shows how much buyers want at each price — usually more when it's cheaper. The supply curve shows how much sellers will offer — usually more when the price is high enough to be worth it. They cross at the equilibrium: the one price where the quantity buyers want exactly matches the quantity sellers provide, so there is neither a shortage nor a surplus. When something shifts a whole curve — a change in incomes, tastes, input costs, or the number of sellers — the equilibrium moves, and price and quantity adjust to clear the market again. Price controls interfere with this. A price ceiling set below equilibrium, like rent control, creates a shortage, because buyers want more than sellers will supply at that low price. A price floor set above equilibrium, like a minimum wage, creates a surplus. Understanding the equilibrium — and how shifts and controls move it — is the foundation of nearly all of microeconomics.

How the simulation shows it

You drag the supply and demand curves and watch the equilibrium price and quantity move in real time, then add a price ceiling or floor and see the resulting shortage or surplus appear. Doing it yourself makes the logic of market-clearing concrete rather than abstract.

Common misconceptions

• A price isn't set by sellers alone. It emerges from both sides at once — the equilibrium is where buyers' willingness to pay meets sellers' willingness to sell.
• A change in price doesn't "shift" the demand curve. Moving along a fixed curve is a change in quantity demanded; the whole curve shifts only when something other than the price changes, like income or tastes.
• Price controls don't just transfer money. A binding ceiling or floor changes the quantity actually traded, creating a shortage or surplus and destroying some of the gains from trade.

Frequently asked questions

What is market equilibrium?

It is the price at which the quantity buyers want to buy exactly equals the quantity sellers want to sell, so there is no shortage and no surplus.

What causes a shortage?

A shortage happens when the price is held below equilibrium — for example by a price ceiling — so buyers want more than sellers are willing to supply at that price.

What is the difference between a shift in demand and a movement along the curve?

A movement along the curve is caused by a price change (a change in quantity demanded). A shift of the whole curve is caused by something else — income, tastes, prices of related goods — changing demand at every price.

Why does a price ceiling cause problems?

If it is set below the equilibrium price, buyers demand more than sellers supply, so the good runs short and some buyers who would happily pay the market price can't get it.

Is a higher price always bad for buyers?

Not necessarily. A higher equilibrium price often reflects genuine scarcity or higher costs; forcing the price down with a ceiling can leave buyers worse off by creating shortages.