2.2.1—Law of supply

The law of supply states that, ceteris paribus, a higher price causes a greater quantity supplied and a lower price causes a smaller quantity supplied over a stated period.

The own-price change alters the producer's incentive to offer units for sale while factor costs, technology, taxes, expectations and firm numbers remain constant. Rising marginal cost is the deeper HL explanation in Objective 2.2.2.

Identify the good's own price change, hold non-price determinants fixed, and predict a change in quantity supplied in the same direction.

If tomatoes rise from $2 to$3 per kilogram while production conditions are unchanged, growers may increase quantity supplied from 500 to 650 kilograms per day.

Supply means willingness and ability to sell, not all physical stock. The law is ceteris paribus and does not explain an observed change when production conditions also change.