Lesson preview · Supply
The Law of Supply
~6 min · Free to read
Now flip the script to the other side of the market: sellers. Imagine you run a lemonade stand. If kids will pay 1 dollar per cup, you might bother setting up on a hot Saturday. If they’ll pay 5 dollars per cup, you’d set up every day, buy nicer lemons, hire your little brother, and expand to a second corner. Higher prices make selling more attractive — so you supply more. That’s the law of supply in a nutshell, and it’s a perfect mirror of the law of demand.
Key Term
Law of Supply
All else equal, as the price of a good rises, the quantity supplied rises. As price falls, quantity supplied falls. Price and quantity supplied move in the same direction — a direct (positive) relationship.
Key Term
Quantity Supplied
The specific amount of a good sellers are willing and able to produce and sell at a particular price. Not ‘what they’d like to sell’ — but what they’d actually put on the market at that exact price.
Linear supply: quantity supplied rises with price ().
Why does supply slope up? Three forces push in the same direction whenever price rises:
Key Term
Rising Marginal Cost
The cost of producing one MORE unit of a good. For most firms, marginal cost rises as output grows — making the 100th pizza is harder than the 10th, and the 1,000th harder still. To cover those higher costs, sellers need a higher price.
Reason 1: Rising marginal costs. Squeezing more output isn’t free — you pay overtime, rent extra equipment, source from farther suppliers. Each extra unit costs more than the last. Higher prices justify those costs.
Reason 2: The profit motive. A higher price turns unprofitable production into profitable production. A bakery that loses money making 500 loaves at 4 dollars each might turn a fine profit at 6 dollars.
Reason 3: Entry of new producers. Sustained high prices attract brand-new firms — high Uber fares pull more drivers online, high oil prices fund new drilling companies. New entrants add to total market supply.
Tip
Three reasons, one direction
Rising marginal costs, the profit motive, and entry of new producers all push quantity supplied in the same direction as price. Supply curves slope up because three separate forces line up to say ‘higher price, more units.’
Tip
Supply vs Demand: opposite shapes
Demand slopes down (buyers want less when price rises). Supply slopes up (sellers offer more when price rises). Drawn together, they form the iconic ‘X’ at the heart of microeconomics — and they meet at the equilibrium price.
Check yourself · no marks
Oil prices jump from 60 dollars to 90 dollars per barrel. According to the law of supply, what happens to the quantity producers want to sell?
Commit to one first. Guessing and being wrong beats reading the answer cold.
Worked Example
Uber surge pricing kicks in during a rainy rush hour, raising the fare multiplier from 1.0× to 2.5×. Ten minutes later, way more drivers are on the road. Identify which of the three reasons for upward-sloping supply best explains what happened — and whether more than one applies.
Practice · 1 / 4
According to the law of supply, if the price of concert tickets rises, quantity supplied:
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