Supply and demand explain how buyers and sellers interact in a market. Demand shows how much people are willing and able to buy at different prices, while supply shows how much producers are willing and able to sell. Together, they help determine the price of goods such as food, phones, gasoline, and concert tickets.
Understanding this model helps students explain shortages, surpluses, price changes, and real-world economic decisions.
On a supply and demand graph, price is shown on the vertical axis and quantity is shown on the horizontal axis. The demand curve usually slopes downward because buyers tend to purchase more when prices are lower. The supply curve usually slopes upward because producers are willing to sell more when prices are higher.
The point where the curves cross is market equilibrium, where quantity demanded equals quantity supplied.
Key Facts
- Demand curve: as price decreases, quantity demanded usually increases.
- Supply curve: as price increases, quantity supplied usually increases.
- Equilibrium occurs where quantity demanded = quantity supplied.
- A shortage occurs when quantity demanded > quantity supplied at a given price.
- A surplus occurs when quantity supplied > quantity demanded at a given price.
- Market revenue can be estimated by Revenue = Price × Quantity sold.
Vocabulary
- Demand
- Demand is the amount of a good or service buyers are willing and able to purchase at different prices.
- Supply
- Supply is the amount of a good or service producers are willing and able to sell at different prices.
- Equilibrium price
- The equilibrium price is the price at which quantity demanded equals quantity supplied.
- Shortage
- A shortage happens when buyers want to buy more of a product than sellers are willing to provide at the current price.
- Surplus
- A surplus happens when sellers want to sell more of a product than buyers are willing to purchase at the current price.
Common Mistakes to Avoid
- Confusing demand with quantity demanded is wrong because demand refers to the whole curve, while quantity demanded is one amount at one price.
- Confusing supply with quantity supplied is wrong because supply refers to the whole curve, while quantity supplied is one amount at one price.
- Saying price always causes the demand curve to shift is wrong because a change in price usually moves along the demand curve, while factors like income, tastes, and substitutes shift it.
- Assuming equilibrium means everyone gets what they want is wrong because equilibrium only means the amount buyers purchase equals the amount sellers sell at that price.
Practice Questions
- 1 At a price of $8, buyers demand 120 sandwiches and sellers supply 80 sandwiches. Is there a shortage or surplus, and how many sandwiches is it?
- 2 A market has an equilibrium price of $15 and an equilibrium quantity of 200 units. Estimate total market revenue at equilibrium using Revenue = Price × Quantity sold.
- 3 A new technology lowers the cost of producing electric bikes. Explain which curve shifts, the direction of the shift, and the likely effect on equilibrium price and quantity.