Supply and demand explain how buyers and sellers interact in markets. This cheat sheet helps students read market graphs, identify equilibrium, and predict how prices and quantities change. It is useful for class notes, homework, test review, and understanding real-world price changes.
The focus is on clear rules, graph interpretation, and common market outcomes.
Key Facts
- The law of demand states that as price rises, quantity demanded falls, and as price falls, quantity demanded rises, all else equal.
- The law of supply states that as price rises, quantity supplied rises, and as price falls, quantity supplied falls, all else equal.
- Market equilibrium occurs where quantity demanded equals quantity supplied, so Qd = Qs.
- A shortage occurs when price is below equilibrium and quantity demanded is greater than quantity supplied, so Qd > Qs.
- A surplus occurs when price is above equilibrium and quantity supplied is greater than quantity demanded, so Qs > Qd.
- A rightward shift of demand increases equilibrium price and equilibrium quantity if supply does not change.
- A rightward shift of supply decreases equilibrium price and increases equilibrium quantity if demand does not change.
- Price elasticity of demand can be estimated as elasticity = percent change in quantity demanded / percent change in price.
Vocabulary
- Demand
- Demand is the quantity of a good or service that consumers are willing and able to buy at different prices.
- Supply
- Supply is the quantity of a good or service that producers are willing and able to sell at different prices.
- Equilibrium Price
- Equilibrium price is the market price where quantity demanded equals quantity supplied.
- Shortage
- A shortage is a market condition where consumers want to buy more than producers are willing to sell at the current price.
- Surplus
- A surplus is a market condition where producers want to sell more than consumers are willing to buy at the current price.
- Elasticity
- Elasticity measures how strongly quantity demanded or supplied responds to a change in price.
Common Mistakes to Avoid
- Confusing a movement along a curve with a shift of the curve is wrong because a price change causes movement, while non-price factors shift the entire curve.
- Labeling the demand curve as upward sloping is wrong because demand usually slopes downward as higher prices lead consumers to buy less.
- Thinking any high price creates a shortage is wrong because a shortage happens when price is below equilibrium, not simply when price feels expensive.
- Forgetting that supply and demand shifts affect both price and quantity is wrong because equilibrium changes in two dimensions on the graph.
- Treating equilibrium as a goal chosen by sellers is wrong because equilibrium is the point where buyers' plans and sellers' plans match in the market.
Practice Questions
- 1 At a price of $6, quantity demanded is 120 units and quantity supplied is 80 units. Is there a shortage or surplus, and how many units is it?
- 2 A market has Qd = 100 - 5P and Qs = 20 + 3P. Find the equilibrium price and quantity.
- 3 If demand for concert tickets increases while supply stays the same, what happens to equilibrium price and equilibrium quantity?
- 4 Explain why a new production technology might lower the market price of a good even if consumer demand does not change.
Understanding Supply, Demand & Market Equilibrium
A market graph is a model of choices, not a record of every sale. Buyers compare a product's value to the money they must give up. Sellers compare the selling price to their costs, including materials, wages, rent, and the value of other work they could do.
The buyers most willing and able to pay tend to purchase first. The sellers able to provide the item at the lowest cost tend to sell first. The market price helps coordinate these separate decisions without one person directing them.
This matters because a price carries information about scarcity. A high price can tell consumers to conserve a limited item and can tell firms that producing more may be worthwhile.
Students often confuse a movement along a curve with a shift of the whole curve. A movement along demand happens when the product's own price changes. A shift happens when another influence changes buyers' willingness or ability to purchase at every price.
Income, tastes, population, expectations, prices of substitutes, and prices of related goods can shift demand. A substitute is an alternative, such as bus rides and train rides. A complement is used together with the product, such as printers and ink.
Supply can shift when input costs, technology, weather, taxes, regulations, expectations, or the number of sellers changes. Before predicting a graph change, identify exactly what changed. This prevents many common mistakes.
Markets do not always reach a balanced outcome instantly. When too many people want an item at the current price, they may wait in lines, search several stores, bid against one another, or accept smaller amounts. Sellers may run out of inventory.
When firms have more goods than customers want, inventory can pile up. Sellers may cut prices, offer discounts, reduce future production, or leave the market. Government rules can slow or change this adjustment.
A price ceiling can keep a price from rising, which may create persistent shortages. A price floor can keep a price from falling, which may leave unsold goods or unused labor. Housing, concert tickets, gasoline, and seasonal food prices show these pressures in daily life.
Elasticity describes how strongly buyers react to a price change. Demand is usually more elastic when close substitutes exist, when the item is not essential, when it takes a large share of a household budget, and when buyers have time to adjust. Restaurant meals are often more elastic than basic medicines.
A small price increase may cause little change in medicine purchases because people need it and may have few alternatives. Businesses pay attention to elasticity because a higher price does not always bring in more total revenue. If quantity purchased falls by a larger percentage than price rises, revenue can fall.
When using percentage changes, compare the size of both changes and keep the direction clear. Elasticity can differ at different prices and for different groups of consumers.