Supply vs Demand
Supply and Demand are two Supply & Demand concepts in AP Economics that students often mix up. Supply is the willingness and ability of producers to sell different quantities of a good at different prices, holding all else constant. Demand is the willingness and ability of consumers to buy different quantities of a good at different prices, holding all else constant. Here is how they compare side by side.
The supply of a good represents the different quantities producers are willing and able to sell at each price level. Supply is determined by factors like technology, input costs, and government policies. The law of supply states that supply curves slope upward, showing a positive relationship between price and quantity supplied.
The demand for a good represents the different quantities consumers are willing and able to purchase at each price level. Demand is determined by factors like consumer income, preferences, and the prices of related goods. The law of demand states that demand curves slope downward, showing an inverse relationship between price and quantity demanded.
Supply vs Demand: The Two Sides of the Same Market
| Supply | Demand | |
|---|---|---|
| Whose behavior it describes | Sellers and producers | Buyers and consumers |
| Relationship with the good's own price | Positive: a higher price brings a larger quantity | Negative: a higher price brings a smaller quantity |
| Slope of the curve | Upward to the right | Downward to the right |
| What a higher price means to that side | A better reward for producing the next unit | A larger sacrifice to obtain the next unit |
| Factors that shift it | Input prices, technology, producer taxes and subsidies, number of sellers | Income, tastes, prices of related goods, expectations, number of buyers |
| Welfare area it generates | Producer surplus, above the curve and below the price | Consumer surplus, below the curve and above the price |
| Effect of an increase in it on equilibrium | Price falls and quantity rises | Price rises and quantity rises |
The two curves answer different questions about the same price
Supply and demand are two separate plans that happen to be drawn on the same axes. Demand answers what buyers would purchase at each possible price. Supply answers what sellers would offer at each possible price. Neither is a single number and neither is a forecast. Both are full schedules that exist at every price at once, which is why the whole curve, rather than one point on it, is the thing you shift. Take an illustrative market for concert tickets where quantity demanded is 900 minus 10 times the price and quantity supplied is 100 plus 10 times the price. At a price of 30, buyers want 600 tickets and sellers offer 400. At a price of 50, buyers want 400 and sellers offer 600. Setting the two expressions equal gives 900 minus 10P equal to 100 plus 10P, so 800 equals 20P, the equilibrium price is 40 and 500 tickets change hands. Those figures are invented to keep the arithmetic clean. Notice what stayed fixed across all three prices. Buyer income, seller costs and the number of people in the market never moved, so only the quantities moved. You can pull the two schedules apart yourself and watch the point of intersection travel at /sandbox/supply-demand.
An increase in supply and an increase in demand push price in opposite directions
Students often fuse the two curves into one idea and then cannot say which way price goes. Keep them apart and the answer falls out. An increase in demand means buyers want more at every price, so that curve moves right, the gap opening at the old price bids price up, and quantity rises. An increase in supply means sellers offer more at every price, so that curve moves right, the gap opening at the old price pushes price down, and quantity still rises. Higher quantity is the shared result. The direction of price is what reveals which curve moved. This gives a fast diagnostic for questions that describe an event without naming a curve. If price and quantity move the same way, demand moved. If they move opposite ways, supply moved. A frost that destroys part of an orange crop lifts price and cuts quantity, so it hit supply. A health report that makes oranges fashionable lifts both, so it hit demand. When both curves move at once, one of the two equilibrium values is indeterminate and the correct answer says so instead of guessing. The full shift lists sit at /glossary/determinants-of-demand and /glossary/determinants-of-supply.
Frequently asked questions
What is the difference between supply and demand?
Supply is the quantity sellers are willing and able to offer at each possible price, and demand is the quantity buyers are willing and able to purchase at each possible price. Supply slopes upward because a higher price rewards the extra cost of producing more, while demand slopes downward because a higher price makes each unit a bigger sacrifice. The two meet at the equilibrium price, the only price where the quantities match.
What happens to price if supply and demand both increase?
Equilibrium quantity definitely rises, but the change in equilibrium price cannot be determined without knowing which curve shifted more. The demand increase pushes price up while the supply increase pushes price down, so the two effects work against each other. On a free response question, write that the price change is indeterminate and explain why.
Why does the supply curve slope up while the demand curve slopes down?
Supply slopes up because each additional unit tends to cost more to produce, so sellers need a higher price before that unit is worth making. Demand slopes down because each additional unit is worth less to a buyer than the one before, and because a higher price makes rival goods look better. Both slopes describe sliding along a curve, never a shift of one.
Live Supply and Demand graph. Drag the curves, or open the full version.
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