Supply and Demand
Supply and demand is the foundational model in economics that explains how prices are determined in a market. When demand increases and supply stays the same, prices rise. When supply increases and demand stays the same, prices fall.
Key Takeaways
- The law of demand says that as price increases, quantity demanded decreases (and vice versa) — the demand curve slopes downward.
- The law of supply says that as price increases, quantity supplied increases — the supply curve slopes upward.
- Market equilibrium occurs where the supply and demand curves intersect — this is the price where quantity supplied equals quantity demanded.
- A shift in demand or supply (caused by external factors like income, technology, or tastes) moves the entire curve, changing both equilibrium price and quantity.
How Supply and Demand Actually Works
Supply and demand is the first model you learn in economics, and it shows up everywhere — from AP exams to university midterms to real-world policy debates. The core idea is simple: buyers and sellers interact in a market, and the price adjusts until the quantity buyers want to purchase equals the quantity sellers want to sell.
The demand side represents consumers. The demand curve slopes downward because as the price of a good rises, fewer people are willing and able to buy it. Think about it: if the price of coffee doubles tomorrow, you might switch to tea or just drink less coffee. That's the law of demand in action.
The supply side represents producers. The supply curve slopes upward because as the price of a good rises, it becomes more profitable for firms to produce it, so they supply more. If coffee prices double, more farmers want to grow coffee beans.
Where the two curves cross is the equilibrium — the market-clearing price. At this price, every unit that a buyer wants to purchase has a willing seller. There's no shortage and no surplus.
The real power of this model comes from understanding shifts. A shift happens when something other than price changes. For example, if consumer income rises, the demand curve shifts right (people want to buy more at every price). If new technology reduces production costs, the supply curve shifts right (firms can produce more at every price). Each shift creates a new equilibrium with a different price and quantity.
How Supply and Demand Shows Up on AP Micro
On the AP Microeconomics exam, supply and demand appears in both multiple-choice and free-response sections. You'll see questions testing whether you can identify the correct shift direction and predict the resulting change in equilibrium price and quantity.
The most common question format gives you a scenario — "The government imposes a new tax on producers" or "Consumer income increases for a normal good" — and asks you to determine what happens to price and quantity. Draw the graph, shift the correct curve, and find the new equilibrium.
For free-response questions (FRQs), always draw a clearly labeled graph. Label your axes (Price on y-axis, Quantity on x-axis), label both curves (S and D), show the initial equilibrium (P₁, Q₁), show the shift, and label the new equilibrium (P₂, Q₂). Graphs earn you points even when your written explanation is incomplete.
Pro tip: when both supply AND demand shift simultaneously, you can determine the direction of change for one variable (price or quantity) but the other is ambiguous. The AP exam loves testing this — don't claim you know both unless the magnitudes are given.
Common Mistakes Students Make
- Confusing a shift with a movement along the curve. A change in the price of the good causes movement along the curve. A change in anything else (income, tastes, technology, input costs) causes the entire curve to shift.
- Shifting the wrong curve. If the scenario affects consumers (income, preferences, population), shift demand. If it affects producers (input costs, technology, regulations), shift supply.
- Forgetting the "ambiguous" answer. When both supply and demand shift, one variable's change is indeterminate unless you know the relative magnitudes of the shifts.
Related Topics
Frequently Asked Questions
A change in quantity demanded is a movement along the demand curve caused by a change in the good's own price. A change in demand is a shift of the entire demand curve caused by external factors like income, tastes, expectations, or the price of related goods.
The supply curve shifts due to changes in input costs (wages, raw materials), technology improvements, government regulations or taxes, number of sellers in the market, and producer expectations about future prices.
Supply and demand is the foundation of roughly 15-20% of the AP Microeconomics exam. It appears directly in Unit 1 and 2 questions, and the concepts underpin nearly every other topic including market structures, factor markets, and government intervention.