Macroeconomics · Unit 1: Basic Economic Concepts · 14 min read · Updated 2026-05-11
Demand — AP Macroeconomics
AP Macroeconomics · Unit 1: Basic Economic Concepts · 14 min read
1. Core Definition and the Law of Demand★☆☆☆☆⏱ 4 min
Demand is the foundational relationship for all market and macroeconomic analysis, making up 10-15% of Unit 1 exam weight. Formally, demand is defined as the relationship between the price of a good and the quantity consumers are *both willing and able* to buy at each possible price, holding all other factors constant (ceteris paribus).
By AP convention, price ($P$) is always plotted on the vertical axis, and quantity demanded ($Q_d$) on the horizontal axis, even though $P$ is the independent variable. It is critical to separate demand (the entire curve) from quantity demanded (a single point on the curve at a specific price).
Q_d = a - bP \quad (a > 0, b > 0)
Exam tip: On AP MCQ, always read the question carefully to see if it asks for 'demand' or 'quantity demanded' — this single word changes what answer is correct.
2. Individual vs. Market Demand★★☆☆☆⏱ 4 min
Market demand is the total quantity demanded by all consumers in a market at every possible price. For private goods (the standard case in Unit 1), market demand is derived by horizontally summing individual quantities demanded at each price. Vertical summation (adding prices for a given quantity) is only used for public goods, not private market demand.
Exam tip: If a question has kinked demand (one consumer drops out at high prices), split the market demand into price segments. For most AP problems, you can directly sum individual functions when all consumers buy positive quantities.
3. Movements Along vs. Shifts of the Demand Curve★★☆☆☆⏱ 6 min
This is the most commonly tested distinction on the AP exam. A movement along the demand curve (called a change in quantity demanded) is caused *only* by a change in the own price of the good (the only endogenous variable on the P-Q axes). A shift of the entire demand curve (called a change in demand) is caused by a change in any non-price (exogenous) determinant of demand.
An increase in own price causes an upward/leftward movement along the curve (lower quantity demanded). A rightward shift means an increase in demand (higher quantity at every price), while a leftward shift means a decrease in demand. The main determinants of shifts are: consumer income, prices of related goods (substitutes/complements), tastes/preferences, future expectations, and number of buyers.
Exam tip: Always do the own-price test first before answering. 90% of student errors on this topic come from mixing up movement and shift, and this test eliminates that error immediately.
Common Pitfalls
Why: Students mix up terminology: 'demand' refers to the entire curve, while 'quantity demanded' refers to a single point on the curve
Why: Students confuse private good market demand with public good demand, which uses vertical summation
Why: Students assume all goods have higher demand when income rises, mixing up normal and inferior good definitions
Why: Students forget AP convention puts $P$ on the vertical axis, so the function is inverted to get $P$ as a function of $Q_d$
Why: Students confuse future price changes with current price changes