Demand — AP Macroeconomics Study Guide

For: AP Macroeconomics candidates sitting AP Macroeconomics.

Covers: Definition of demand, the law of demand, movement along vs. shifts of the demand curve, determinants of demand shifts, individual vs. market demand, and core concepts tested on both AP Macroeconomics MCQ and FRQ sections.

You should already know: Scarcity and the concept of willingness and ability to pay; basic graphing of linear curves on price-quantity axes; the difference between endogenous and exogenous variables.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Macroeconomics style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Demand?

Demand is the foundational relationship of all market analysis in AP Macroeconomics, and makes up approximately 10-15% of Unit 1 (Basic Economic Concepts) exam weight, translating to 2-4 MCQ points and often a component of early FRQ questions. This topic appears in both MCQ and FRQ sections, and serves as a building block for all subsequent analysis, from market equilibrium to aggregate demand. Formally, demand is defined as the relationship between the price of a specific good or service and the quantity of that good or service consumers are both willing and able to buy at each possible price, holding all other factors constant (ceteris paribus). A key distinction: demand is not just a desire for a good, it requires the ability to pay as well. 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 typically the independent variable in the demand function. You may see the term "inverse demand" on the exam, which just means P is written as a function of $Q_d$, instead of the other way around. It is critical to separate demand (the entire curve) from quantity demanded (a single point on the curve at a specific price).

2. Law of Demand and Individual Demand

The law of demand is the core empirical relationship that underpins all demand analysis: holding all other factors constant, as the price of a good increases, quantity demanded of the good decreases, and as price decreases, quantity demanded increases. This creates an inverse relationship between price and quantity demanded, which is why demand curves slope downward. Two explanations for the law of demand are the substitution effect (higher prices make consumers switch to cheaper alternative goods) and the income effect (higher prices reduce consumers’ real purchasing power, so they can buy less of the good). For an individual consumer, a linear demand function is almost always written as: $$Q_d=a-bP$$ where $a$ is the horizontal intercept (quantity demanded when price = $0),and$b$isapositiveconstantthatgivestheinverserelationship(theslopeof$Q_d$ with respect to P is negative).

Worked Example

Lila buys bakery cookies each week. Her demand schedule for cookies is:

Write Lila’s linear demand function and confirm it follows the law of demand.

1. Start with the standard linear individual demand form: $Q_d=a-bP$, where $b>0$.
2. Calculate the slope coefficient $b$: $b=-\frac{\Delta Q_d}{\Delta P}=-\frac{7-9}{2-1}=2$, so the form becomes $Q_d=a-2P$.
3. Solve for the intercept $a$ by plugging in a known pair of values: when $P=1$, $Q_d=9$, so $9=a-2(1)\to a=11$.
4. Verify the function $Q_d=11-2P$ fits all other points: for $P=4$, $Q_d=11-8=3$, which matches the schedule.
5. The coefficient on P is -2, which is negative: as price increases, quantity demanded decreases, confirming the function follows the law of demand.

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.

3. Individual vs. Market Demand

Market demand is the total quantity demanded by all consumers in a market at every possible price. To get market demand from individual demand curves, you horizontally sum individual quantities demanded at each price. This means you add up how much all consumers want to buy at a given price, rather than adding prices for a given quantity (that is vertical summation, used only for public goods, not private market demand). If individual demand functions are linear, the market demand function is simply the sum of the individual demand functions, resulting in a new linear function with a larger intercept and steeper (more negative) slope when plotted on the standard P-Q axes. This concept is often tested in both MCQ and early FRQ parts that require deriving market demand.

Worked Example

In a small town, there are only two consumers buying cookies: Lila, with demand $Q_{d,L}=11-2P$, and Ben, with demand $Q_{d,B}=9-P$. Derive the market demand function for cookies, assuming all prices are low enough that both consumers buy positive quantities.

1. Market demand follows the rule: $Q_{d,market}=Q_{d,L}+Q_{d,B}$, because we sum quantities at each price.
2. Substitute the individual demand functions into the equation: $Q_{d,market}=(11-2P)+(9-P)$.
3. Combine like terms to simplify: $(11+9)+(-2P-P)=20-3P$.
4. Verify with an example: at $P=2$, Lila buys 7, Ben buys 7, so total market quantity is 14. The market function gives $20-3(2)=14$, which matches.
5. The final market demand function is $Q_{d,market}=20-3P$.

Exam tip: If the question gives you a kinked individual demand (where one consumer drops out of the market at high prices), split the market demand into segments, but for almost all AP problems, you can just sum the functions directly when both consumers buy positive quantities.

4. Movements Along vs. Shifts of the Demand Curve

This is the most commonly tested distinction for demand on the AP exam. A movement along the demand curve (also called a change in quantity demanded) is caused only by a change in the own price of the good in question — the only endogenous variable on the demand graph axes. An increase in own price causes an upward/leftward movement along the curve (lower quantity demanded), while a decrease in own price causes a downward/rightward movement (higher quantity demanded). A shift of the entire demand curve (also called a change in demand) is caused by a change in any non-price (exogenous) determinant of demand. A rightward shift means an increase in demand (higher quantity demanded at every price), while a leftward shift means a decrease in demand (lower quantity demanded at every price). The main determinants of shifts tested on the AP exam are: consumer income, prices of related goods (substitutes/complements), consumer tastes/preferences, expectations of future prices/income, and the number of buyers in the market.

Worked Example

For each event below, identify whether it causes a movement along or a shift of the demand curve for artisanal bread, and which direction. (i) The price of wheat, an input to bread, falls, lowering the price of artisanal bread. (ii) Consumer incomes rise, and artisanal bread is a normal good. (iii) The price of sourdough starter, a complement to baking homemade bread (a substitute for store-bought artisanal bread), falls.

1. First apply the core test: does the event change the own price of artisanal bread? If yes → movement; if no → shift.
2. Event (i): The price of artisanal bread itself falls, so this is a change in quantity demanded, causing a downward/rightward movement along the existing demand curve.
3. Event (ii): Income is a non-price determinant. For a normal good, higher income increases demand at every price, so this causes a rightward shift of the entire demand curve.
4. Event (iii): Price of a complement for a substitute changed, not the own price of artisanal bread. Lower price of sourdough starter increases demand for homemade bread, so consumers buy less store-bought artisanal bread at every price. This causes a leftward shift of the demand curve for artisanal bread.

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.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Calling a change in quantity demanded caused by a price change a "shift in demand". Why: Students mix up terminology: "demand" refers to the entire curve, while "quantity demanded" refers to a single point on the curve, so vocabulary is swapped. Correct move: Always apply the "own price" test: if the change is to the own price of the good, it is a change in quantity demanded (movement along the curve); if not, it is a change in demand (shift of the curve).
• Wrong move: Summing individual demand vertically to get market demand. Why: Students confuse private good market demand with public good demand, which uses vertical summation of individual willingness to pay. Correct move: Always sum quantity demanded at each price (horizontal summation) for market demand of private goods, the only context for demand in Unit 1.
• Wrong move: Stating that an increase in consumer income shifts demand for an inferior good right. Why: Students assume all goods have higher demand when income rises, mixing up normal and inferior good definitions. Correct move: Memorize the rule: inferior goods = demand falls when income rises; normal goods = demand rises when income rises, and write the rule on scrap paper if you get confused.
• Wrong move: Drawing a demand curve with a positive slope because the demand function is $Q_d=a-bP$. Why: Students forget AP convention puts P on the vertical axis, so the function is inverted to get P as a function of Qd. Correct move: Remember AP convention: P = y-axis, Qd = x-axis, standard demand curves always slope downward, so draw it with a negative slope.
• Wrong move: Claiming an expected future increase in the price of gasoline shifts current demand for gasoline left. Why: Students confuse future price changes with current price changes. Correct move: If consumers expect future price to rise, they buy more now at current lower prices, so current demand shifts right; if they expect future price to fall, current demand shifts left.

6. Practice Questions (AP Macroeconomics Style)

Question 1 (Multiple Choice)

Which of the following events will cause a rightward shift in the current demand curve for electric bicycles (e-bikes)? A) A decrease in the cost of lithium batteries reduces the market price of new e-bikes. B) A new government subsidy reduces the price of gasoline, a substitute for e-bikes. C) A new city law requires all university students to use e-bikes for on-campus transportation. D) Both B and C are correct.

Worked Solution: First, a rightward shift of demand requires a non-price determinant that increases demand at every current price. Option A is a change in the own price of e-bikes, so it causes a movement along the demand curve, not a shift, so A is wrong. Option B: a lower price of a substitute (gasoline for gas-powered bikes) means consumers will buy fewer e-bikes at every price, shifting demand left, so B is wrong. Option C: the new law increases the number of buyers in the market for e-bikes, a non-price determinant that increases demand at every price, shifting demand right. The correct answer is C.

Question 2 (Free Response)

The market for iced coffee on a college campus has only two consumers: Javi and Priya. Javi’s weekly demand for iced coffee is $Q_{d,J}=12-2P$, and Priya’s weekly demand is $Q_{d,P}=8-P$. (a) Derive the market demand function for iced coffee. (2 points) (b) Identify the intercept of the market demand curve on the price (vertical) axis and the quantity (horizontal) axis. (2 points) (c) Explain how a decrease in the price of coffee syrup (a complement to iced coffee) will shift the market demand curve for iced coffee, and why. (1 point)

Worked Solution: (a) Market demand is the horizontal sum of individual quantities demanded at each price: $$Q_{d,market}=Q_{d,J}+Q_{d,P}=(12-2P)+(8-P)=20-3P$$ The market demand function is ${\mathbf{Q}}_{\mathbf{d}}=20-3\mathbf{P}$.

(b) To find the price intercept, set $Q_d=0$ and solve for $P$: $0=20-3P\to P=\frac{20}{3}\approx 6.67$. To find the quantity intercept, set $P=0$ and solve for $Q_d$: $Q_d=20-0=20$. The price intercept is $\$6.67$ on the vertical axis, and the quantity intercept is 20 iced coffees per week on the horizontal axis.

(c) Complements are consumed together, so a lower price of coffee syrup increases quantity demanded of iced coffee at every price of iced coffee. Therefore, the market demand curve for iced coffee will shift right.

Question 3 (Application / Real-World Style)

In 2024, a major social media trend popularized "quiet quitting" working multiple jobs, leading more office workers to work from home full-time permanently. Prior to the trend, 70% of office workers commuted to the office 5 days a week. Using demand concepts, explain how this trend changed the current demand for gasoline (used for commuting) and identify the determinant that caused the change.

Worked Solution: This trend did not change the current price of gasoline, so it causes a shift of the entire demand curve for gasoline rather than a movement along the curve. The non-price determinant here is a change in consumer tastes and lifestyle preferences, which reduced the quantity of gasoline demanded at every current price, as fewer workers commute to the office by car daily. As a result, the demand for gasoline decreased, shifting the entire demand curve for gasoline left. In context, this shift explains why average gasoline prices fell in the years following the rise of permanent remote work.

7. Quick Reference Cheatsheet

8. What's Next

This chapter on demand is the foundational prerequisite for all market and macroeconomic analysis in AP Macroeconomics. Immediately after this, you will study supply, then combine supply and demand to build the market equilibrium model, which is used to analyze how prices and quantities adjust to shocks. Without mastering the distinction between movements and shifts in demand, you will not be able to correctly predict market outcomes, which will trip you up on both supply-demand questions and later aggregate demand-aggregate supply analysis. Demand also directly feeds into the concept of aggregate demand, the core of the AD-AS model that explains inflation, unemployment, and long-run economic growth. All the rules for demand shifts learned here extend directly to the determinants of aggregate demand, so mastering this chapter makes that later topic far easier.

• Supply
• Market Equilibrium
• Price Elasticity of Demand
• Aggregate Demand