Demand

AP Macroeconomics· AP Macroeconomics CED — 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).

📘 Definition

Law of Demand

Holding all other factors constant, there is an inverse relationship between the price of a good and quantity demanded: as price increases, quantity demanded decreases, and vice versa. This explains why all standard demand curves slope downward.

Example:

A $1 increase in the price of coffee reduces the number of coffees consumers buy per week.

Q_d = a - bP \quad (a > 0, b > 0)

📐 Worked Example

Lila buys bakery cookies each week. Her demand schedule for cookies is: | Price per cookie ($) | Quantity Demanded (cookies per week) | |----------------------|----------------------------------------| | 1 | 9 | | 2 | 7 | | 3 | 5 | | 4 | 3 | Write Lila's linear demand function and confirm it follows the law of demand.

1
Start with the standard linear individual demand form, where $b > 0$ for the law of demand:
$Q_d = a - bP$
2
Calculate the slope coefficient $b$ :
$b = -\frac{\Delta Q_d}{\Delta P} = -\frac{7-9}{2-1} = 2$
3
Solve for the intercept $a$ by plugging in the known pair $P = 1, Q_{d} = 9$ :
$9 = a - 2(1) \rightarrow a = 11$
4
Verify with $P = 4$ : $Q_{d} = 11 - 2 (4) = 3$ , which matches the schedule. The negative coefficient on $P$ confirms 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.

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.

📐 Worked Example

In a small town, there are only two consumers buying cookies: Lila, with demand $Q_{d, L} = 11 - 2 P$ , 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 for private goods is the sum of individual quantities at each price:
$Q_{d,market} = Q_{d,L} + Q_{d,B}$
2
Substitute the individual functions:
$Q_{d,market} = (11 - 2P) + (9 - P)$
3
Combine like terms to simplify:
$(11 + 9) + (-2P - P) = 20 - 3P$
4
Verify at $P = 2$ : Lila buys 7, Ben buys 7, total 14. The function gives $20 - 3 (2) = 14$ , which matches. Final market demand: $Q_{d, ma r k e t} = 20 - 3 P$

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.

📐 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): The change affects a substitute for artisanal bread, not the own price. Lower sourdough starter price increases demand for homemade bread, so consumers buy less store-bought artisanal bread at every price, causing a leftward shift.

✓ Quick check

Test your understanding with this AP-style MCQ:

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.
Reveal answer
C —
A is a change in own price, so it causes a movement along the curve, not a shift. B reduces demand for e-bikes (shifts left), so only C is correct. C increases the number of buyers, a non-price determinant that shifts demand right.

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.

4. Common Pitfalls

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

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

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

Wrong move:

Drawing a demand curve with a positive slope because the demand function is $Q_{d} = a - b P$

Why:

Students forget AP convention puts $P$ on the vertical axis, so the function is inverted to get $P$ as a function of $Q_{d}$

Correct move:

Remember AP convention: $P$ = y-axis, $Q_{d}$ = x-axis, standard demand curves always slope downward

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

5. Quick Reference Cheatsheet

Category	Formula/Rule	Notes
Individual Linear Demand	$Q_{d} = a - b P$	$a$ = quantity when $P = 0$ ; $b > 0$ for downward slope
Market Demand (2 consumers)	$Q_{d, ma r k e t} = Q_{d 1} + Q_{d 2}$	Sum quantities at each price (horizontal summation) for private goods
Change in Quantity Demanded	Movement along curve	Caused only by change in the good's own price
Change in Demand	Shift of entire curve	Caused by change in any non-price determinant; right = increase, left = decrease
Normal Good (Income Change)	↑Income → ↑Demand	Shift right when income rises; shift left when income falls
Inferior Good (Income Change)	↑Income → ↓Demand	Shift left when income rises; shift right when income falls
Substitute (Related Good Price)	↑Price of substitute → ↑Demand	Shift right for the original good
Complement (Related Good Price)	↑Price of complement → ↓Demand	Shift left for the original good
Expected Future Price Change	↑Expected future price → ↑Current demand	Consumers buy more now to avoid higher future prices

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · MCQ
Identify shift vs movement in demand
2022 · FRQ
Derive market demand from individual demand

What's Next

Demand is the foundational prerequisite for all market and macroeconomic analysis in AP Macroeconomics. Mastering the distinction between movements and shifts in demand is critical to correctly predicting market outcomes, and this knowledge extends directly to later topics like aggregate demand. Without a solid understanding of demand concepts, you will struggle to correctly analyze market shocks and macroeconomic fluctuations on both MCQ and FRQ sections. Next, you will build on this knowledge to study supply, then combine supply and demand to model market equilibrium, the core framework for analyzing price and quantity changes in any market. All rules for demand shifts you learned here will also apply when you study aggregate demand later in the course.