Demand

AP Microeconomics· AP Microeconomics CED — Supply and Demand· 14 min read

1. Core Definition of Demand★☆☆☆☆⏱ 3 min

Demand is the foundational concept of AP Microeconomics Unit 2, which makes up 18-20% of total AP exam score. It is a building block for equilibrium analysis, consumer surplus, elasticity, and most other microeconomic topics tested on both MCQ and FRQ.

📘 Definition

Demand

The relationship between the price of a good/service and the maximum quantity consumers are willing AND able to purchase at that price, holding all other factors constant (ceteris paribus).

Example:

A demand curve describes the full set of price-quantity combinations for coffee, not just one single point.

Two standard notations are commonly used for demand:

Q_D = c - dP

This is direct demand, which expresses quantity demanded as a function of price. For graphing (with price on the vertical axis), we use inverse demand:

P = a - bQ_D

Demand can be presented as a table (demand schedule), a graph (demand curve), or a mathematical function, and is categorized as individual (one consumer) or market (all consumers in a market).

2. The Law of Demand and Quantity Demanded★★☆☆☆⏱ 4 min

📘 Definition

Quantity Demanded

The specific amount of a good consumers are willing and able to buy at one specific price point, in contrast to demand which describes the entire relationship between all prices and corresponding quantities.

The Law of Demand is the core principle of this topic: holding all other factors constant, there is an inverse relationship between a good's own price and its quantity demanded. When price increases, quantity demanded decreases; when price decreases, quantity demanded increases.

This inverse relationship (which gives a downward-sloping demand curve on a standard graph) comes from three complementary sources:

Substitution effect: Higher prices make consumers switch to cheaper substitute goods
Income effect: Higher prices reduce consumers' real purchasing power, so they buy less
Diminishing marginal utility: Each additional unit of the good gives less extra satisfaction than the last, so consumers will only buy more if price falls

📐 Worked Example

A college student’s monthly demand for bubble tea is given by the direct demand function: $Q_{D} = 30 - 3 P$ where $Q_{D}$ is number of bubble teas and $P$ is price in dollars per bubble tea. What is the quantity demanded when $P = $4$ ? If price rises to $$6$ , how does quantity demanded change, and is this consistent with the law of demand?

1
Recall that quantity demanded is a point-specific value, calculated by substituting the given price into the demand function.
2
For $P = 4$ , substitute into the function:
3
$Q_D = 30 - 3(4) = 30 - 12 = 18$
4
So quantity demanded is 18 bubble teas per month.
5
For $P = 6$ , substitute again:
6
$Q_D = 30 - 3(6) = 30 - 18 = 12$
7
Quantity demanded is 12 bubble teas per month.
8
When price increased by $2, quantity demanded fell by 6 units. This matches the inverse relationship required by the law of demand.

Exam tip:

Always separate the terms 'demand' and 'quantity demanded' on FRQs. AP graders routinely dock points for using these terms interchangeably, as the distinction between the two is the most heavily tested concept in this topic.

3. Movements Along vs Shifts of the Demand Curve★★★☆☆⏱ 4 min

The most frequently tested distinction on AP Micro demand questions is between a movement along the existing demand curve and a shift of the entire demand curve. The rule is simple: only a change in the good’s own price causes a movement along the demand curve. This movement represents a change in quantity demanded, not a change in the overall relationship between price and quantity.

A shift of the entire demand curve (a change in demand) is caused by a change in one or more non-price determinants of demand. A right shift means demand increased (quantity demanded is higher at every price), and a left shift means demand decreased (quantity demanded is lower at every price).

📐 Worked Example

For each of the following changes in the market for organic apples, state whether the result is a movement along the demand curve or a shift of the demand curve, and the direction of the change: (a) The price of organic apples falls; (b) The price of peanut butter (a complement for apples) rises; (c) Average consumer income rises, and organic apples are a normal good.

1
Apply the core rule: only changes in a good's own price cause movements along the demand curve.
2
(a) This is a change in the own price of organic apples, so it causes a downward-right movement along the existing demand curve (increase in quantity demanded, no change in overall demand).
3
(b) Peanut butter is a complement to apples. When the price of a complement rises, consumers buy fewer apples at every apple price, so demand decreases, shifting the entire demand curve left.
4
(c) Organic apples are a normal good, so higher average income increases demand at every price, shifting the entire demand curve right.

Exam tip:

For any MCQ asking if a change is a shift or movement, first check if the change is to the good’s own price. If yes, it is a movement, and you can immediately eliminate all shift options to cut your work in half.

4. Individual vs Market Demand★★★★☆⏱ 5 min

Individual demand describes the quantity demanded by a single consumer at each price, while market demand describes the total quantity demanded by all consumers in the market at each price. To get market demand for private goods (the standard case in Unit 2), you calculate the horizontal sum of all individual demand curves: for any given price, add up the quantity demanded by each individual consumer to get the total market quantity demanded.

This is different from vertical summation (used for public goods), where you add prices instead of quantities — this is a common source of error on AP exams. Because some consumers will drop out of the market at higher prices (they will not buy any units when price exceeds their maximum willingness to pay), market demand is often a piecewise function, which is explicitly tested on AP FRQs.

📐 Worked Example

There are only two consumers in the local market for homemade bread: Elena and Raj. Elena’s demand is $Q_{E} = 10 - 2 P$ , and Raj’s demand is $Q_{R} = 8 - P$ , where $Q$ is number of loaves per month and $P$ is price per loaf in dollars. Derive the market demand function for homemade bread.

1
Recall that market demand for private goods is the sum of individual quantities at each price.
2
Add the two individual demand functions:
3
$Q_{market} = Q_E + Q_R = (10 - 2P) + (8 - P) = 18 - 3P$
4
Identify price ranges where consumers buy positive quantity: Elena will buy loaves only when $P < 5$ , and Raj will buy only when $P < 8$ . For prices between $5 and $8, only Raj buys positive quantity, so market demand equals Raj’s demand. For prices $\geq 8$ , no one buys any bread.
5
Write the full piecewise market demand function:
6
$Q_{market} = \begin{cases} 18 - 3P & \text{if } 0 \leq P < 5 \\ 8 - P & \text{if } 5 \leq P < 8 \\ 0 & \text{if } P \geq 8 \end{cases}$

✓ Quick check

Test your understanding with this AP-style multiple choice question:

Which of the following events will cause a rightward shift in the market demand curve for plant-based burgers, a normal good?
- A) A decrease in the price of plant-based burgers
- B) An increase in the price of beef burgers, a substitute for plant-based burgers
- C) A decrease in average consumer income during a recession
- D) A new manufacturing process that lowers the marginal cost of producing plant-based burgers
Reveal answer
B —
First, eliminate options that do not shift demand: Option A is a change in own price, so it only causes a movement along the curve. Option C: lower income for a normal good shifts demand left. Option D: lower production costs affect supply, not demand. Only B is correct: higher substitute price increases demand for plant-based burgers, shifting the curve right.

Exam tip:

Always write the full piecewise segments for market demand, even if the question doesn’t explicitly ask. AP FRQs almost always award an extra point for correctly accounting for consumers that drop out at high prices.

5. Common Pitfalls

Wrong move:

Calling an own-price change that changes quantity demanded a 'shift in demand'

Why:

Students confuse the definition of demand (the entire price-quantity relationship) and quantity demanded (a single point on the curve)

Correct move:

Any time the change is to the good's own price, describe the change as a 'change in quantity demanded' and a 'movement along the demand curve'.

Wrong move:

Shifting demand right when income rises for an inferior good

Why:

Students mix up normal and inferior good definitions, forgetting the direction of shift for inferior goods

Correct move:

First label the good as normal or inferior, then apply the rule: income up → demand up for normal, income up → demand down for inferior.

Wrong move:

Vertically summing individual demand curves to get market demand for private goods

Why:

Students confuse private good market demand with public good demand, which uses vertical summation

Correct move:

For private goods (the standard Unit 2 case), always sum quantities at each price (horizontal summation), never sum prices.

Wrong move:

Claiming the law of demand says demand falls when income falls

Why:

Students misremember what the law of demand describes, confusing non-price determinants with the core price-quantity relationship

Correct move:

Memorize that the law of demand only describes the inverse relationship between a good’s own price and its quantity demanded, ceteris paribus.

Wrong move:

Drawing an upward-sloping demand curve when the question does not explicitly label it a Giffen or Veblen good

Why:

Students remember Giffen goods are an exception to the law of demand and over-apply the exception

Correct move:

Assume all goods follow the law of demand (downward-sloping demand) unless the question explicitly identifies the good as an exception.

6. Quick Reference Cheatsheet

Category	Formula / Rule	Notes
Law of Demand	\frac{\Delta Q_D}{\Delta P} < 0	Inverse relationship between own price and quantity demanded, ceteris paribus
Direct Demand Function	Q_D = c - dP	$c, d > 0$ for downward-sloping demand
Inverse Demand Function (graphing)	P = a - bQ_D	$P$ on vertical axis, $Q_{D}$ on horizontal axis, $a, b > 0$
Market Demand (private good)	Q_{D,market} = \sum Q_{D,i}	Horizontal summation: add quantities at each price
Normal Good Income Change	\text{Income} \uparrow \implies \text{Demand} \uparrow	Right shift of demand curve
Inferior Good Income Change	\text{Income} \uparrow \implies \text{Demand} \downarrow	Left shift of demand curve
Own Price Change	Change in quantity demanded, movement along curve	Never shifts the demand curve
Non-Price Determinant Change	Change in demand, shift of entire curve	Right = increase in demand, left = decrease in demand

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 demand shift determinant
2022 · FRQ
Derive piecewise market demand

What's Next

Demand is the foundation for all of Unit 2 and nearly all of AP Microeconomics. Mastering the critical distinction between shifts and movements of demand is required to correctly predict equilibrium price and quantity changes, a core skill tested on every AP Micro exam. Next, you will learn the parallel concept of supply, then combine demand and supply to find market equilibrium, analyze how market outcomes adjust to external shocks, and calculate consumer and producer surplus. Demand also underpins all later concepts, including elasticity, consumer choice, monopoly pricing, and welfare analysis, so a solid understanding here is critical for earning a high score on the exam.