AP Macroeconomics

## Learning Outcomes

- Explain how changes in demand and supply create a new market equilibrium
- Distinguish between a curve shift and a movement along a curve
- Predict how equilibrium price and equilibrium quantity change after single shifts
- Analyze simultaneous shifts in demand and supply, including indeterminate outcomes
- Apply demand and supply logic to AP-style scenarios involving income, related goods, expectations, input costs, and subsidies
- Solve simple equilibrium problems using both graphs and equations

## AP Macroeconomics Syllabus

For the AP Macroeconomics, you are expected to understand how supply and demand shifts change equilibrium, with a focus on the following syllabus points:

- Know that market equilibrium occurs where quantity demanded equals quantity supplied
- Identify the difference between a change in demand or supply and a change in quantity demanded or quantity supplied
- Explain how an increase or decrease in demand changes equilibrium price and equilibrium quantity
- Explain how an increase or decrease in supply changes equilibrium price and equilibrium quantity
- Understand the role of demand determinants such as income, tastes, expectations, number of buyers, and related goods
- Understand the role of supply determinants such as input costs, technology, taxes or subsidies, expectations, and number of sellers
- Analyze simultaneous shifts in both curves and identify which equilibrium result is determinate and which is indeterminate
- Solve algebraic equilibrium questions by setting Qd equal to Qs

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. If demand increases and supply stays constant, what happens to equilibrium price and equilibrium quantity?
2. What is the difference between a change in demand and a change in quantity demanded?
3. If wages for avocado farm workers rise, which curve shifts in the avocado market?
4. Why is quantity often indeterminate when both demand and supply shift at the same time?
5. How would you find equilibrium if given equations for Qd and Qs?

## Introduction

In AP Macroeconomics Unit 1, supply and demand are tested as core tools for thinking. Even though these graphs often look simple, many exam questions are built around small wording differences: _demand_ versus _quantity demanded_, _shift_ versus _movement_, and _single shifter_ versus _double shifter_. If you miss those distinctions, you can get the whole question wrong.

This subtopic focuses on what happens after a market is disturbed. A new preference, a tax, improved technology, or a change in income can shift one or both curves. Your task is to identify which curve moves, in which direction, and what happens to equilibrium price and equilibrium quantity.

> **Key Term: market equilibrium**
> The point where quantity demanded equals quantity supplied, shown by the intersection of the demand and supply curves.

### Start with equilibrium and disequilibrium

Before you can analyze a new equilibrium, you need a firm baseline. On a standard market graph, **price** is on the vertical axis and **quantity** is on the horizontal axis. The **demand** curve slopes downward and the **supply** curve slopes upward. Their intersection gives the equilibrium price and equilibrium quantity.

At equilibrium, the market clears. Buyers want to buy exactly the amount sellers want to sell. There is no pressure for price to change.

If price is above equilibrium, quantity supplied exceeds quantity demanded. That creates a **surplus**. Sellers cannot sell everything they bring to market, so price tends to fall.

If price is below equilibrium, quantity demanded exceeds quantity supplied. That creates a **shortage**. Buyers compete for too few goods, so price tends to rise.

> **Key Term: surplus**
> A market condition in which quantity supplied is greater than quantity demanded, usually because price is above equilibrium.
>
> **Key Term: shortage**
> A market condition in which quantity demanded is greater than quantity supplied, usually because price is below equilibrium.

A classic numerical example makes this clearer. Suppose equilibrium is at **$5** and **300 units**. If the price were **$8**, buyers might want only **100 units** while sellers offer **350 units**. That gives a surplus of **250 units**. If the price were **$3**, buyers might want **500 units** while sellers offer only **250 units**. That gives a shortage of **250 units**. In a competitive market, those imbalances create pressure back toward equilibrium.

This matters because when demand or supply shifts, you are not just moving to a random new point. You are finding the new market-clearing price and quantity.

> **Test Tip:** On AP questions, first ask whether the market is at equilibrium, in shortage, or in surplus. Many wrong answers come from forgetting that prices above equilibrium create surplus and prices below equilibrium create shortage.

### Single shifts in demand and supply

The most common AP questions give one change and ask for the new equilibrium. Use this four-step method:

1. Identify whether the event affects demand or supply
2. Decide whether it increases or decreases that curve
3. Shift the correct curve right or left
4. Compare the old and new equilibrium

A rightward shift means an increase. A leftward shift means a decrease.

#### Demand shifts

A **change in demand** means the whole demand curve shifts because of a non-price factor.

> **Key Term: change in demand**
> A shift of the entire demand curve caused by a non-price determinant such as income, tastes, expectations, related goods, or number of buyers.

If demand increases and supply stays the same:

- equilibrium price rises
- equilibrium quantity rises

Why? Buyers want more at every price. At the old price, there is now a shortage. That shortage pushes price upward. As price rises, sellers move up along the supply curve and offer more, so quantity sold also rises.

If demand decreases and supply stays the same:

- equilibrium price falls
- equilibrium quantity falls

Why? Buyers want less at every price. At the old price, there is now a surplus. Sellers respond by cutting price, and the lower equilibrium also reduces the quantity sold.

Common demand shifters include:

- tastes or preferences
- income
- number of buyers
- expectations
- prices of related goods

Two demand-side distinctions are tested a lot:

- **Normal good**: higher income increases demand
- **Inferior good**: higher income decreases demand

- **Substitutes**: if the price of one rises, demand for the other rises
- **Complements**: if the price of one rises, demand for the other falls

Example 1: A new health study says avocados improve heart health. That raises consumer preference for avocados. Demand shifts right. Result: equilibrium price rises and equilibrium quantity rises.

Example 2: Good X is an inferior good and household income rises. Demand for X falls. Result: equilibrium price falls and equilibrium quantity falls.

#### Supply shifts

A **change in supply** means the whole supply curve shifts because of a non-price factor.

> **Key Term: change in supply**
> A shift of the entire supply curve caused by a non-price determinant such as input costs, technology, taxes, subsidies, expectations, or number of sellers.

If supply increases and demand stays the same:

- equilibrium price falls
- equilibrium quantity rises

Why? Sellers now offer more at every price. At the old price, that creates a surplus. Price is driven downward, and at the lower price more units are exchanged.

If supply decreases and demand stays the same:

- equilibrium price rises
- equilibrium quantity falls

Why? Sellers offer less at every price. At the old price, that creates a shortage. Price rises, but fewer units are sold overall.

Common supply shifters include:

- input or resource costs
- technology
- taxes and subsidies
- expectations
- number of sellers

Example 3: Steel producers face higher coal costs. Coal is an input, so production becomes more expensive. Supply shifts left. Result: equilibrium price rises and equilibrium quantity falls.

Example 4: The government subsidizes corn production. A subsidy lowers effective production cost, so supply shifts right. Result: equilibrium price falls and equilibrium quantity rises.

> **Test Tip:** If the price of the good itself changes, do not shift the curve. That causes a movement along the curve only. Non-price changes shift the whole curve.

### Shift versus movement along the curve

This is one of the most tested distinctions in Unit 1.

> **Key Term: change in quantity demanded**
> A movement along a demand curve caused only by a change in the good’s own price.
>
> **Key Term: change in quantity supplied**
> A movement along a supply curve caused only by a change in the good’s own price.

Students often say “demand increased” when they really mean “quantity demanded increased.” That wording error often reveals a graphing error too.

Here is the rule:

- **Own price changes** → movement along the curve
- **Non-price determinant changes** → shift of the whole curve

Suppose the price of coffee falls. That does **not** shift demand for coffee. It causes an increase in **quantity demanded** along the same demand curve.

Suppose consumer incomes rise and coffee is a normal good. That **does** shift the demand curve right.

The same logic applies to supply:

- a higher market price for oranges causes an increase in **quantity supplied**
- lower fertilizer costs cause an increase in **supply**

Why this matters for equilibrium questions: when one curve shifts, the other side usually experiences a movement along its unchanged curve. For example, if demand rises, supply does not shift. But the higher equilibrium price causes a higher **quantity supplied**.

Students often miss this in written explanations. They wrongly say “supply increased” when they should say “quantity supplied increased.”

Side-by-side comparison:

- Consumer preference for SUVs rises
  - Correct: demand shifts right
  - Incorrect: quantity demanded increases because of SUV price

- Price of SUVs rises
  - Correct: quantity demanded falls
  - Incorrect: demand shifts left

- Wages for orange pickers rise
  - Correct: supply shifts left
  - Incorrect: quantity supplied falls because orange price changed

- Price of oranges rises
  - Correct: quantity supplied rises
  - Incorrect: supply shifts right

### Simultaneous shifts and algebraic equilibrium

Some of the best AP questions use **double shifters**. Both curves move at once. In those cases, one equilibrium result is often clear while the other is ambiguous.

> **Key Term: indeterminate**
> An equilibrium outcome that cannot be known for certain because the relative size of two opposing shifts is not given.

The key logic is simple: if both shifts push one variable in the same direction, that result is certain. If they push a variable in opposite directions, that result is indeterminate.

#### Double-shift patterns

If **demand increases** and **supply increases**:

- quantity definitely rises
- price is indeterminate

If **demand decreases** and **supply decreases**:

- quantity definitely falls
- price is indeterminate

If **demand increases** and **supply decreases**:

- price definitely rises
- quantity is indeterminate

If **demand decreases** and **supply increases**:

- price definitely falls
- quantity is indeterminate

Why? Take demand up plus supply down. Demand up pushes price up and quantity up. Supply down pushes price up and quantity down. Since both raise price, price is certain. Quantity is pulled in opposite directions, so it is uncertain.

Example: Demand for bananas rises because of consumer tastes, while supply falls because storms damage crops. Price must rise. Quantity could rise, fall, or stay the same depending on which shift is stronger.

> **Test Tip:** For double-shifter questions, do not guess both outcomes. Work each shift separately, then combine them. If the two shifts disagree on one variable, mark it indeterminate.

#### Worked algebra example

Sometimes AP-style practice gives equations instead of graphs.

Suppose:

- $Q_d = 100 - 2P$
- $Q_s = 20 + 3P$

At equilibrium, quantity demanded equals quantity supplied:

$$
100 - 2P = 20 + 3P
$$

Step 1: move price terms to one side

$$
80 = 5P
$$

Step 2: solve for price

$$
P = 16
$$

Step 3: substitute back into either equation

$$
Q = 100 - 2(16)
$$

$$
Q = 100 - 32 = 68
$$

So the equilibrium is:

- **equilibrium price = 16**
- **equilibrium quantity = 68**

Now suppose demand increases, so the new demand equation is:

$$
Q_d = 120 - 2P
$$

Supply stays:

$$
Q_s = 20 + 3P
$$

Set them equal:

$$
120 - 2P = 20 + 3P
$$

$$
100 = 5P
$$

$$
P = 20
$$

Substitute back:

$$
Q = 120 - 2(20) = 80
$$

New equilibrium:

- **price rises from 16 to 20**
- **quantity rises from 68 to 80**

That matches the graph rule for an increase in demand.

A quick formula shortcut, if equations are in the standard forms $Q_d = a - bP$ and $Q_s = c + dP$, is:

$$
P^* = \frac{a-c}{b+d}
$$

But only use that if you are confident. The safer exam method is to set $Q_d = Q_s$ and solve step by step.

Common student errors:

- shifting a curve when only own price changed
- forgetting that supply increase means right shift, not left
- claiming both price and quantity are always known after double shifts
- reading the new equilibrium at the old intersection
- confusing substitutes with complements
- forgetting that income affects normal and inferior goods differently

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Market equilibrium occurs where quantity demanded equals quantity supplied
- A price above equilibrium creates a surplus and downward pressure on price
- A price below equilibrium creates a shortage and upward pressure on price
- A change in demand or supply is a curve shift caused by a non-price determinant
- A change in quantity demanded or quantity supplied is a movement along a curve caused by the good’s own price
- An increase in demand raises equilibrium price and equilibrium quantity
- A decrease in demand lowers equilibrium price and equilibrium quantity
- An increase in supply lowers equilibrium price and raises equilibrium quantity
- A decrease in supply raises equilibrium price and lowers equilibrium quantity
- For substitutes, a rise in the price of one increases demand for the other
- For complements, a rise in the price of one decreases demand for the other
- In double-shift questions, one outcome is often indeterminate because the shifts push it in opposite directions

## Key Terms and Concepts

- market equilibrium
- surplus
- shortage
- change in demand
- change in supply
- change in quantity demanded
- change in quantity supplied
- indeterminate


Market Equilibrium, Disequilibrium, and Changes in Equilibrium - Supply and demand change equilibrium