# Market Equilibrium, Disequilibrium, and Changes in Equilibrium

> AP Macroeconomics · Unit 1: Basic Economic Concepts
> Source: https://www.owlsprep.com/study/ap-macroeconomics-u1-market-equilibrium-disequilibrium-and-changes/

This guide covers core concepts of market equilibrium, disequilibrium, and comparative statics for AP Macroeconomics, including calculation methods, shift analysis, and common exam pitfalls to help you master this foundational Unit 1 topic.

**Prerequisites:** Law of demand and linear demand curves; Law of supply and linear supply curves; Solving systems of linear equations

## Learning objectives

- Define market equilibrium, disequilibrium, surplus, and shortage
- Calculate equilibrium price and quantity using graphical and algebraic methods
- Analyze price adjustment from disequilibrium back to equilibrium
- Predict changes in equilibrium after single and simultaneous shifts of supply and demand
- Avoid common exam pitfalls on equilibrium analysis questions

## Core Definitions: Equilibrium and Disequilibrium

Market equilibrium is a foundational economic concept describing a market state where the quantity of a good demanded by consumers exactly equals the quantity supplied by producers. At equilibrium, there is no inherent pressure for price or output to change, because both buyers and sellers are satisfied with the market outcome at the prevailing price. Graphically, equilibrium occurs at the intersection of the downward-sloping demand curve and upward-sloping supply curve, with coordinates for equilibrium price ($P_e$) and equilibrium quantity ($Q_e$).

**Disequilibrium** — Any market state where quantity demanded does not equal quantity supplied, resulting in either a shortage (excess demand) or surplus (excess supply). Disequilibrium creates automatic price pressure that pushes the market back toward equilibrium.

*Example:* A price set above equilibrium creates a surplus of unsold goods.

This subtopic is part of AP Macroeconomics Unit 1, which accounts for 12-15% of your total exam score. It appears in both multiple-choice (MCQ) and free-response (FRQ) sections, and often acts as a building block for later policy analysis questions.

## Calculating Equilibrium Price and Quantity

The first core skill tested on the AP exam is calculating equilibrium price ($P_e$) and equilibrium quantity ($Q_e$) using both graphical and algebraic methods. Graphically, you simply plot the downward-sloping demand curve and upward-sloping supply curve on a graph with price on the vertical axis and quantity on the horizontal axis; the intersection point gives you $P_e$ as the y-coordinate and $Q_e$ as the x-coordinate.

For algebraic problems, which are common in FRQ opening parts, you start with the fundamental equilibrium condition:

$$Q_d = Q_s$$

For linear demand and supply functions, the most common form on the AP exam, this is straightforward to solve. Standard linear demand is written as $Q_d = a - bP$, where $a$ is the quantity demanded when price is 0, and $b$ is a positive slope coefficient (the negative sign reflects the law of demand). Standard linear supply is written as $Q_s = c + dP$, where $d$ is the positive slope coefficient reflecting the law of supply.

**Worked example:** Given the demand for artisanal iced coffee is $Q_d = 200 - 15P$ and supply is $Q_s = 50 + 10P$, where $Q$ is cups per week and $P$ is price per cup in dollars. Find equilibrium price and equilibrium quantity.

1. 1. Write the equilibrium condition: set quantity demanded equal to quantity supplied
2. $$Q_d = Q_s$$
3. 2. Substitute the given functions into the condition:
4. $$200 - 15P_e = 50 + 10P_e$$
5. 3. Rearrange terms to isolate $P_e$:
6. $$200 - 50 = 15P_e + 10P_e \rightarrow 150 = 25P_e \rightarrow P_e = 6$$
7. 4. Plug $P_e = 6$ back into the demand function to find $Q_e$:
8. $$Q_e = 200 - 15(6) = 200 - 90 = 110$$
9. 5. Confirm with the supply function to check for algebra errors:
10. $$Q_e = 50 + 10(6) = 50 + 60 = 110$$

> **Exam tip:** Always plug your calculated equilibrium price back into both supply and demand to confirm your quantity matches; this check catches 90% of common sign-flipping algebra errors.

## Disequilibrium: Surpluses and Shortages

Disequilibrium occurs whenever the actual market price is not equal to the equilibrium price, so $Q_d \neq Q_s$. There are two distinct types of disequilibrium, each creating automatic pressure for price to adjust back to equilibrium:

- **Shortage (excess demand):** Occurs when the actual market price $P < P_e$, so $Q_d > Q_s$. More buyers want to purchase the good than sellers are willing to supply at the low price, so unsatisfied buyers bid up the price, pushing price upward toward $P_e$.
- **Surplus (excess supply):** Occurs when the actual market price $P > P_e$, so $Q_s > Q_d$. Sellers are left with unsold inventory, so they cut prices to clear excess stock, pushing price downward toward $P_e$.

The size of a disequilibrium is the absolute difference between quantity demanded and quantity supplied at the given market price, which is always a positive value.

**Worked example:** Using the same iced coffee market from the previous example ($Q_d = 200 - 15P$, $Q_s = 50 + 10P$, $P_e = 6$), if local cafes set a price of \$8 per cup, identify if the market is in equilibrium, surplus, or shortage, and calculate the size of the disequilibrium.

1. 1. Compare the actual price to equilibrium: $P = 8 > P_e = 6$, so this is a surplus.
2. 2. Calculate quantity demanded at $P = 8$:
3. $$Q_d = 200 - 15(8) = 200 - 120 = 80$$
4. 3. Calculate quantity supplied at $P = 8$:
5. $$Q_s = 50 + 10(8) = 50 + 80 = 130$$
6. 4. Calculate the size of the surplus:
7. $$\text{Surplus} = Q_s - Q_d = 130 - 80 = 50$$
8. 5. Explain adjustment: Sellers will cut prices to clear unsold iced coffee, pushing price back down to \$6 per cup until the surplus is eliminated.

> **Exam tip:** On FRQ, always explicitly explain how price adjustment eliminates a surplus or shortage; AP graders require you to connect the disequilibrium to the direction of price change, not just state what the disequilibrium is.

## Comparative Statics: Single Shifts in Supply or Demand

Comparative statics is the process of comparing the original equilibrium to the new equilibrium after a shift in supply, demand, or both. When only one curve shifts (the other remains constant), you can always predict the direction of change for both equilibrium price and quantity, with no ambiguity. The simple rule for single shifts is:

- Increase in demand (demand shifts right): $P_e \uparrow$, $Q_e \uparrow$
- Decrease in demand (demand shifts left): $P_e \downarrow$, $Q_e \downarrow$
- Increase in supply (supply shifts right): $P_e \downarrow$, $Q_e \uparrow$
- Decrease in supply (supply shifts left): $P_e \uparrow$, $Q_e \downarrow$

**Worked example:** The market for electric bicycles has original supply $Q_s = 5000 + 5P$ and demand $Q_d = 20000 - 10P$, where $Q$ is bikes per year and $P$ is price per bike in dollars. A major improvement in battery technology reduces production costs, shifting supply to $Q_s' = 8000 + 5P$, with no change to demand. What is the effect on equilibrium price and quantity?

1. 1. Identify the shift: Lower production costs are a non-price determinant that increases supply, so supply shifts right, demand remains unchanged.
2. 2. Apply the equilibrium condition to the new supply:
3. $$Q_d = Q_s' \rightarrow 20000 - 10P_e' = 8000 + 5P_e'$$
4. 3. Solve for the new equilibrium price:
5. $$12000 = 15P_e' \rightarrow P_e' = \$800$$
6. 4. Calculate new equilibrium quantity:
7. $$Q_e' = 20000 - 10(800) = 12000$$
8. 5. Compare to original equilibrium ($P_e = \$1000$, $Q_e = 10000$): Equilibrium price decreases by \$200, and equilibrium quantity increases by 2000 bikes, matching the single shift rule.

> **Exam tip:** On FRQ that require drawing a shifted curve, always label your original curves $S_1/D_1$ and new curves $S_2/D_2$, and label original and new equilibrium points $E_1$ and $E_2$; AP graders take points off for unlabeled graphs.

## Comparative Statics: Simultaneous Shifts in Both Curves

When both supply and demand shift at the same time, only one outcome (either $P_e$ or $Q_e$) has a predictable direction; the other is ambiguous, meaning you cannot predict its change without knowing the relative magnitude of the two shifts. The rule for simultaneous shifts is:

- Both demand and supply shift right: $Q_e \uparrow$, $P_e$ ambiguous
- Demand shifts right, supply shifts left: $P_e \uparrow$, $Q_e$ ambiguous
- Both demand and supply shift left: $Q_e \downarrow$, $P_e$ ambiguous
- Demand shifts left, supply shifts right: $P_e \downarrow$, $Q_e$ ambiguous

This works because the outcome that moves in the same direction from both shifts is definite, while the outcome that moves opposite directions from each shift depends on how big each shift is.

**Worked example:** A new public health study finds that regular oat milk consumption reduces the risk of heart disease, increasing consumer preference for oat milk. At the same time, drought destroys a large share of the North American oat crop, increasing the cost of producing oat milk. What is the effect on equilibrium price and quantity in the oat milk market?

1. 1. Identify shifts: Higher consumer preference increases demand (shifts demand right), higher input costs decrease supply (shifts supply left).
2. 2. Check direction of change for each outcome: A right demand shift pushes $P_e$ up and $Q_e$ up; a left supply shift pushes $P_e$ up and $Q_e$ down.
3. 3. $P_e$ moves in the same direction (up) from both shifts, so $P_e$ is definitely predicted to increase. $Q_e$ moves up from demand and down from supply, so the net change in $Q_e$ depends on the size of each shift.
4. 4. Conclusion: Equilibrium price will definitely increase; the change in equilibrium quantity is ambiguous, and cannot be determined without additional information on the magnitude of the two shifts.

**Check your understanding**

Test your understanding with this AP-style MCQ:

1. The market for holiday turkeys has an original equilibrium at $P_e = \$20$, $Q_e = 1,000,000$ turkeys. Demand for turkeys increases in November, and at the same time, turkey farmers increase the number of turkeys they bring to market (supply increases). What is the effect on equilibrium price and quantity?

   - A) Equilibrium price increases, equilibrium quantity decreases
   - B) Equilibrium price decreases, equilibrium quantity increases
   - C) Equilibrium quantity increases, change in equilibrium price is ambiguous
   - D) Equilibrium price increases, change in equilibrium quantity is ambiguous

   *Why:* Correct! Both shifts increase quantity, so $Q_e$ definitely increases. Demand pushes price up, supply pushes price down, so price change is ambiguous.

> **Exam tip:** On MCQ questions about double shifts, any answer that claims both price and quantity have a definite change is almost always wrong; eliminate it immediately unless you are given explicit information about the size of each shift.

## Common pitfalls

- **Wrong:** Shifting the entire demand curve when the only change is a change in the price of the good itself
  - Why it fails: Students confuse a movement along the curve (change in quantity demanded/supplied) with a shift of the entire curve (change in demand/supply)
  - Correct: Always ask: Is this change caused by the price of this good? If yes, it is a movement along the curve; if it is any other factor, it is a shift of the entire curve
- **Wrong:** Reporting the size of a surplus or shortage as a negative number
  - Why it fails: Students subtract the larger quantity from the smaller, leading to a negative value
  - Correct: Always calculate surplus as $Q_s - Q_d$ and shortage as $Q_d - Q_s$, so the result is always positive, since disequilibrium size is a measure of excess quantity
- **Wrong:** Predicting both price and quantity for a simultaneous double shift without being given the size of each shift
  - Why it fails: Students memorize single shift rules and apply them incorrectly to double shifts
  - Correct: For double shifts, check if the change in P is in the same direction from both shifts (then P is definite, Q ambiguous) before answering
- **Wrong:** Solving for equilibrium by setting $P_d = P_s$ instead of $Q_d = Q_s$ for functions written as Q in terms of P
  - Why it fails: Students mix up standard (Q as a function of P) and inverse (P as a function of Q) forms
  - Correct: Always write the equilibrium condition as $Q_d = Q_s$ when your functions are written with Q as the dependent variable
- **Wrong:** Claiming that a shortage means no goods are available for sale, or a surplus means no goods are sold
  - Why it fails: Students confuse disequilibrium with zero quantity traded
  - Correct: In disequilibrium, the quantity traded is always the smaller of $Q_d$ and $Q_s$; there are just more or fewer goods desired than are available at the current price
- **Wrong:** Drawing a new equilibrium after a left shift in demand with a higher equilibrium quantity than the original
  - Why it fails: Students confuse left and right directions on the quantity axis
  - Correct: Remember that right on the x-axis means higher quantity, so a right shift means higher equilibrium quantity, and a left shift means lower equilibrium quantity

## Cheatsheet

| Shift Type | Direction of $P_e$ Change | Direction of $Q_e$ Change |
| --- | --- | --- |
| Increase in demand (right shift) | Increase | Increase |
| Decrease in demand (left shift) | Decrease | Decrease |
| Increase in supply (right shift) | Decrease | Increase |
| Decrease in supply (left shift) | Increase | Decrease |
| Demand right, supply right | Ambiguous | Increase |
| Demand right, supply left | Increase | Ambiguous |
| Demand left, supply left | Ambiguous | Decrease |
| Demand left, supply right | Decrease | Ambiguous |

## What's next

Market equilibrium analysis is the foundational building block for all further supply and demand analysis in AP Macroeconomics. The core logic of shifts and equilibrium adjustment you learned here directly extends to the aggregate demand-aggregate supply (AD-AS) model that forms the core of Units 2 through 4, and is required for all analysis of fiscal and monetary policy. Mastering the rules for single and simultaneous shifts here will make it much easier to avoid mistakes on more advanced macroeconomic policy questions that make up a large share of your exam score.

- [Unit 1: Basic Economic Concepts Overview](https://www.owlsprep.com/study/ap-macroeconomics-u1-overview/)
- [Economic Indicators and the Business Cycle](https://www.owlsprep.com/study/ap-macroeconomics-u2-overview/)
- [The Circular Flow and GDP](https://www.owlsprep.com/study/ap-macroeconomics-u2-the-circular-flow-and-gdp/)

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