# Supply

> AP Microeconomics · Unit 2: Supply and Demand
> Source: https://www.owlsprep.com/study/ap-microeconomics-u2-supply/

This module covers core supply concepts tested on AP Microeconomics, including the law of supply, individual vs market supply, supply curve shifts vs movements, and price elasticity of supply, with worked examples aligned to the College Board exam framework.

**Prerequisites:** Basic linear function graphing; Comparative statics analysis; Understanding of economic variables: price vs quantity

## Learning objectives

- Define supply and state the Law of Supply
- Distinguish between individual and market supply, and calculate market supply from individual supply curves
- Differentiate between movements along and shifts of the supply curve
- Calculate price elasticity of supply using the midpoint method
- Identify and avoid common exam pitfalls related to supply

## Core Definition of Supply

Supply describes the relationship between the market price of a good or service and the quantity of that good that producers are willing and able to sell at every possible price, holding all other relevant factors constant (ceteris paribus). Unlike a single quantity supplied at one price, supply refers to the entire schedule of quantities across all price levels. It is a core component of Unit 2, which makes up 20-25% of the total AP Micro exam score, with supply itself accounting for ~4-6% of total exam weight.

**Supply** — The full schedule of quantities of a good producers are willing and able to sell at every possible market price, holding all non-price factors constant.

*Notation:* $Q_s$ = quantity supplied, $S$ = overall supply relationship

> **info**
>
> Standard notation on the AP exam uses $Q_s$ for quantity supplied (number of units), $P$ for market price per unit, and $S$ to represent the entire supply relationship (curve or schedule).

## Law of Supply and Market Supply Aggregation

The core principle of supply is the Law of Supply, which holds that ceteris paribus, an increase in the price of a good leads to an increase in quantity supplied, and a decrease in price leads to a decrease in quantity supplied. This upward slope arises from increasing marginal costs of production: producing more output requires increasingly costly inputs at the margin, so producers only supply more at higher prices.

We distinguish between individual supply (for a single producer) and market supply (for all producers in a market). Market supply is calculated by horizontally summing individual supply curves: add the quantity supplied by each producer at a given price to get total market quantity supplied. The general form of a linear supply function is:

$$Q_s = mP + b$$

By the law of supply, the slope $m$ is always positive. The intercept $b$ is almost always negative, as producers require a positive minimum price to supply any quantity.

**Worked example:** Suppose there are two craft bread bakeries in a small neighborhood: Bakery X has individual supply $Q_{sX} = 3P - 6$, and Bakery Y has individual supply $Q_{sY} = 2P - 4$. What is the market supply function for loaves of craft bread in this neighborhood, and what is the total quantity supplied when $P = \$4$ per loaf?

1. Confirm each individual supply follows the law of supply: both slope coefficients (3 and 2) are positive, which matches our rule.
2. Market supply is the horizontal sum of individual quantities, so we add the two individual supply functions:

   $$Q_s = Q_{sX} + Q_{sY}$$
3. Substitute the functions and simplify:

   $$Q_s = (3P - 6) + (2P - 4) = 5P - 10$$
4. Plug in $P = 4$ to find total quantity supplied:

   $$Q_s = 5(4) - 10 = 10$$
5. The market supply function is $Q_s = 5P - 10$, and total quantity supplied at $P = \$4$ is 10 loaves per day.

> **Exam tip:** When adding individual supply curves to get market supply, always add quantities (the $Q$ terms), not prices. This is the opposite of vertical summing for public goods, a common point of confusion later in the course.

## Movements Along vs. Shifts of the Supply Curve

This is the most frequently tested distinction on the AP Micro exam, and a common source of lost points in FRQ terminology. A movement along the supply curve (called a change in quantity supplied) is caused only by a change in the price of the good itself, with all non-price factors held constant, so the entire curve does not move. A shift of the entire supply curve (called a change in supply) is caused by a change in any non-price determinant of supply, meaning quantity supplied changes at every possible price.

Key non-price determinants of supply include: prices of inputs, production technology, number of sellers, producer expectations of future prices, government policies (taxes, subsidies, regulations), and prices of related goods in production. An increase in supply shifts the curve rightward; a decrease shifts it leftward.

**Worked example:** For each of the following events in the market for skateboards, state whether it causes a movement along the supply curve or a shift of the supply curve, and the direction of any shift: (a) The market price of skateboards rises from \$80 to \$100; (b) A new subsidy is given to skateboard manufacturers; (c) The price of maple wood (a key input for skateboard decks) increases.

1. **(a)**: The only change is to the price of the good itself. This causes an upward movement along the existing supply curve (an increase in quantity supplied, no shift of the curve).
2. **(b)**: The subsidy reduces production costs, a non-price determinant of supply. Producers supply more at every price, so this causes a rightward shift of the entire supply curve (an increase in supply).
3. **(c)**: Higher input prices increase production costs, a non-price determinant of supply. Producers supply less at every price, so this causes a leftward shift of the entire supply curve (a decrease in supply).

> **Exam tip:** On MCQ questions asking to identify shifts vs movements, first ask: is the change to the price of the good in question? If yes = movement. If no = shift. This eliminates 50% of wrong answers immediately.

## Price Elasticity of Supply

Price elasticity of supply (PES, denoted $E_s$) measures how responsive quantity supplied is to a change in the price of the good. It is used to predict how much quantity supplied changes when price shifts, which is critical for analyzing policy and demand shocks. Because of the law of supply, $E_s$ is always positive. The basic formula for PES is:

$$E_s = \frac{\% \Delta Q_s}{\% \Delta P}$$

- **Elastic**: $E_s > 1$, quantity supplied changes more than proportionally to price
- **Inelastic**: $E_s < 1$, quantity supplied changes less than proportionally to price
- **Unit elastic**: $E_s = 1$, quantity supplied changes proportionally to price
- **Perfectly inelastic**: $E_s = 0$, quantity supplied is fixed regardless of price (vertical curve)
- **Perfectly elastic**: $E_s = \infty$, any quantity supplied at a single price (horizontal curve)

To calculate PES between two points, the AP exam expects use of the midpoint method to avoid the endpoint problem (where elasticity changes based on direction of movement). The midpoint formula is:

$$E_s = \frac{(Q_2 - Q_1)/\left(\frac{Q_2 + Q_1}{2}\right)}{(P_2 - P_1)/\left(\frac{P_2 + P_1}{2}\right)}$$

Key determinants of PES: longer time horizons, easier input access, and better storage capacity all make supply more elastic.

**Worked example:** When the price of organic blueberries rises from \$4 per pint to \$6 per pint, the total quantity supplied by Oregon blueberry farmers increases from 10 million pints to 14 million pints per month. Calculate PES using the midpoint method, and state if supply is elastic, inelastic, or unit elastic.

1. Label values: $P_1 = 4$, $P_2 = 6$, $Q_1 = 10$, $Q_2 = 14$.
2. Calculate percentage change in quantity supplied:

   $$\% \Delta Q_s = \frac{14 - 10}{(14 + 10)/2} = \frac{4}{12} \approx 0.333$$
3. Calculate percentage change in price:

   $$\% \Delta P = \frac{6 - 4}{(6 + 4)/2} = \frac{2}{5} = 0.4$$
4. Calculate PES and categorize:

   $$E_s = \frac{0.333}{0.4} \approx 0.83$$
5. Since $0.83 < 1$, supply is inelastic in this range.

> **Exam tip:** Remember that PES is always positive, unlike price elasticity of demand which is usually reported as a negative number. If you get a negative PES, you mixed up supply and demand, so double-check your calculation immediately.

## AP-Style Concept Check

**Check your understanding**

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

1. Which of the following events will cause a rightward shift of the supply curve for handmade ceramic mugs?

   - A) An increase in the price of handmade ceramic mugs
   - B) A decrease in the price of clay, the key input used to make ceramic mugs
   - C) An increase in the minimum wage that raises wages paid to ceramic artists
   - D) A new Instagram trend increases consumer willingness to pay for handmade ceramic mugs

   *Why:* A change in the price of the good itself only causes a movement along the supply curve, not a shift. Higher input prices shift supply left, and a change in consumer willingness to pay shifts the demand curve, not supply. Lower input prices reduce production costs, so producers supply more at every price, shifting supply rightward.

## Common pitfalls

- **Wrong:** Adding individual supply curves vertically instead of horizontally to get market supply
  - Why it fails: Students confuse horizontal summing for private good market supply with the vertical summing used for public goods later in the course.
  - Correct: Always add quantity values at each price when calculating market supply, which is horizontal summing.
- **Wrong:** Calling a change in the price of the good a 'shift in supply' instead of a 'movement along the supply curve'
  - Why it fails: Students mix up the terminology for change in quantity supplied vs change in supply, the most common terminology error on the exam.
  - Correct: Always first check if the change is to the price of the good itself: if yes = movement along the curve (change in quantity supplied); if no = shift of the entire curve (change in supply).
- **Wrong:** Getting a negative price elasticity of supply and keeping it as the final answer
  - Why it fails: Students carry over the negative elasticity convention from demand to supply, forgetting the law of supply leads to a positive relationship between P and Q.
  - Correct: If you calculate a negative PES, reverse the order of the change terms to get a positive elasticity, as P and Q always move in the same direction for supply.
- **Wrong:** Stating that a rightward shift of the supply curve is a decrease in supply
  - Why it fails: Students confuse the direction of shifts by mixing up axis positioning.
  - Correct: A right shift means higher quantity supplied at every price = that is an *increase* in supply; a left shift is a *decrease* in supply. Remember: 'right is rise, left is less' for supply shifts.
- **Wrong:** Treating a change in the price of a substitute in production as a movement along the original good's supply curve
  - Why it fails: The substitute is a different good, so its price is a non-price determinant of the original good's supply.
  - Correct: Any change in price of a related good in production is a non-price change, so it shifts the supply curve for the original good, not a movement along it.

## Cheatsheet

| Category | Formula / Rule | Notes |
| --- | --- | --- |
| Linear Supply Function | $Q_s = mP + b$ | $m>0$ (law of supply, upward slope); $b<0$ (minimum positive price required) |
| Market Supply (multiple producers) | $Q_s = Q_{s1} + Q_{s2} + ... + Q_{sn}$ | Horizontal sum: add quantities at each price |
| Point PES | $E_s = \frac{\% \Delta Q_s}{\% \Delta P}$ | Always positive due to law of supply |
| Midpoint PES | $E_s = \frac{(Q_2-Q_1)/\frac{Q_1+Q_2}{2}}{(P_2-P_1)/\frac{P_1+P_2}{2}}$ | Avoids endpoint bias for calculations between two points |
| Elastic Supply | $E_s > 1$ | Quantity supplied changes more than proportionally to price |
| Inelastic Supply | $E_s < 1$ | Quantity supplied changes less than proportionally to price |
| Unit Elastic Supply | $E_s = 1$ | Quantity supplied changes proportionally to price |
| Perfectly Inelastic Supply | $E_s = 0$ | Vertical supply curve, quantity fixed regardless of price |
| Perfectly Elastic Supply | $E_s = \infty$ | Horizontal supply curve, any quantity supplied at one price |

## What's next

Mastering supply is foundational for all core topics in AP Microeconomics. After combining supply with demand to study market equilibrium, you will use these concepts to analyze consumer and producer surplus, the impact of government policies like taxes and price controls, and eventually build up to firm production and cost theory in later units. This topic is heavily tested on both multiple-choice and free-response sections of the exam, so reviewing common pitfalls and practicing elasticity calculations is key to earning full points. Recognizing supply shifters is also critical for correctly drawing and labeling graphs on FRQ questions, which account for half of your total exam score.

- [Demand](https://www.owlsprep.com/study/ap-microeconomics-u2-demand/)
- [Unit 2 Overview](https://www.owlsprep.com/study/ap-microeconomics-u2-overview/)
- [Price Elasticity of Demand](https://www.owlsprep.com/study/ap-microeconomics-u2-price-elasticity-of-demand/)

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