Supply — AP Microeconomics
1. Core Definition of Supply ★★☆☆☆ ⏱ 3 min
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.
2. Law of Supply and Market Supply Aggregation ★★★☆☆ ⏱ 4 min
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.
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.
3. Movements Along vs. Shifts of the Supply Curve ★★★☆☆ ⏱ 4 min
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.
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.
4. Price Elasticity of Supply ★★★★☆ ⏱ 5 min
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.
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.
5. AP-Style Concept Check ★★★☆☆ ⏱ 2 min
Common Pitfalls
Why: Students confuse horizontal summing for private good market supply with the vertical summing used for public goods later in the course.
Why: Students mix up the terminology for change in quantity supplied vs change in supply, the most common terminology error on the exam.
Why: 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.
Why: Students confuse the direction of shifts by mixing up axis positioning.
Why: The substitute is a different good, so its price is a non-price determinant of the original good's supply.