1.5 Supply

Q: What does the Law of Supply state regarding the relationship between price and quantity supplied?

The Law of Supply states that there is a direct (positive) relationship between price ($ P $) and quantity supplied ($ Q_s $), meaning they move in the same direction.

Q: What is the typical slope and axis labeling for a supply curve?

A supply curve is upward-sloping , with Price ($ P $) on the vertical Y-axis and Quantity ($ Q $) on the horizontal X-axis.

Q: What is the only factor that causes a "Change in Quantity Supplied" (movement along the curve)?

A change in quantity supplied is caused ONLY by a change in the price of the product itself .

Q: What constitutes a "Change in Supply" and how is it represented on a graph?

A change in supply is a shift of the entire curve caused by external factors (determinants); a shift right is an increase, while a shift left is a decrease.

Q: How do subsidies and taxes differently affect the supply curve?

A subsidy increases supply (shifts Right), while a tax decreases supply (shifts Left).

Q: How does an expectation of higher future prices affect current supply?

If sellers expect prices to rise in the future, they often decrease current supply (shift Left) to save inventory for later sale at the higher price.

Q: In the context of supply, what is the difference between "Price" and "Cost"?

Price is the money a consumer pays to the seller, while Cost is the money a business spends to produce the good.

AP Macroeconomics · AP (Advanced Placement) · 4 min read · Updated: 7 Sept 2026

1.5 Supply

Welcome to the other half of the market! In chapter 1.4, we looked at things from the perspective of the consumer (Demand). Now, we are switching hats to look at the world through the eyes of the producer (the business or the seller). Understanding supply is essential because it explains how firms decide how much of a product to bring to the market and at what price.

The Law of Supply

The Law of Supply states that there is a direct (positive) relationship between price and quantity supplied, assuming all other factors remain constant (ceteris paribus).

In simple terms:

When the price ($ P $) of a good increases, the quantity supplied ($ Q_s $) increases.
When the price ($ P $) of a good decreases, the quantity supplied ($ Q_s $) decreases.

Why does this happen? Think like a business owner. If the price of the cookies you bake goes from \$1 to \$5, you’ll want to bake as many as possible to make more profit! At a higher price, even less efficient producers find it profitable to enter the market.

Key Takeaway: Producers love high prices! The Law of Supply shows that they are willing to provide more of a good when they can sell it for more money.

The Supply Curve

On a graph, the supply curve is upward-sloping (going from the bottom left to the top right). On the AP Exam, you must always label your axes correctly:

Vertical Axis (Y): Price ($ P $)
Horizontal Axis (X): Quantity ($ Q $)

If you see a table showing different prices and the quantities a firm is willing to sell, that is a Supply Schedule. When you plot those points on a graph, you get the Supply Curve (usually labeled $ S $).

Movement vs. Shift: The Most Important Distinction

This is where many students lose points on the AP Macro exam. You must distinguish between a change in quantity supplied and a change in supply.

1. Change in Quantity Supplied (Movement)

This is caused ONLY by a change in the price of the product itself. On a graph, you simply move from one point to another along the existing supply curve.

Example: If the price of coffee rises, farmers will harvest more beans. This is a movement along the curve.

2. Change in Supply (Shift)

This is caused by external factors (determinants). The entire curve moves to the left or the right.

Increase in Supply: The curve shifts to the Right ($ S \rightarrow S_1 $). At every price, producers are now willing to sell more.
Decrease in Supply: The curve shifts to the Left ($ S \leftarrow S_1 $). At every price, producers are now willing to sell less.

Pro-tip: Don't think "up" or "down" for shifts. Think "Right is More, Left is Less."

Determinants of Supply (The Shifters)

What makes a supply curve shift? You can remember the main determinants using the mnemonic S-T-I-G-E-R:

1. Subsidies and Taxes (Government Actions)
A subsidy is like a "gift" from the government to a business to encourage production. This increases supply (shifts Right). A tax is a cost imposed by the government, which decreases supply (shifts Left).

2. Technology
Improvements in technology make production faster and cheaper. This increases supply (shifts Right).

3. Input Prices (Resource Costs)
This is the most common shifter. If the price of raw materials, labor (wages), or energy goes up, it becomes more expensive to produce the good. This decreases supply (shifts Left). If inputs become cheaper, supply increases (shifts Right).

4. Expectations of Future Prices
If a seller expects the price of their product to rise significantly next month, they might hold back their current stock to sell it later. This decreases current supply (shifts Left).

5. Related Goods (Prices of other goods)
If a farmer can grow either wheat or corn, and the price of wheat skyrockets, the farmer will shift resources to wheat. Therefore, the supply of corn will decrease (shifts Left).

6. size of the market (Number of Sellers)
As more firms enter an industry, the total market supply increases (shifts Right). If firms go out of business, supply decreases (shifts Left).

Common Mistakes to Avoid

Confusing Supply with Demand: Remember, supply is about the producer. If a question says "Consumers like the product more," that is a Demand shift, not a Supply shift!
Mixing up Price and Cost: "Price" is what the customer pays; "Cost" is what the business pays to make the item. An increase in cost (input prices) shifts supply to the left.
Directional Confusion: When supply decreases, it shifts to the left. On an upward-sloping curve, a leftward shift can look like it's moving "up," but it represents a lower quantity at every price. Stick to "Left" and "Right."

Quick Review Box

The Law: $ P \uparrow, Q_s \uparrow $ and $ P \downarrow, Q_s \downarrow $
The Curve: Upward sloping.
Price Change: Only causes movement along the curve (Change in Quantity Supplied).
S-T-I-G-E-R Change: Causes a shift of the whole curve (Change in Supply).
Next Step: In Chapter 1.6, we will put Supply and Demand together to find the Market Equilibrium!

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What does the Law of Supply state regarding the relationship between price and quantity supplied?

The Law of Supply states that there is a direct (positive) relationship between price ($ P $) and quantity supplied ($ Q_s $), meaning they move in the same direction.

What is the typical slope and axis labeling for a supply curve?

A supply curve is upward-sloping, with Price ($ P $) on the vertical Y-axis and Quantity ($ Q $) on the horizontal X-axis.

What is the only factor that causes a "Change in Quantity Supplied" (movement along the curve)?

A change in quantity supplied is caused ONLY by a change in the price of the product itself.

What constitutes a "Change in Supply" and how is it represented on a graph?

A change in supply is a shift of the entire curve caused by external factors (determinants); a shift right is an increase, while a shift left is a decrease.

How do subsidies and taxes differently affect the supply curve?

A subsidy increases supply (shifts Right), while a tax decreases supply (shifts Left).

What is the effect of an increase in input prices (resource costs) on supply?

An increase in input prices makes production more expensive, which decreases supply and shifts the curve to the Left.

How does an expectation of higher future prices affect current supply?

If sellers expect prices to rise in the future, they often decrease current supply (shift Left) to save inventory for later sale at the higher price.

In the context of supply, what is the difference between "Price" and "Cost"?

Price is the money a consumer pays to the seller, while Cost is the money a business spends to produce the good.

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