Supply is the quantity of a good producers are willing and able to sell at each price. The law of supply says price and quantity supplied move in the same direction, which makes the market supply curve upward sloping.
Why This Matters for the AP Microeconomics Exam
Supply is half of the supply and demand model, which is the backbone of Unit 2 and shows up across the whole course. On the exam, you will need to draw a correctly labeled supply curve, show movements along it versus shifts of it, and explain why producers respond to prices, costs, and technology the way they do.
This topic sets up later work on market equilibrium, surplus, elasticity, and government policy. If you can confidently shift a supply curve in the right direction and explain the cause, you are ready to analyze price and quantity changes when supply and demand are combined.
Key Takeaways
- The law of supply: when own-price rises, quantity supplied rises; when own-price falls, quantity supplied falls. They move in the same direction.
- A change in the good's own price causes a movement along the supply curve, not a shift.
- The market supply curve is the horizontal summation of individual producers' supply curves and is upward sloping.
- A change in a determinant of supply shifts the curve: right for an increase, left for a decrease.
- Supply is tied to producers' costs of production, which connects to marginal cost in later units.
- Always graph with price on the vertical axis and quantity on the horizontal axis, and label both curves and axes.
What Supply Means
Supply is the quantity of a good or service that producers are willing and able to offer for sale at each possible price during a set time period. Supply is the seller side of the market. Buyers bring demand, and sellers bring supply.
A supply schedule is just a table that lists how much producers will supply at each price. The supply curve is the graph of that schedule.
The Law of Supply
The law of supply states that, holding everything else constant (ceteris paribus), a higher own-price leads to a higher quantity supplied, and a lower own-price leads to a lower quantity supplied. Price and quantity supplied move in the same direction.
This is why the supply curve slopes upward. Higher prices make it more profitable to produce and sell more, so producers offer larger quantities. As you will see in the production and cost unit, the supply curve is connected to a firm's marginal cost of production, so producing more usually costs more, and a higher price is needed to justify it.
The Supply Curve
The supply curve goes in the standard market graph: quantity on the horizontal axis and price on the vertical axis. Because quantity supplied rises as price rises, the curve slopes upward to the right. It can be a straight line or a curve, but it always increases.
The market supply curve comes from adding up the quantities that all individual producers supply at each price. This horizontal summation gives you the upward-sloping market supply curve.
When the good's own price changes, you move from one point on the supply curve to another. That is a movement along the curve, also called a change in quantity supplied. The curve itself does not move.
Determinants of Supply
When something other than the good's own price changes, the entire supply curve shifts. A shift means that at every price, producers now want to supply a different quantity. A rightward shift is an increase in supply; a leftward shift is a decrease in supply.
The main determinants of supply:
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Input costs (prices of factors of production): If the resources used to make the good get more expensive, producing it costs more, so supply decreases (shifts left). Cheaper inputs increase supply (shift right).
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Technology and productivity: Better technology lets producers make more with the same resources, which increases supply and shifts the curve right.
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Taxes and subsidies: A tax on production raises producers' costs and tends to decrease supply. A subsidy lowers their costs and tends to increase supply.
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Producer expectations of future prices: If producers expect higher prices later, they may hold back current supply. If they expect worse conditions ahead, they may change how much they offer now.
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Number of sellers: More producers in the market increase supply (shift right). Fewer producers decrease supply (shift left).
Government regulations can also act as constraints that affect how much producers can or will supply.
How to Use This on the AP Microeconomics Exam
Free Response
- Draw the supply curve in a correctly labeled graph: price on the vertical axis, quantity on the horizontal axis, and the curve clearly labeled (S or S1).
- When a determinant changes, shift the curve and label the new curve (S2). State the direction clearly: rightward is an increase, leftward is a decrease.
- Explain the cause and effect. Do not just say supply increased; say why, such as "lower input costs reduced production costs, so supply increased and the curve shifted right."
MCQ
- Watch the wording. "Price of the good itself changed" means a movement along the curve. Any other cause means a shift.
- Match each determinant to the correct shift direction before you pick an answer.
Common Trap
- Confusing a change in quantity supplied (movement along the curve from an own-price change) with a change in supply (a shift from a determinant) is one of the most common point losers. Sort the cause first, then decide movement versus shift.
Common Misconceptions
- A change in the good's own price shifts the supply curve. It does not. An own-price change moves you along the existing curve. Only non-price determinants shift the curve.
- Supply means a single quantity. Supply is the whole relationship between price and quantity at every price, shown by the entire curve, not one number.
- The supply curve slopes up because of substitution and income effects. Those explain demand. Supply slopes up because higher prices make producing more profitable, which ties to rising marginal cost.
- Rightward always looks like "up." A rightward shift is an increase in supply, but the curve moves right, meaning more is supplied at every price. Read the axes, not just the visual direction.
- Expected higher future prices always raise supply now. Producers may actually reduce current supply to sell later at the higher expected price, so check the direction the expectation pushes.
Related AP Microeconomics Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
determinants of supply | Factors other than price that affect the quantity of a good producers are willing to supply, including technology, input costs, and producer expectations. |
incentives | Factors that motivate economic actors to make particular choices or take specific actions. |
individual supply curves | The supply curves of individual producers showing the quantity each producer is willing to supply at different price levels. |
law of supply | The economic principle that states the quantity supplied of a good increases when its price increases and decreases when its price decreases, assuming all other factors remain constant. |
market supply curve | The horizontal summation of all individual supply curves, showing the total quantity supplied by all producers at each price level. |
movement along a supply curve | A change in quantity supplied caused by a change in the good's own price, represented by moving along the existing supply curve rather than shifting the curve itself. |
own-price | The price of a good itself, as opposed to prices of other goods; the primary factor causing movement along a supply curve. |
price | The amount of money required to purchase a good or service in a market. |
quantity supplied | The amount of a good or service that producers are willing and able to offer for sale at a specific price. |
supply curve | A graph showing the relationship between the price of a good and the quantity that producers are willing to supply at each price level. |
technology | Methods and tools used in production that can affect the efficiency and cost of producing goods, thereby influencing supply decisions. |
upward-sloping | The characteristic shape of a supply curve, indicating that quantity supplied increases as price increases. |
Frequently Asked Questions
What is the law of supply?
The law of supply says that, holding other factors constant, a higher own-price leads to a higher quantity supplied and a lower own-price leads to a lower quantity supplied. Price and quantity supplied move in the same direction.
Why does the supply curve slope upward?
The supply curve slopes upward because higher prices give producers more incentive to make and sell a good. In later AP Micro topics, this connects to rising marginal cost as firms produce more output.
What causes movement along a supply curve?
A change in the good's own price causes movement along the supply curve. That is called a change in quantity supplied, not a change in supply.
What shifts the supply curve?
A non-price determinant shifts supply. Common shifters include input costs, technology, productivity, taxes, subsidies, producer expectations, regulations, and the number of sellers.
What is the difference between supply and quantity supplied?
Supply is the whole relationship between price and quantity shown by the entire curve. Quantity supplied is one amount producers sell at one specific price.
How do I graph supply on the AP Micro exam?
Put price on the vertical axis and quantity on the horizontal axis, draw an upward-sloping curve, and label it clearly. If a determinant changes, shift the whole curve right or left and explain the cause.