Demand and Supply Review for AP Economics (page 3)

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By — McGraw-Hill Professional
Updated on Mar 4, 2011


Main Topics: Law of Supply, Increasing Marginal Costs, Supply Curves, Determinants of Supply

Fred and Wilma, Woodward and Bernstein, Ross and Rachel, Bill and Monica, Mercedes and Benz, Ben and Jerry, Sex Symbol and Economist. What do they have in common? They are all famous, or infamous, partners. If there are three words that you need to have in your arsenal for the AP exams, they are "Demand and Supply," or "Supply and Demand" if you are the rebellious type. The previous section has covered the demand half of this duo and so it stands to reason that we should spend a little time studying the other side. Unlike demand, few of us have ever had up close and personal experience as suppliers. Lacking such personal experience with supply, it is helpful to try to put yourself in the shoes of someone who wishes to profit from the production and sale of a product. If something happens that would increase your chances of earning more profit, you increase your supply of the product. If something happens that will hurt your profit opportunities, you decrease your supply of the product.

Law of Supply

Drumroll, please. The Law of Supply is commonly described as: "Holding all else equal, when the price of a good rises, suppliers increase their quantity supplied for that good." In other words, there is a direct, or positive, relationship between the price and the quantity supplied of a good.

Again, we insist on qualifying our law with the phrase, "Holding all else equal." Similar to the demand model, the supply model is a simplified version of real behavior. In addition to the price, there are several factors that influence how many units of a good producers supply. In order to predict how producers respond to fluctuations in one variable (price), we must assume that all other relevant factors are held constant. Before we talk about these external supply determinants, let's examine what is happening behind the scenes of the Law of Supply.

Increasing Marginal Costs

The more you do something (e.g., a physical activity), the more difficult it becomes to do the next unit of that activity. Anyone who has run laps around a track, lifted weights, or raked leaves in the yard understands this. If you were asked to rake leaves, as more hours of raking are supplied, it becomes physically more and more difficult to rake the next hour. We also include the opportunity cost of the time involved in the raking, and you surely know that time is precious to a student. If you have a paper to write or an exam to cram for, raking leaves for an hour comes at a dear cost. In terms of forgone opportunities, the marginal cost of raking leaves rises as you postpone that paper or study session.

When we discussed production possibilities in Chapter 5, we addressed a key economic concept: as more of a good is produced, the greater is its marginal cost.

  • As suppliers increase the quantity supplied of a good, they face rising marginal costs.
  • As a result, they only increase the quantity supplied of that good if the price received is high enough to at least cover the higher marginal cost.

The Supply Curve

A small town has a thriving summer sidewalk lemonade stand industry. Table 6.3 summarizes the daily quantity of lemonade cups offered for sale at several prices, holding constant all other factors that might influence the overall supply of lemonade. This table is sometimes referred to as a supply schedule.

The values in this table reflect the Law of Supply: "Holding all else equal, when the price of a cup of lemonade rises, suppliers increase their quantity supplied for lemonade." Remember those profit opportunities? If kids can sell more cups of lemonade at a higher price, they will do so. It is often quite useful to convert a supply schedule like the one in Table 6.3 into a graphical representation, the supply curve (Figure 6.5).


Quantity Supplied versus Supply

The Law of Supply predicts an upward (or positive) sloping supply curve (Figure 6.5). When the price moves from $1 to $1.25, and all other factors are held constant, we observe an increase in the quantity supplied from 100 cups to 120 cups. Just as with demand, it is important to place special emphasis on "quantity supplied." When the price of the good changes, and all other factors are held constant, the supply curve is held constant; we simply observe the producer moving along the fixed supply curve. If one of the external factors changes, the entire supply curve shifts to the left or right.

Determinants of Supply

Lemonade producers are willing and able to supply more lemonade if something happens that promises to increase their profit opportunities. In addition to the price of the product itself, there are a number of variables, or determinants of supply, that account for the total supply of a good like lemonade. These factors are:

  • Cost of Inputs

If the cost of sugar, a key ingredient in lemonade, unexpectedly falls, it has now become less costly to produce lemonade and so we should expect producers all over town, seeing the profit opportunity, to increase the supply of lemonade at all prices. This results in a graphical rightward shift in the entire supply curve.

  • An increase in supply is viewed as a rightward shift in the supply curve. There are two ways to think about this shift.
  1. At all prices, the producer is willing and able to supply more units of the good. In Figure 6.6 you can see that at the constant price of $1, the quantity supplied has risen from two to three.
  2. At all quantities, the marginal cost of production is lower, so producers are willing and able to accept lower prices for the good.
  • Of course the opposite is true of a decrease in supply, or leftward shift of the supply curve. In Figure 6.6 you can see that at the constant price of $1, the quantity supplied has fallen from two to one.


  • Technology or Productivity

A technological improvement usually decreases the marginal cost of producing a good, thus allowing the producer to supply more units, and is reflected by a rightward shift in the supply curve. If kids all over town began using electric lemon squeezers rather than their sticky bare hands, the supply of lemonade would increase.

  • Taxes and Subsidies

A per unit tax is treated by the firm as an additional cost of production and would therefore decrease the supply curve, or shift it leftward. Mayor McScrooge might impose a 25 cent tax on every cup of lemonade, decreasing the entire supply curve. A subsidy is essentially the anti-tax, or a per unit gift from the government because it lowers the per unit cost of production.

  • Price Expectations

A producer's willingness to supply today might be affected by an expectation of tomorrow's price. If it were the 2nd of July, and lemonade producers expected a heat wave and a 4th of July parade in two days, they might hold back some of their supply today and hope to sell it at an inflated price on the holiday. Thus today's quantity supplied at all prices would decrease.

  • Price of Other Outputs

Firms can use the same resources to produce different goods. If the price of a milkshake were rising and profit opportunities were improving for milkshake producers, the supply of lemonade in a small town would decrease and the supply of milkshakes would increase.

  • Number of Suppliers

When more suppliers enter a market, we expect the supply curve to shift to the right. If several of our lemonade entrepreneurs are forced by their parents to attend summer camp, we would expect the entire supply curve to move leftward. Fewer cups of lemonade would be supplied at each and every price.

The review questions for this study guide can be found at:

Demand, Supply, Market Equilibrium, and Welfare Analysis Review Questions for AP Economics

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