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2. KRUGMAN’S
Economics
for AP®
S E C O N D E D I T I O N

3. Section 9
Module 47

4. What You Will Learn in this Module
Explain the difference between elastic and inelastic demand
Describe the relationship between elasticity and total revenue
Illustrate how the price elasticity changes along a demand curve
Identify the factors that determine price elasticity of demand
What You Will Learn in this Module
Section 9 | Module 47

5. What the Price Elasticity of Demand Tells Us
How elastic is elastic?
Demand is perfectly inelastic when the quantity demanded does not respond at all to changes in the price.
When demand is perfectly inelastic, the demand curve is a vertical line.
Demand is perfectly elastic when any price increase will cause the quantity demanded to drop to zero.
When demand is perfectly elastic, the demand curve is a horizontal line.
Section 9 | Module 47

6. Two Extreme Cases of Price Elasticity of Demand
Figure Caption:
Figure 11.1 (47.1): Two Extreme Cases of Price Elasticity of Demand
Panel (a) shows a perfectly inelastic demand curve, which is a vertical line. The quantity of shoelaces demanded is always 1 billion pairs, regardless of price. As a result, the price elasticity of demand is zero—the quantity demanded is unaffected by the price.
Section 9 | Module 47

7. Interpreting the Price Elasticity of Demand
Demand is elastic if the price elasticity of demand is greater than 1.
Demand is inelastic if the price elasticity of demand is less than 1.
Demand is unit-elastic if the price elasticity of demand is exactly 1.
PEofD = (% Change in Quantity Demanded)/(% Change in Price)
Section 9 | Module 47

8. Unit-Elastic Demand, Inelastic Demand, and Elastic Demand
Figure Caption:
Figure 11.2 (47.2): Unit-Elastic Demand, Inelastic Demand, and Elastic Demand
Panel (a) shows a case of unit-elastic demand: a 20% increase in price generates a 20% decline in quantity demanded, implying a price elasticity of demand of 1.
Unit-Elastic Demand, Inelastic Demand, and Elastic Demand
Section 9 | Module 47

9. How Elastic is Elastic?
Because this classification predicts how changes in the price of a good will affect the total revenue earned by producers from the sale of that good.
The total revenue is defined as the total value of sales of a good or service, i.e.
Total Revenue = Price × Quantity Sold
Section 9 | Module 47

10. Total Revenue Figure Caption: Figure 11.3(47.3):
The green rectangle represents total revenue generated from 1,100 drivers who each pay a toll of $0.90.
Section 9 | Module 47

11. How Elastic is Elastic?
When a seller raises the price of a good, there are two countervailing effects in action (except in the rare case of a good with perfectly elastic or perfectly inelastic demand):
A price effect: After a price increase, each unit sold sells at a higher price, which tends to raise revenue.
A quantity effect: After a price increase, fewer units are sold, which tends to lower revenue.
Section 9 | Module 47

12. How Elastic is Elastic?
If demand for a good is elastic (the price elasticity of demand is greater than 1), an increase in price reduces total revenue.
In this case, the quantity effect is stronger than the price effect.
If demand for a good is inelastic (the price elasticity of demand is less than 1), a higher price increases total revenue.
In this case, the price effect is stronger than the quantity effect.
If demand for a good is unit-elastic (the price elasticity of demand is 1), an increase in price does not change total revenue.
In this case, the sales effect and the price effect exactly offset each other.
Section 9 | Module 47

13. Price Elasticity of Demand and Total Revenue
Section 9 | Module 47

14. The Price Elasticity of Demand Changes Along the Demand Curve
Figure Caption:
Figure 11.4: The Price Elasticity of Demand Changes Along the Demand Curve
The upper panel shows a demand curve corresponding to the demand schedule in the table. The lower panel shows how total revenue changes along that demand curve: at each price and quantity combination, the height of the bar represents the total revenue generated. You can see that at a low price, raising the price increases total revenue. So demand is inelastic at low prices. At a high price, however, a rise in price reduces total revenue. So demand is elastic at high prices.
The Price Elasticity of Demand Changes Along the Demand Curve
Section 9 | Module 47

15. What Factors Determine the Price Elasticity of Demand?
Price elasticity of demand is determined by:
whether close substitutes are available
whether the good is a necessity or a luxury
share of income spent on the good
time
Section 9 | Module 47

16. Section 9 | Module 47

17. Summary
The responsiveness of the quantity demanded to price can range from perfectly inelastic demand, where the quantity demanded is unaffected by the price, to perfectly elastic demand, where there is a unique price at which consumers will buy as much or as little as they are offered.
When demand is perfectly inelastic, the demand curve is a vertical line; when it is perfectly elastic, the demand curve is a horizontal line.
The price elasticity of demand is classified according to whether it is more or less than 1.
If it is greater than 1, demand is elastic; if it is less than 1, demand is inelastic; if it is exactly 1, demand is unit-elastic. This classification determines how total revenue, the total value of sales, changes when the price changes.
Section 9 | Module 47