Unit 2: Supply and Demand

📚Study Guide: Supply and Demand

Unit 2: Supply and Demand

Overview: The supply and demand model is the cornerstone of microeconomic analysis and provides the essential framework for understanding how prices and quantities are determined in competitive markets. This unit requires students to thoroughly understand the distinction between a change in demand—a shift of the entire demand curve caused by changes in income, tastes, prices of related goods, expectations, or the number of buyers—and a change in quantity demanded, which is a movement along a fixed demand curve caused solely by a change in the good's own price. Similarly, students must distinguish between a change in supply and a change in quantity supplied. The equilibrium price and quantity occur where the supply and demand curves intersect, creating market-clearing conditions where quantity demanded equals quantity supplied. The unit extensively covers elasticity, which measures the responsiveness of quantity demanded or supplied to changes in price or other determinants. Price elasticity of demand determines whether a price increase raises or lowers total revenue: when demand is elastic, price and total revenue move in opposite directions; when inelastic, they move together. The concepts of consumer surplus and producer surplus provide measures of market efficiency and the gains from trade. Students also analyze government interventions in markets, including price ceilings, which create shortages when set below equilibrium; price floors, which create surpluses when set above equilibrium; and per-unit taxes, which create deadweight loss by reducing the quantity traded below the efficient level. Understanding tax incidence—who bears the burden of a tax—is determined by the relative price elasticities of supply and demand; the more inelastic side of the market bears a larger share of the tax burden.

Key Concepts

• Law of Demand: There is an inverse relationship between price and quantity demanded, other things equal, resulting in a downward-sloping demand curve.
• Law of Supply: There is a direct relationship between price and quantity supplied, other things equal, resulting in an upward-sloping supply curve.
• Market Equilibrium: The price and quantity at which the quantity demanded equals the quantity supplied. At equilibrium, there is neither a surplus nor a shortage.
• Price Elasticity of Demand: A measure of how much the quantity demanded responds to a change in price. Calculated as the percentage change in quantity demanded divided by the percentage change in price. Demand is elastic when the absolute value exceeds 1, inelastic when below 1, and unit elastic when equal to 1.
• Total Revenue Test: If demand is elastic, an increase in price decreases total revenue. If demand is inelastic, an increase in price increases total revenue.
• Consumer Surplus: The difference between the maximum price consumers are willing to pay and the actual price they pay, represented graphically as the area below the demand curve and above the price.
• Producer Surplus: The difference between the actual price producers receive and the minimum price they are willing to accept, represented as the area above the supply curve and below the price.
• Tax Incidence and Deadweight Loss: A per-unit tax drives a wedge between the price paid by consumers and the price received by producers, creating a deadweight loss triangle representing lost gains from trade. The side with the more inelastic curve bears more of the tax burden.

Vocabulary

• Demand: The relationship between the price of a good and the quantity consumers are willing and able to purchase at various prices.
• Supply: The relationship between the price of a good and the quantity producers are willing and able to sell at various prices.
• Equilibrium Price: The price at which the quantity demanded equals the quantity supplied.
• Shortage: A situation in which quantity demanded exceeds quantity supplied, occurring when the price is below equilibrium.
• Surplus: A situation in which quantity supplied exceeds quantity demanded, occurring when the price is above equilibrium.
• Elastic: A condition in which the absolute value of the price elasticity of demand is greater than 1, indicating high responsiveness.
• Inelastic: A condition in which the absolute value of the price elasticity of demand is less than 1, indicating low responsiveness.
• Price Ceiling: A legal maximum price that can be charged for a good, effective only when set below the equilibrium price.
• Price Floor: A legal minimum price that can be charged for a good, effective only when set above the equilibrium price.
• Deadweight Loss: The reduction in total surplus resulting from a market distortion such as a tax, price control, or monopoly pricing.

Essential Formulas and Graphs

• Price Elasticity of Demand: %ΔQd / %ΔP (use midpoint formula for precise calculations)
• Midpoint Formula: [(Q2 - Q1) / ((Q2 + Q1) / 2)] / [(P2 - P1) / ((P2 + P1) / 2)]
• Cross-Price Elasticity: Positive for substitutes, negative for complements
• Income Elasticity: Positive for normal goods, negative for inferior goods
• Graph: Supply and demand graph showing equilibrium, surpluses, shortages, taxes, price ceilings, and price floors.

Common Mistakes

• Confusing a shift in demand with a movement along the demand curve. Only changes in the good's own price cause movements; all other factors shift the curve.
• Misinterpreting elasticity values. Remember that |Ed| > 1 means elastic, |Ed| < 1 means inelastic, and |Ed| = 1 means unit elastic.
• Drawing price ceilings above equilibrium or price floors below equilibrium. Such non-binding controls have no effect on the market outcome.
• Believing that the statutory incidence of a tax determines the economic burden. The distribution of the burden depends on relative elasticities, not on whom the tax is legally levied.

AP Exam Strategies

• Memorize the demand shifters using the acronym TIREN: Tastes, Income, Prices of Related goods, Expectations, Number of buyers.
• Memorize the supply shifters using TIPENS: Technology, Input prices, Prices of related goods in production, Expectations, Number of sellers, Government policies (taxes, subsidies, regulations).
• When analyzing taxes, draw the vertical wedge between the supply and demand curves and clearly label the price consumers pay, the price producers receive, and the tax per unit.
• To determine tax incidence, compare the slopes or elasticities: the steeper (more inelastic) curve bears the larger burden.

Real-World Applications

• Rent Control: New York City and San Francisco impose rent ceilings that create housing shortages, reduce landlord incentives to maintain properties, and lead to black markets and waiting lists.
• Gasoline Taxes: Because demand for gasoline is relatively inelastic in the short run, consumers bear most of the burden of gasoline taxes at the pump.
• Minimum Wage: A price floor in the labor market that can create unemployment (surplus of labor) when set above the equilibrium wage, particularly affecting low-skilled workers.