AP Microeconomics Unit 2 ReviewSupply and Demand

AP Microeconomics Unit 2, Supply and Demand, is the unit where you learn the market model that drives the entire course. The single biggest idea is that prices are not arbitrary; they emerge from the interaction of buyers and sellers, and they automatically push competitive markets toward an equilibrium where quantity supplied equals quantity demanded. Everything else in the unit (elasticity, surplus, price controls, taxes, tariffs) is a tool for measuring how that equilibrium behaves when conditions change. At 20-25% of the exam, this is the heaviest unit in AP Micro.

What this unit covers

The two laws and what shifts the curves

• The law of demand says price and quantity demanded move in opposite directions. When a good's own price falls, buyers move down along the demand curve to a higher quantity demanded.
• The law of supply says price and quantity supplied move in the same direction. Higher prices give producers an incentive to make more, so the supply curve slopes upward.
• Market demand and market supply are just the horizontal sums of individual buyers' and sellers' curves. The market curve is the individuals added together at each price.
• The single most-tested distinction in this unit is shift vs. movement. A change in the good's own price moves you along a curve. A change in anything else (income, tastes, input costs, technology, prices of related goods, expectations, number of buyers or sellers) shifts the whole curve.
• Markets only work this way when property rights are well defined and people can respond to incentives within their constraints (income, time, laws).

Elasticity, the responsiveness toolkit

• Price elasticity of demand is %ΔQd / %ΔP. If the magnitude is greater than 1, demand is elastic; less than 1, inelastic; exactly 1, unit elastic.
• Slope is not elasticity. Along a straight-line demand curve, elasticity changes from elastic at high prices to inelastic at low prices, even though the slope never changes.
• The total revenue test ties it together. If demand is elastic, a price increase lowers total revenue. If demand is inelastic, a price increase raises total revenue. At unit elasticity, revenue is maximized.
• Price elasticity of supply is %ΔQs / %ΔP, with the same elastic/inelastic benchmark of 1. Supply tends to be more elastic when producers have more time to adjust.
• Income elasticity of demand (%ΔQd / %Δincome) tells you whether a good is normal (positive) or inferior (negative). Cross-price elasticity (%ΔQd of good A / %ΔP of good B) tells you whether goods are substitutes (positive) or complements (negative).

Equilibrium, surplus, and what happens when markets get shocked

• Equilibrium is where the demand and supply curves cross. The market clears, meaning buyers want to buy exactly the quantity sellers want to sell, with no shortage or surplus.
• Consumer surplus is the area below the demand curve and above the price. Producer surplus is the area above the supply curve and below the price. Together they measure total economic surplus, the gains from trade in that market.
• If price sits above equilibrium, a surplus appears and pushes price down. If price sits below equilibrium, a shortage appears and pulls price up. Market forces self-correct toward equilibrium.
• When a determinant shifts demand or supply, price, quantity, consumer surplus, and producer surplus all change. How much price changes versus quantity depends on the elasticities of the two curves.

Government intervention and international trade

• A price ceiling is a legal maximum price. A binding ceiling (set below equilibrium) creates a shortage. A price floor is a legal minimum price. A binding floor (set above equilibrium) creates a surplus.
• Taxes shift behavior by driving a wedge between what buyers pay and what sellers receive. Subsidies do the opposite, encouraging more production and consumption. Both affect government revenue or spending.
• Tax incidence (who actually bears the burden) depends on relative elasticities. The more inelastic side of the market pays more of the tax, regardless of who legally sends the check.
• Any intervention in a market already producing the efficient quantity shrinks total surplus and creates deadweight loss, the value of mutually beneficial trades that no longer happen.
• Opening to trade replaces the autarky (no-trade) price with the world price. If the world price is lower, the country imports, consumer surplus grows, producer surplus shrinks, and total surplus rises. A tariff raises the domestic price toward autarky, generates government revenue, and creates deadweight loss. Quotas restrict import quantity directly with similar effects but no tariff revenue for the government.

Unit 2, Supply and Demand at a glance

Why Unit 2, Supply and Demand matters in AP Micro

Supply and demand is the default model of the whole course. Almost every later question starts by setting up a market with these two curves, then adds a complication. If you can draw a correctly labeled market graph, shift a curve, and read off the new price and quantity, you have the core skill AP Micro tests over and over.

• The efficiency standard built here (maximizing total surplus at equilibrium) is the benchmark every later unit measures against, from monopoly to externalities.
• Elasticity is the course's measuring stick for responsiveness, and it returns when you analyze tax incidence, monopoly pricing, and wage outcomes.
• The shift vs. movement distinction is the most common graphing error on the exam, and this unit is where you build that fluency.

How this unit connects across the course

• Scarcity, incentives, and marginal decision making from Basic Economic Concepts (Unit 1) explain why demand slopes down and supply slopes up. Demand is really marginal benefit; supply is really marginal cost.
• The supply curve you draw here gets a microfoundation in Production, Cost, and the Perfect Competition Model (Unit 3), where you learn that a competitive firm's supply curve is its marginal cost curve above average variable cost.
• Imperfect Competition (Unit 4) uses elasticity constantly. A monopolist never produces on the inelastic part of its demand curve, and the total revenue test from Topic 2.3 explains why.
• The deadweight loss and surplus analysis from government intervention returns in Market Failure and the Role of Government (Unit 6), where taxes and subsidies become tools to fix externalities instead of distortions. Factor Markets (Unit 5) reuses the whole model with labor, where a minimum wage is just a price floor in the labor market.

Key models and graphs to know

• The supply and demand graph is the workhorse. Label price on the vertical axis, quantity on the horizontal, and find equilibrium where the curves cross.
• Double-shift scenarios show what happens when both curves move at once. One of price or quantity changes for sure, and the other is indeterminate without knowing the sizes of the shifts.
• Consumer and producer surplus areas, usually triangles you compute with (1/2)(base)(height) from a graph or table.
• The total revenue test, linking PED to whether a price change raises or lowers TR = P × Q.
• Price ceiling and price floor graphs, showing the resulting shortage or surplus, the transferred surplus, and the deadweight loss triangle.
• Per-unit tax and subsidy graphs, with the tax wedge between buyer price and seller price, government revenue as a rectangle, and deadweight loss as a triangle.
• The international trade graph, with a horizontal world price line, imports or exports as the gap between domestic quantity demanded and supplied, and a tariff raising the world price line.
• Elasticity formulas: PED = %ΔQd / %ΔP, PES = %ΔQs / %ΔP, income elasticity = %ΔQd / %Δincome, cross-price elasticity = %ΔQd of A / %ΔP of B.

Unit 2, Supply and Demand on the AP exam

This unit carries 20-25% of the exam, the largest share of any unit, so expect it everywhere in the multiple-choice section and as a regular anchor for free-response questions. The skills tested map directly to the verbs in the unit. You define and identify (which curve shifts when income falls and the good is inferior), you explain using graphs (why a binding price ceiling causes a shortage and what happens to consumer surplus), and you calculate from a graph or table (elasticity values, surplus areas, tax revenue, deadweight loss).

Free-response questions in AP Micro typically ask you to draw a correctly labeled graph, shift a curve in response to a described shock, and show new equilibrium price and quantity with clear labels. Common setups include a market hit by a supply or demand shock, a per-unit tax where you identify buyer price, seller price, revenue, and deadweight loss, and a trade scenario where a tariff changes domestic price, imports, and surplus areas. Multiple-choice questions lean heavily on shift vs. movement, the total revenue test, interpreting elasticity values, and predicting indeterminate outcomes in double-shift problems. Practice drawing fast, fully labeled graphs; correct labels earn points even when the explanation is brief.

Essential questions

• How do prices coordinate the decisions of millions of buyers and sellers without anyone being in charge?
• Why do some price changes barely affect behavior while others change it dramatically, and how do we measure that?
• When governments intervene in a market with price controls, taxes, or tariffs, who wins, who loses, and what value disappears entirely?
• Why does opening a market to international trade raise total surplus even though it creates losers within the country?

Key terms to know

• Law of demand: A rise in a good's own price reduces quantity demanded, moving you along the demand curve.
• Law of supply: A rise in a good's own price increases quantity supplied, moving you along the supply curve.
• Determinants of demand: Non-price factors (income, tastes, prices of related goods, expectations, number of buyers) that shift the entire demand curve.
• Price elasticity of demand: The percentage change in quantity demanded divided by the percentage change in price, measuring buyer responsiveness.
• Total revenue test: A quick check that uses how total revenue responds to a price change to classify demand as elastic, inelastic, or unit elastic.
• Cross-price elasticity of demand: The percentage change in quantity demanded of one good divided by the percentage change in the price of another; positive means substitutes, negative means complements.
• Income elasticity of demand: The percentage change in quantity demanded divided by the percentage change in income; positive means normal good, negative means inferior good.
• Consumer surplus: The difference between what buyers are willing to pay and what they actually pay, shown as the area below demand and above price.
• Producer surplus: The difference between the price sellers receive and their minimum acceptable price, shown as the area above supply and below price.
• Price ceiling: A legal maximum price that, when set below equilibrium, creates a persistent shortage.
• Price floor: A legal minimum price that, when set above equilibrium, creates a persistent surplus.
• Deadweight loss: Total surplus lost when a market produces more or less than the efficient quantity.
• Tax incidence: How the burden of a tax is split between buyers and sellers, determined by relative elasticities, not by who writes the check.
• Tariff: A tax on imports that raises the domestic price, generates government revenue, helps domestic producers, and hurts consumers.

Common mix-ups

• A change in price never shifts that good's own demand or supply curve. Price changes cause movements along a curve; only the non-price determinants shift it.
• Slope is not elasticity. A linear demand curve has constant slope but elasticity that falls from elastic to inelastic as you move down it.
• "Surplus" means two different things in this unit. A market surplus is excess quantity supplied at a too-high price. Consumer and producer surplus are measures of benefit, the areas on the graph.
• Price ceilings go below equilibrium to bind, and price floors go above. A ceiling set above equilibrium or a floor set below it does nothing to the market.