AP Microeconomics Unit 2 Notes: Equilibrium, Efficiency, and Policy in Markets

Market Equilibrium

A market is any setting where buyers and sellers interact to exchange a good or service. In AP Microeconomics, you model that interaction with demand (buyers) and supply (sellers). The key idea is that prices are not just numbers—they are signals that coordinate decisions. When the price is “right,” the amount people want to buy matches the amount firms want to sell, and the market is in balance.

What equilibrium is

Market equilibrium is the situation where quantity demanded equals quantity supplied. The equilibrium price is the price at which buyers and sellers are both satisfied in aggregate (not every individual, but the market totals). The equilibrium quantity is the amount actually bought and sold.

In symbols:

Q_d = Q_s

Why this matters: equilibrium is the benchmark for almost everything else in this unit. Consumer and producer surplus, the effects of taxes, price controls, and trade policy are all typically measured relative to what would happen in equilibrium without intervention.

How markets move toward equilibrium (the logic)

If the market price is not at equilibrium, forces push it back:

  • If price is above equilibrium, sellers want to sell a lot but buyers don’t want to buy much. That creates a surplus (excess supply). Firms compete to get rid of unsold inventory by cutting prices, and the price tends to fall.
  • If price is below equilibrium, buyers want to buy more than firms want to sell. That creates a shortage (excess demand). Buyers compete for the limited goods, bidding the price up (or sellers raise prices), and the price tends to rise.

A common misconception is thinking “a surplus means the price is too low.” It’s the opposite: a surplus happens when price is too high (too much produced relative to what consumers will buy).

Shifts vs. movements along curves

A lot of errors come from mixing up changes in quantity demanded/supplied (movements along a curve caused by price changes) with changes in demand/supply (shifts of the entire curve caused by non-price determinants).

  • A shift in demand (from income, tastes, prices of related goods, expectations, number of buyers) changes the equilibrium price and quantity.
  • A shift in supply (from input prices, technology, taxes/subsidies, expectations, number of sellers, regulations, natural conditions) also changes equilibrium.

Worked example: finding equilibrium with equations

Suppose a market has:

Q_d = 100 - 2P
Q_s = 20 + 2P

Step 1: Set Q_d = Q_s

100 - 2P = 20 + 2P

Step 2: Solve for price

80 = 4P
P = 20

Step 3: Plug back in to get quantity

Q = 100 - 2(20) = 60

So equilibrium is P = 20 and Q = 60.

Exam Focus

  • Typical question patterns
    • Given a graph or equations, find equilibrium price and quantity.
    • Predict the direction of change in P and Q after a demand or supply shift.
    • Identify whether a scenario causes a movement along a curve or a shift.
  • Common mistakes
    • Calling a shortage a “surplus” (or vice versa) because you forget which side is bigger.
    • Shifting the wrong curve (for example, treating a change in price as a demand shift).
    • In algebra questions, solving for Q before finding the equilibrium P and mixing up which equation to substitute into.

Consumer and Producer Surplus

Equilibrium tells you where the market lands; surplus tells you how well the market performs for buyers and sellers. In AP Micro, consumer and producer surplus are the core measures of gains from voluntary exchange.

Consumer surplus: what it is and why it matters

Consumer surplus is the difference between what consumers are willing to pay and what they actually pay. It measures the net benefit consumers receive.

On a standard demand-supply graph, the demand curve represents marginal willingness to pay. If the market price is lower than what some consumers would have paid, they gain surplus.

Why it matters: consumer surplus is a building block of total surplus (overall market welfare) and helps you evaluate whether policies help or harm consumers.

Producer surplus: what it is and why it matters

Producer surplus is the difference between the price producers receive and the minimum price they’d be willing to accept to produce (their marginal cost, represented by the supply curve in the standard model).

Why it matters: producer surplus captures firms’ gains from selling in the market. Policies can shift surplus between consumers, producers, and government.

Total surplus and efficiency

Total surplus is:

TS = CS + PS

In the basic competitive market model, equilibrium maximizes total surplus because all trades with benefits greater than costs happen, and no trades with costs greater than benefits occur.

A common misconception: producer surplus is not the same as profit. Profit subtracts fixed costs and other accounting costs; producer surplus is based on the supply curve (marginal costs) and price.

Computing surplus (the triangle idea)

When curves are straight lines and you have a single market price, surplus areas are often triangles.

Triangle area formula:

Area = \frac{1}{2} \times base \times height

  • For consumer surplus, the “height” is (max willingness to pay at Q=0 minus market price), and the base is the market quantity.
  • For producer surplus, the “height” is (market price minus minimum acceptable price at Q=0), and the base is the market quantity.

Worked example: surplus from a graph using intercepts

Suppose demand hits the price axis at P = 50 and supply hits the price axis at P = 10. The market equilibrium is P = 30 and Q = 40.

Consumer surplus:

  • Height: 50 - 30 = 20
  • Base: 40

CS = \frac{1}{2} \times 40 \times 20 = 400

Producer surplus:

  • Height: 30 - 10 = 20
  • Base: 40

PS = \frac{1}{2} \times 40 \times 20 = 400

Total surplus:

TS = 400 + 400 = 800

Exam Focus

  • Typical question patterns
    • Calculate CS, PS, and TS from a graph (often with triangles/rectangles).
    • Show how a policy changes CS and PS and identify deadweight loss.
    • Interpret CS/PS conceptually (who benefits from a price change?).
  • Common mistakes
    • Using the wrong “height” (for CS, it’s demand intercept minus price; for PS, it’s price minus supply intercept).
    • Forgetting that the base is the quantity actually traded, which can change under interventions.
    • Confusing total revenue (a rectangle) with surplus (often triangles).

Price Ceilings and Floors

Price controls are legal minimums or maximums set by the government. They matter because they can prevent the market from reaching equilibrium, creating shortages or surpluses and changing who gets the gains from trade.

Price ceilings

A price ceiling is a legal maximum price. It only matters if it is binding, meaning it is set below equilibrium price.

  • Binding price ceiling → shortage because at the low price, quantity demanded rises and quantity supplied falls.
  • Nonbinding price ceiling (set above equilibrium) → no effect.

Why it matters: ceilings are often intended to make goods “affordable” (rent control, price caps on necessities), but the shortage creates non-price rationing—waiting lines, reduced quality, favoritism, or black markets.

A key AP idea: the “problem” is not just that fewer units are sold; it’s that mutually beneficial trades that would have occurred at equilibrium no longer happen.

Price floors

A price floor is a legal minimum price. It only matters if it is binding, meaning it is set above equilibrium price.

  • Binding price floor → surplus because quantity supplied rises and quantity demanded falls.
  • Nonbinding price floor → no effect.

Common examples include minimum wage (a labor market floor) and agricultural price supports.

Welfare effects and deadweight loss

When a binding price control reduces the quantity exchanged below the efficient equilibrium quantity, it creates deadweight loss (DWL): total surplus that disappears because trades that would have benefited both sides do not happen.

Conceptually:

  • Some consumer and/or producer surplus is transferred (for example, consumers who still get the good at a lower price may gain).
  • But the “missing trades” generate a net loss—DWL.

You usually identify DWL as a triangle between supply and demand over the range of quantities that are no longer traded.

Worked example: identifying shortage/surplus and the traded quantity

Suppose equilibrium is P = 10 and Q = 100.

1) Government sets a price ceiling at P = 6 (binding).

  • At P = 6, assume Q_d = 140 and Q_s = 60.
  • Shortage is:

Shortage = Q_d - Q_s = 140 - 60 = 80

  • The quantity actually traded is limited by the smaller of the two (what sellers bring to market): Q = 60.

2) Government sets a price floor at P = 14 (binding).

  • At P = 14, assume Q_d = 70 and Q_s = 130.
  • Surplus is:

Surplus = Q_s - Q_d = 130 - 70 = 60

  • Quantity actually traded is limited by the smaller number (what buyers will buy): Q = 70.

A common mistake is to use Q_d under a ceiling as “the quantity sold.” Under a shortage, sellers can’t (or won’t) supply that much at the capped price.

Exam Focus

  • Typical question patterns
    • Determine whether a price ceiling/floor is binding and identify shortage or surplus.
    • Compute the size of shortage/surplus and the quantity actually exchanged.
    • Shade CS/PS/DWL on a graph after a price control.
  • Common mistakes
    • Saying “price ceiling causes surplus” or “price floor causes shortage.” Remember: ceiling → shortage; floor → surplus.
    • Forgetting that the traded quantity is the smaller of Q_s and Q_d at the controlled price.
    • Treating nonbinding controls as if they change the market outcome.

Taxes and Subsidies

Taxes and subsidies are powerful because they change incentives directly. In the supply-demand model, they show up as wedges between what buyers pay and what sellers receive.

Per-unit taxes: what they are

A per-unit tax is a fixed amount charged on each unit sold (for example, 1 dollar per gallon). It can be levied on buyers or sellers, but the key AP result is that legal incidence (who is required to pay) is not the same as economic incidence (who actually bears the burden through higher prices paid or lower prices received).

Define:

  • P_b = price paid by buyers
  • P_s = price received by sellers
  • t = tax per unit

The wedge is:

P_b - P_s = t

How a tax changes the market outcome

A tax reduces the quantity traded because it effectively raises the cost of buying/selling.

Graphically:

  • If the tax is on sellers, the supply curve shifts up by the amount of the tax (because sellers need a higher price to supply the same quantity).
  • If the tax is on buyers, you can think of demand shifting down by the amount of the tax.

Either way, you end up with:

  • Higher P_b
  • Lower P_s
  • Lower equilibrium quantity

Tax incidence depends on elasticities

The side of the market that is more inelastic bears more of the tax burden.

Intuition: if you are less responsive to price changes, you can’t “escape” the tax easily, so more of it sticks to you.

  • More inelastic demand → consumers bear more of the tax (buyers’ price rises a lot).
  • More inelastic supply → producers bear more of the tax (sellers’ received price falls a lot).

A frequent misconception is “the seller always pays a tax imposed on sellers.” In competitive markets, prices adjust; the burden is shared based on elasticities.

Government revenue and deadweight loss

Tax revenue is a rectangle:

Tax\ Revenue = t \times Q_{after\ tax}

A tax also creates deadweight loss because trades that would have happened at the no-tax equilibrium no longer happen.

A qualitative rule: bigger taxes and more elastic supply/demand generally create larger DWL because quantity falls more.

Subsidies: the mirror image

A subsidy is a payment that encourages production or consumption.

For a per-unit subsidy s:

  • Buyers may pay a lower effective price.
  • Sellers may receive a higher effective price.
  • Quantity traded increases.

The wedge becomes:

P_s - P_b = s

Subsidies usually require government spending and can also generate DWL if they push the market beyond the efficient quantity (leading to overproduction/overconsumption relative to marginal cost and marginal benefit).

Worked example: solving for post-tax prices and quantity

Using the earlier market:

Q_d = 100 - 2P
Q_s = 20 + 2P

Equilibrium was P = 20 and Q = 60.

Now impose a per-unit tax t = 10 on sellers.

Step 1: Use the wedge relationship
Sellers receive P_s, buyers pay P_b, and:

P_b - P_s = 10

Supply depends on P_s, demand depends on P_b.

Step 2: Write quantities using the correct prices

Q_d = 100 - 2P_b
Q_s = 20 + 2P_s

Step 3: Set Q_d = Q_s and substitute P_b = P_s + 10

100 - 2(P_s + 10) = 20 + 2P_s

100 - 2P_s - 20 = 20 + 2P_s

80 - 2P_s = 20 + 2P_s

60 = 4P_s

P_s = 15

Then:

P_b = 25

Step 4: Find quantity after tax

Q = 20 + 2(15) = 50

So the tax causes quantity to fall from 60 to 50, buyers to pay 25, and sellers to receive 15. Notice the tax burden is split: buyers pay 5 more, sellers receive 5 less.

Government revenue:

Tax\ Revenue = 10 \times 50 = 500

Exam Focus

  • Typical question patterns
    • Compute P_b, P_s, and Q after a per-unit tax; calculate tax revenue.
    • Determine which side bears more burden using relative elasticities.
    • Identify CS, PS, tax revenue, and DWL regions on a graph.
  • Common mistakes
    • Using one price when the tax creates two prices (forgetting the wedge).
    • Claiming the side legally taxed bears all the incidence.
    • Computing revenue using the pre-tax quantity instead of Q_{after\ tax}.

International Trade and Tariffs

International trade extends the supply-demand model by introducing a new “price”: the world price. Once a country opens to trade, domestic buyers and sellers can transact with the rest of the world, often at a price different from the closed-economy equilibrium.

World price and trade direction

The world price P_w is the price at which a good is bought and sold internationally.

Compare P_w to the domestic equilibrium price P_{dom} (the price without trade):

  • If P_w < P_{dom}, the country becomes an importer. Consumers can buy cheaper from abroad; domestic producers face lower prices.
  • If P_w > P_{dom}, the country becomes an exporter. Domestic producers can sell at higher prices; domestic consumers face higher prices.

In the small-country model used in AP Micro, the country takes P_w as given (it’s too small to affect the world price).

How to find imports or exports on a graph

When the domestic price is forced to equal P_w:

  • Domestic quantity demanded is read from the demand curve at P_w.
  • Domestic quantity supplied is read from the supply curve at P_w.

If importing:

Imports = Q_d(P_w) - Q_s(P_w)

If exporting:

Exports = Q_s(P_w) - Q_d(P_w)

Why this matters: many trade questions are really “read the two quantities off the graph and take the difference.”

Gains from trade (who wins and who loses)

When a country opens to trade, total surplus typically rises, but it is redistributed:

  • As an importer (lower price): consumers gain surplus; producers lose surplus.
  • As an exporter (higher price): producers gain surplus; consumers lose surplus.

The gains come from getting more of the good from the lower-cost source (imports) or selling to higher willingness-to-pay buyers abroad (exports).

A common misconception is “trade helps everyone domestically.” In this simplified model, one group gains and the other loses, even though total surplus increases.

Tariffs: what they are

A tariff is a tax on imported goods.

If the world price is P_w and the tariff is t per unit, then the domestic price with the tariff becomes:

P_{tariff} = P_w + t

(For an importing country in the small-country model.)

How tariffs change the market

Starting from free trade as an importer:

  • Domestic price rises from P_w to P_w + t.
  • Domestic quantity supplied rises (local firms produce more at the higher price).
  • Domestic quantity demanded falls (consumers buy less at the higher price).
  • Imports shrink.

Welfare effects under a tariff:

  • Consumers lose surplus (higher price, lower quantity consumed).
  • Producers gain surplus (higher price, more domestic production).
  • Government collects tariff revenue:

Tariff\ Revenue = t \times Imports_{after\ tariff}

  • There is deadweight loss from two inefficiencies:
    • Production inefficiency: domestic firms produce units that foreign producers could have produced at lower cost.
    • Consumption inefficiency: some consumers who value the good above its true resource cost stop buying because of the tariff.

Worked example: imports and tariff revenue

Assume a country is an importer under free trade.

  • World price: P_w = 8
  • At P = 8: Q_d = 120 and Q_s = 40

Imports under free trade:

Imports = 120 - 40 = 80

Now impose a tariff t = 2, so:

P_{tariff} = 10

At P = 10: suppose Q_d = 100 and Q_s = 60.

Imports after tariff:

Imports_{after} = 100 - 60 = 40

Tariff revenue:

Tariff\ Revenue = 2 \times 40 = 80

Interpretation: the tariff reduces imports (from 80 to 40) and raises government revenue, but it does so by raising the domestic price and reducing consumption.

Exam Focus

  • Typical question patterns
    • Given P_w, determine whether the country imports or exports; calculate the amount of imports/exports.
    • Show gains from trade on a graph (changes in CS and PS) when moving from autarky to world price.
    • Analyze a tariff: new domestic price, new imports, tariff revenue, and DWL.
  • Common mistakes
    • Mixing up importer vs. exporter cases (always compare P_w to the domestic equilibrium price).
    • Calculating imports as Q_s - Q_d even when importing (sign error).
    • Using free-trade import quantity when computing tariff revenue instead of Imports_{after\ tariff}.