Supply and Demand

The Core Model

Most people assume someone decides what gasoline costs. A government agency, maybe. Some pricing czar in a back office. They're wrong. The price of a gallon of gas emerges from millions of uncoordinated decisions made by buyers who want fuel and stations that want profit. Nobody is in charge.

Supply and demand is how economists describe that messy, decentralized process. Buyers want lower prices. Sellers want higher ones. Somewhere in the middle, those two pressures cancel out and you land on an equilibrium, the price where the quantity people want to buy matches the quantity producers want to sell. Concert tickets work this way. So do hourly wages, apartment rents, and the going rate for a dozen eggs at your grocery store.

The 2022 AP Micro exam leaned heavily on this framework. If you walked in unable to draw a basic supply-and-demand diagram, you were already behind.

Demand: The Buyer's Side

How many iPhones would people buy at $500? At $1,200? At $200? Demand is the full schedule of those answers — every possible price matched with the quantity consumers would purchase at that price.

The Law of Demand says price and quantity demanded move in opposite directions. Sneakers at $80, you grab a pair. At $200, you walk past. Clearance rack at $40, you buy two. That inverse relationship explains why the demand curve slopes downward on the graph.

Now, a *change in quantity demanded* and a *change in demand* sound like the same thing. They are not. If you write on a free-response that "demand decreased" when the question describes a price increase, you will lose points. A price change slides you along the existing curve. The whole curve only shifts when something other than the good's own price changes:
- Income goes up, and you eat out more (that's a *normal good*); meanwhile your ramen purchases might drop, because ramen is an *inferior good* for most people once they can afford alternatives
- Substitute prices matter — Pepsi drops to 99 cents and suddenly fewer people reach for Coke
- Cultural shifts or viral trends (the 2023 Stanley cup craze sent demand for those tumblers through the roof practically overnight)
- Expectations about future prices — rumors of a tariff on electronics next month push people to buy laptops today
- Market size — 10,000 new college students move into a city each fall, and cheap pizza demand jumps

Supply: The Seller's Side

Put yourself in a wheat farmer's boots. Wheat prices double overnight. You plant more wheat. Maybe you convert a few hundred acres that were growing corn. The profit motive pulls resources toward whatever's paying well, and that logic is the entire foundation of the supply side.

The Law of Supply captures it simply: higher prices lead to higher *quantity supplied*. The supply curve slopes upward because production becomes more attractive as the price climbs.

What shifts the entire supply curve left or right? Several things:
- A spike in input costs — steel prices rise and car manufacturers supply fewer vehicles at every price level
- Technology breakthroughs — fracking technology in the mid-2000s unlocked massive oil reserves across North Dakota and Texas, shifting U.S. oil supply dramatically to the right
- More sellers entering (a wave of new coffee shops in your neighborhood)
- Government policy can go either direction: a $0.02-per-ounce tax on sugary drinks shrinks supply, while a federal subsidy on solar panels expands it
- A freeze in Florida devastates the orange crop — supply shifts sharply left, and orange juice prices spike at the grocery store within weeks

Equilibrium

Where the supply and demand curves cross, you get the equilibrium price and equilibrium quantity. At that price, the amount buyers want to purchase exactly matches what sellers want to produce. No unsold inventory stacking up in warehouses. No frustrated shoppers leaving empty-handed.

What happens when the price sits above equilibrium? Sellers stock shelves that don't empty. That unsold inventory — a *surplus* — pressures them to cut prices, run sales, and discount. The price drifts back down.

Below equilibrium, the opposite. Buyers show up and the product is gone. That *shortage* lets sellers raise prices, or buyers outbid each other. The price climbs back up.

Economists call equilibrium a "resting point" for this reason. The market doesn't stay away from it for long. External forces like government price controls can pin the price somewhere else, but the pressure to return never disappears. On the 2019 AP Micro free-response, students who couldn't explain this self-correcting mechanism lost easy points on an otherwise straightforward question.

Shifts vs. Movements Along the Curve

One question sorts this out: *did the good's own price change, or did something else change?*

Own price changed → you get a movement along the curve. The curve stays put. You slide to a different point on it. Gasoline goes from $3.50 to $4.00 a gallon, so you drive a bit less. That's movement *along* the demand curve.

Something else changed → the entire curve shifts. Tesla releases a $25,000 electric car, and millions of drivers stop caring about gasoline at *any* price. The gas demand curve shifts left.

If a free-response question says "the price of corn increased, so demand for corn decreased" and you agree with that phrasing, you just told the grader you don't understand the model. Price of corn increasing means *quantity demanded* decreased — a movement along the curve. Demand itself didn't budge. The distinction sounds pedantic. On the AP exam, it is worth real points. Every single time.

A useful filter: if the cause is the good's own price, it's a movement. If the cause is income, preferences, input costs, technology, substitutes, complements, expectations, or policy, it's a shift.

Worked Example

Given: Demand: P = 100 - 2Q and Supply: P = 20 + Q. Find equilibrium.

Step 1: Set the equations equal. At equilibrium, the price on both sides matches:

100 - 2Q = 20 + Q

Step 2: Solve for Q.

100 - 20 = Q + 2Q
80 = 3Q
Q = 26.67 units

Step 3: Plug Q into either equation for P. Using supply:

P = 20 + 26.67 = $46.67

Verify with demand: P = 100 - 2(26.67) = 100 - 53.33 = $46.67. Same answer, so we're good.

In plain English: at $46.67, buyers want exactly 26.67 units and sellers want to produce exactly 26.67 units. No surplus, no shortage.

*Bonus application:* What if the exam asks about a surplus or shortage at P = $60? Plug $60 into both equations. Demand gives Q = 20; supply gives Q = 40. Quantity supplied exceeds quantity demanded by 20 units, so you've got a surplus of 20.

Price Controls

Sometimes a government decides the market price is too high or too low and steps in. The results are predictable, and predictably messy.

A price ceiling is a legal *maximum*. New York City's rent stabilization program caps what landlords can charge on roughly one million apartments. When that cap sits below the equilibrium rent, more tenants want apartments than landlords are willing to offer at the capped price. The result is a *shortage*. Long waitlists, under-the-table payments, and deteriorating building conditions follow.

A price floor is a legal *minimum*. The federal minimum wage — $7.25 per hour since 2009 — is the textbook example. Set it above the equilibrium wage, and more workers want jobs than firms want to fill. The resulting *surplus* of labor is unemployment.

If you write on the AP exam that a price ceiling creates a surplus, that answer is worth zero points. Floors go *under* something to hold it up — they prop the price above equilibrium, so quantity supplied exceeds quantity demanded, producing surpluses. Ceilings press *down* on the price, holding it below equilibrium, so quantity demanded exceeds quantity supplied, producing shortages.

Both binding controls generate deadweight loss. Transactions that would have made both buyer and seller better off simply never happen.