Supply and Demand

The Core Model

Nobody in Washington sits in a room and decides what gas should cost. The price you see at the pump comes out of millions of individual decisions happening all at once, drivers choosing whether to fill up or wait until Thursday, refineries calculating how much crude to run through, and the gas station on the corner watching what the Chevron across the street just posted on its sign. The whole system is completely decentralized, and honestly it works way better than you'd expect.

Supply and demand is how economists make sense of all that chaos. Buyers want prices low, sellers want them high, and the back-and-forth settles on an equilibrium, the price where the quantity people want to buy lines up exactly with what sellers want to produce. Concert tickets, apartment rents, hourly wages, the price of a dozen eggs at Kroger. If it has a market price, this model explains how it got there.

The 2022 AP Micro exam leaned on this framework hard. Walking in without being able to draw a supply-and-demand diagram was basically like showing up to a calculus final without knowing what a fraction is.

Demand: The Buyer's Side

Demand is the full schedule of how many units consumers would buy at every possible price. Not one price. Every price. When Apple prices the iPhone at $500, they sell a certain number. At $1,200, fewer people bite. At $200, everyone and their grandma grabs one.

The Law of Demand says price and quantity demanded move in opposite directions, and that's what gives you the downward-sloping demand curve on the graph. Sneakers at $80, people grab a pair. Same sneakers at $200, most people walk right past. Slap a $40 clearance sticker on them and some people buy two.

Here's an AP exam pitfall that costs students points every single year: mixing up change in quantity demanded with change in demand. If you write on a free-response that "demand decreased" when the question is describing a price increase, the grader takes off points. And honestly they should, because those are completely different things. A price change just 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 people start eating out more (restaurant meals are a normal good), but their ramen purchases might actually drop since ramen is an inferior good for most people once they can afford real food
- Substitute prices: Pepsi drops to 99 cents a can and Coke starts losing customers
- Cultural shifts or viral trends, like the 2023 Stanley tumbler craze that sent demand through the roof in about two weeks flat
- Expectations about future prices: rumors of a tariff on electronics next month and suddenly everyone rushes to buy laptops right now
- Market size: 10,000 new college students pour into a town every August, and cheap pizza demand spikes overnight

Supply: The Seller's Side

A wheat farmer checks the futures market one morning and sees wheat prices have doubled overnight. What does she do? She plants more wheat, maybe converts a few hundred acres that had been growing corn. Profit pulls resources toward whatever pays well, and that basic logic is the entire foundation of the supply side.

The Law of Supply: higher prices mean higher quantity supplied. The supply curve slopes upward because producing stuff gets more attractive as prices climb. Pretty intuitive when you think about it.

What shifts the entire supply curve?
- A spike in input costs: steel prices jump 40% and automakers supply fewer vehicles at every price level
- Technology breakthroughs: fracking in the mid-2000s unlocked massive oil reserves across North Dakota and Texas, and U.S. oil supply shifted dramatically to the right
- More sellers entering the market (suddenly a wave of new coffee shops opens in your neighborhood)
- Government policy cuts both ways: a $0.02-per-ounce tax on sugary drinks shrinks supply, while a federal subsidy on solar panels expands it
- Natural disasters: a freeze hits Florida and wrecks the orange crop, supply shifts left, and OJ prices at the store jump within weeks

Equilibrium

The equilibrium price and equilibrium quantity sit right where the supply and demand curves cross. At that exact price, the number of units buyers want to purchase matches what sellers want to produce. No leftovers, no empty shelves.

Above equilibrium, sellers stock shelves that don't clear. That unsold inventory (a surplus) pressures them to cut prices, run sales, start marking things down. The price drifts back toward equilibrium on its own.

Below equilibrium, buyers show up and the product is already gone. That shortage gives sellers room to raise prices, or buyers start outbidding each other. Price climbs back up.

This self-correcting mechanism is why economists call equilibrium a "resting point." The market doesn't stay away from it for long unless something external like a government price control pins the price somewhere else. And even then, the underlying pressure to snap back to equilibrium never actually goes away. On the 2019 AP Micro free-response, students who couldn't walk through this adjustment process lost easy points on what should have been a straightforward question.

Shifts vs. Movements Along the Curve

There's one question that sorts this out every single time: did the good's own price change, or did something else change?

Own price changed means you get a movement along the curve. The curve stays exactly where it is, you just slide to a different point on it. Gas 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 means the entire curve shifts. Tesla releases a $25,000 electric car and millions of drivers stop caring about gasoline at any price, so the gas demand curve shifts left.

If a free-response prompt 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. The price of corn going up means quantity demanded decreased, which is movement along the curve. Demand itself didn't budge. This distinction sounds pedantic, but on the AP exam it costs people real points every single year.

Quick filter that always works: 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 government policy, it's a shift.

Worked Example

You're 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 has to match:

100 - 2Q = 20 + Q

Step 2: Solve for Q.

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

Step 3: Plug Q back into either equation to get P. Using supply:

P = 20 + 26.67 = $46.67

Check it with demand: P = 100 - 2(26.67) = 100 - 53.33 = $46.67. Same answer both ways, so you know there's no algebra mistake.

Translation: at $46.67, buyers want exactly 26.67 units and sellers want to produce exactly 26.67 units. No surplus, no shortage, everyone's happy.

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

Price Controls

Sometimes the government looks at the market price and decides it's too high or too low, so they step in. The results follow a pretty predictable pattern every time.

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, and you get a shortage. Long waitlists, under-the-table payments, buildings that don't get maintained. The usual fallout.

A price floor is a legal minimum. The federal minimum wage (stuck at $7.25 per hour since 2009, which is kind of wild) 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 what economists call unemployment.

There's a memory trick that saves AP students every year: floors go under something to hold it up, so they prop the price above equilibrium (surplus). Ceilings press down from above, holding the price below equilibrium (shortage). If you write on the exam that a price ceiling creates a surplus, that answer earns zero points.

Both binding controls generate deadweight loss, because transactions that would have made both buyer and seller better off just never happen. The deals are there, the gains from trade exist, but the price control blocks them.