MGEA02H3 Chap.2 Supply, Demand and Price Controls
Supply, Demand and Price Controls
Markets in equations
The course writes demand and supply as equations in either price form or quantity form and solves equilibrium by equating them.
Determinants other than own price sit in the constant, so a change in income, a related price or input costs shifts the curve in parallel, while a change in own price moves along it.
Controls and the short side
A binding ceiling or floor stops the market from clearing.
The quantity traded is set by the short side of the market, the resale or black-market price is read from demand at that quantity, and the lost trades form a deadweight loss triangle.
Past tests also use a grid of simultaneous shifts in which one outcome is determinate and the other ambiguous.
What the answer keys reward
For shift questions the keys name the curve that moves for each event and its direction before combining the effects, and they label any coordinate that depends on relative shift sizes as uncertain rather than guessing.
For price controls they compute both quantities at the controlled price, take the smaller as the quantity traded and read any black-market price from demand. Inferior goods and substitutes in production are the two traps the keys flag most often.
How this topic connects to later chapters
Every later market chapter starts from these two curves.
Demand is reinterpreted as marginal utility in chapter 3, supply as marginal cost in chapters 6 and 7, and the deadweight triangle of a ceiling has the same shape as the excess burden of a tax in chapter 4, the monopoly loss in chapter 8 and the tariff triangles in chapter 11. Getting the short-side logic right here makes those later areas straightforward.
What this chapter covers
- 01
Movement along a curve versus a shift
- 02
Determinants absorbed into the demand intercept
- 03
Solving linear equilibrium algebraically
- 04
Simultaneous shifts and ambiguous outcomes
- 05
Binding price ceilings and black markets
- 06
Price floors, minimum wages and quotas
Worked example · free
A binding ceiling and its black market
- 2Equate the curves: Q = 100 and P = $40, so the $28 ceiling binds.
- 2At $28, Qs = 60 and Qd = 124, a shortage of 64; the short side trades 60.
- 2Demand at Q = 60 gives a black-market price of $60.
- 2Deadweight loss is ½ × (60 − 28) × (100 − 60) = $640.
Key terms
- Equilibrium Price
- The equilibrium price is the price at which quantity demanded equals quantity supplied.
- Price Ceiling
- A price ceiling is a legal maximum price, which causes a shortage when set below equilibrium.
- Price Floor
- A price floor is a legal minimum price, which causes excess supply when set above equilibrium.
- Black Market
- A black market is illegal resale of a controlled good, at a price set by buyers’ willingness to pay for the limited quantity.
- Substitute in Production
- A substitute in production is another good the same producers could make, so a rise in its price lowers supply of the first good.
Supply, Demand and Price Controls FAQ
How can I tell whether a ceiling actually binds?
Solve the equilibrium first. A ceiling binds only if it is below the equilibrium price, and a floor only if it is above. A non-binding control changes nothing, and an option reporting a shortage at a non-binding ceiling is a distractor.
What happens when demand and supply both shift?
Work out the effect of each shift separately. Where the two effects push one variable the same way, that result is determinate; where they push in opposite directions, the outcome depends on which shift is larger and should be reported as indeterminate.
Why does a ceiling create a black market price above the original price?
Only the quantity sellers offer at the ceiling reaches buyers. With so few units available, the last buyer values a unit at the height of demand at that small quantity, which can exceed the original equilibrium price.
Is a quota the same as a price ceiling?
No. A quota fixes quantity, and the price rises to what buyers will pay for that limited quantity, so the gain goes to the sellers who hold the quota. A ceiling fixes price below equilibrium, so the limited quantity sells cheaply and any gain goes to the buyers who get the units.
Exam move
Pick a pair of linear curves, solve equilibrium, then test a ceiling, a floor and a quota against it. For each, write the short side, the quantity traded and the deadweight loss, and check that the surplus changes add up. When two events happen at once, write a two-row table of each event’s separate effect on price and quantity before combining them.
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