CB2400 Chap.3 Supply, Equilibrium and Price Controls
Supply, Equilibrium and Price Controls
Supply follows the producer's constraint
A supply schedule records quantities sellers are willing and able to offer at each own price while input prices, technology, expectations and related-output prices stay fixed. A higher own price usually raises quantity supplied and creates movement along the curve.
Cheaper inputs or better technology shift supply right because more can be offered at every market price.
Equilibrium is a condition and a process
Market clearing requires quantity demanded to equal quantity supplied. A price above equilibrium creates excess supply, giving sellers an incentive to lower price. A price below equilibrium creates excess demand, creating upward pressure.
Solving the two functions identifies the intersection; describing shortage or surplus explains why another price does not persist in an uncontrolled market.
Price controls interrupt adjustment
A maximum price below equilibrium is a binding ceiling and creates a shortage. A minimum price above equilibrium is a binding floor and creates a surplus. A ceiling above equilibrium or a floor below it does not bind.
The policy changes the permitted price, not the underlying demand and supply functions, so both quantities must be evaluated at the controlled price.
What this chapter covers
- 01
supply determinants
- 02
horizontal market supply
- 03
market-clearing equation
- 04
shortage and surplus
- 05
comparative static shifts
- 06
binding ceilings and floors
Worked example · free
Solve equilibrium and a ceiling
- 3Set Qd equal to Qs and solve for price.
- 3Substitute into both functions to verify quantity.
- 4Evaluate the ceiling and calculate the shortage.
Key terms
- Supply Function
- A relationship between quantity supplied and its own-price and non-price determinants.
- Market Clearing
- The condition in which planned purchases equal planned sales.
- Shortage
- The excess of quantity demanded over quantity supplied at a stated price.
- Surplus
- The excess of quantity supplied over quantity demanded at a stated price.
- Binding Control
- A legal price rule that prevents the market from reaching its uncontrolled equilibrium.
Supply, Equilibrium and Price Controls FAQ
How do I calculate market equilibrium?
Set the market demand function equal to the market supply function and solve for price. Substitute that price into both equations and verify that they produce the same quantity. That cross-check detects algebra errors before a later policy calculation builds on the wrong intersection.
What makes a price ceiling binding?
A ceiling binds only when it is set below the equilibrium price. At the controlled price, quantity demanded exceeds quantity supplied and a shortage remains because price cannot rise to clear the market. A ceiling above equilibrium leaves the market outcome unchanged.
What happens when demand and supply both increase?
Both shifts raise equilibrium quantity, so quantity increases. The demand shift pushes price upward while the supply shift pushes it downward. Without the relative magnitudes, price is ambiguous. An unchanged price is one possibility, but it cannot be asserted from directions alone.
Exam move
Solve one baseline equilibrium and label its price and quantity on a diagram. Evaluate one price below it and one above it, calculate the gap and explain the pressure on price. Then impose a ceiling and a floor, checking whether each rule is binding before naming the resulting shortage or surplus. Create a four-line solution template: functions, equilibrium condition, substitution check and economic verdict.
Substitution into both schedules should return the same equilibrium quantity. At a controlled price, keep quantity demanded and quantity supplied separate before taking their difference. A ceiling below equilibrium is potentially binding and produces excess demand; a floor above equilibrium is potentially binding and produces excess supply. Rules placed on the other side of equilibrium do not constrain the market outcome.
For shift questions, trace demand and supply changes independently. If both shifts push quantity the same way, its direction is certain; if their price effects conflict, price is ambiguous without magnitudes. Draw the old and new intersections before writing the conclusion, and never treat ambiguity as unchanged. Finally, narrate the adjustment mechanism.
A shortage creates upward price pressure because buyers compete for scarce units, while a surplus creates downward pressure as sellers try to clear unsold output. This explanation is the causal bridge between the calculated gap and the equilibrium model. Compare the smaller planned quantity with the amount actually tradable under a binding control, since the long side's plan cannot be completed without extra units.
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