Price Determination Under Perfect Competition
How market demand and market supply jointly fix the equilibrium price, and what happens when either curve shifts.
Definition
Equilibrium price under perfect competition is the single price at which the quantity buyers wish to purchase exactly equals the quantity sellers wish to offer, leaving neither a surplus nor a shortage.
Simple Explanation
It is the price where the buyer's curve and the seller's curve cross — the one price both sides are happy to trade at, with nothing left over and nothing missing.
Detailed Explanation
Market demand (downward sloping) and market supply (upward sloping), drawn on the same diagram, intersect at exactly one point — this point fixes both the equilibrium price and the equilibrium quantity actually traded in the market.
Away from equilibrium, market forces push price back towards it. At any price above equilibrium, quantity supplied exceeds quantity demanded, creating excess supply (a surplus); unsold stock pressures sellers to cut price, pulling it back down. At any price below equilibrium, quantity demanded exceeds quantity supplied, creating excess demand (a shortage); competition among buyers for the limited stock pushes price back up. This self-correcting tendency is why equilibrium, once disturbed, tends to be restored.
A shift of the demand curve (caused by income, tastes, or prices of related goods changing — not the good's own price) with supply unchanged moves the equilibrium along the supply curve: a rightward demand shift raises both equilibrium price and quantity; a leftward shift lowers both.
A shift of the supply curve (caused by cost of production, technology, or government policy changing) with demand unchanged moves the equilibrium along the demand curve: a rightward supply shift lowers equilibrium price but raises equilibrium quantity; a leftward shift raises price but lowers quantity.
Simple applications extend this same demand-supply framework to real situations: an indirect tax (like GST) effectively shifts the supply curve leftward/upward by raising the cost sellers must recover, raising equilibrium price and lowering equilibrium quantity; a subsidy does the reverse, shifting supply rightward/downward, lowering price and raising quantity. The same logic explains price floors (minimum support price, set above equilibrium, creating a surplus the government must buy up) and price ceilings (price control, set below equilibrium, creating a shortage).
Real-life Examples
Onion price crash
A bumper onion harvest shifts the market supply curve rightward; with demand unchanged, the new intersection settles at a lower price and a higher quantity traded.
Festive demand surge
Demand for sweets shifts rightward just before a festival; with supply unchanged in the short run, the new equilibrium settles at both a higher price and a higher quantity.
Step-by-step Graph
- 1
Draw demand and supply curves
Plot the downward-sloping market demand curve (DD) and upward-sloping market supply curve (SS) on the same diagram.
- 2
Mark the intersection
Identify point E where DD and SS cross — this fixes equilibrium price (OP) and equilibrium quantity (OQ).
- 3
Show disequilibrium
At a price above OP, mark the horizontal gap between SS and DD as excess supply; below OP, mark the gap as excess demand.
- 4
Shift one curve
To analyse a change, shift only the affected curve (demand or supply) and read off the new intersection point as the new equilibrium.
Tables
| Change | Equilibrium Price | Equilibrium Quantity |
|---|---|---|
| Demand increases (shifts right) | Rises | Rises |
| Demand decreases (shifts left) | Falls | Falls |
| Supply increases (shifts right) | Falls | Rises |
| Supply decreases (shifts left) | Rises | Falls |
| Intervention | Effect on supply curve | Effect on equilibrium price |
|---|---|---|
| Indirect tax | Shifts left/up (cost rises) | Price rises, quantity falls |
| Subsidy | Shifts right/down (cost falls) | Price falls, quantity rises |
| Price floor (above equilibrium) | No shift; price held above E | Creates a persistent surplus |
| Price ceiling (below equilibrium) | No shift; price held below E | Creates a persistent shortage |
Common Mistakes
- Shifting both curves when only one determinant (e.g. cost of production) has actually changed.
- Confusing excess demand (price below equilibrium) with excess supply (price above equilibrium).
- Treating a tax as a shift of the demand curve instead of the supply curve — a tax raises the seller's cost, so it shifts supply.
Exam Tips
- Always draw the 'before' and 'after' curves on the same diagram, with the new equilibrium clearly relabelled E1.
- For surplus/shortage questions, explicitly mark the price line and read off the horizontal distance between the two curves at that price.
- For tax/subsidy applications, state clearly which curve shifts and why, before reading off the new equilibrium.
Summary
- Equilibrium price is where market demand equals market supply.
- Prices above equilibrium create excess supply; prices below create excess demand; both are self-correcting.
- A demand shift moves price and quantity in the same direction.
- A supply shift moves price and quantity in opposite directions.
- Taxes, subsidies, and price controls all operate by shifting a curve or fixing price away from equilibrium.
Practice MCQs
1. At a price above the equilibrium price, a perfectly competitive market experiences:
2. If market demand for a good increases while supply stays unchanged, equilibrium price and quantity will:
3. An indirect tax imposed on a good typically:
4. A government-imposed price floor set above the equilibrium price creates:
5. If supply increases (shifts rightward) while demand stays unchanged, equilibrium price will:
6. A subsidy granted to producers typically: