Market equilibrium and market forces

What market equilibrium is

• Market equilibrium is the price and quantity where quantity demanded equals quantity supplied.
  • Written Qd = Qs, at the equilibrium price (Pe) and equilibrium quantity (Qe).
  • On the model it is the single point where the demand curve crosses the supply curve.
• At equilibrium the market clears — every consumer willing to pay Pe gets the good, and every producer willing to supply at Pe sells it.
• There is no shortage and no surplus, so nothing is pushing the price to change.

Demand is not quantity demanded

This is the single most-corrected error in the whole standard, so get it exact.

• Demand is the whole curve — the entire relationship between price and quantity at every possible price.
  • Demand changes when something other than price changes: income, tastes, the price of a substitute, population, expectations.
  • A change in demand shifts the whole curve left or right.
• Quantity demanded is one point on the curve — the amount bought at one particular price.
  • Quantity demanded changes when the price of the good itself changes.
  • A change in quantity demanded is a movement along the curve.
• The same distinction applies exactly to supply and quantity supplied.

If this changes	What moves	The right words
The good's own price	Along the curve	"quantity demanded falls"
Income, tastes, substitutes	The whole curve	"demand decreases"
The good's own price	Along the curve	"quantity supplied rises"
Costs of production, technology	The whole curve	"supply increases"

How market forces clear a shortage

• A shortage exists when the price is set below equilibrium, so Qd > Qs.
  • The horizontal gap between Qs and Qd at that price is the shortage.

• The mechanism runs through affordability and profitability, and you must name both:
  • Consumers who miss out bid the price up — they are willing to pay more rather than go without.
  • As the price rises, the good becomes less affordable, so some consumers drop out and quantity demanded falls (a movement along the demand curve).
  • As the price rises, production becomes more profitable, so producers supply more and quantity supplied rises (a movement along the supply curve).
  • The two movements close the gap from both ends until Qd = Qs again at Pe.

How market forces clear a surplus

• A surplus exists when the price is set above equilibrium, so Qs > Qd.

• The mechanism is the mirror image:
  • Producers left with unsold stock cut the price rather than not sell at all.
  • As the price falls the good becomes more affordable, so quantity demanded rises.
  • As the price falls production becomes less profitable, so quantity supplied falls.
  • The gap closes until Qd = Qs at Pe.

Why this page is worth so many marks

• The 2025 Assessment Report says outright: "How market forces work to restore equilibrium has consistently been in the exam in some form."
• It also names exactly what a good answer contains: "the basic principles of affordability and profitability and laws of demand and supply".
• An answer that says "the price will go back to equilibrium" describes the result but never explains the mechanism, and cannot get past Achieved.