Equilibrium is a state in which conflicting forces are in balance. In equilibrium, it will be possible for both buyers and sellers to realize their goals simultaneously.
The following graph depicts the market supply and demand for concert tickets at Madison Square Garden in New York City.
Equilibrium price and quantity are where the supply and demand curves intersect. Draw a horizontal line from the intersection to the price axis. This is equilibrium price: $60. Draw a vertical line from the intersection to the quantity axis. This is equilibrium quantity: 300. It is equilibrium because quantity demanded equals quantity supplied at $60 per ticket. At this price, there is neither surplus (excess supply) nor shortage (excess demand), so there is no downward or upward pressure for the price to change.
Surplus will push prices downward towards equilibrium.
Similarly, shortages push prices upward towards equilibrium.
Because the price rises if it is below equilibrium, falls if it is above equilibrium, and remains constant if it is at equilibrium, the price is pulled toward equilibrium and remains there until some event changes the equilibrium. We refer to such an equilibrium as being stable because whenever price is disturbed away from the equilibrium, it tends to converge back to that equilibrium.
An unstable equilibrium is an equilibrium that is not restored if disrupted by an external force. While most equilibria studied in economics are of the stable variety, a few cases of unstable equilibria do emerge from time to time, in limited circumstances.
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