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By now, we are familiar with graphs of supply curves and demand curves. To find market equilibrium, we combine the two curves onto one graph. The point of intersection of supply and demand marks the point of equilibrium. Unless interfered with, the market will settle at this price and quantity. Why is this? At this point of intersection, buyers and sellers agree on both price and quantity. For instance, in the graph below, we see that at the equilibrium price p*, buyers want to buy exactly the same amount that sellers want to sell.

Figure %: Market Equilibrium

Figure %: Price Higher than Equilibrium Price

We can also see what happens when one of the curves shifts up or down in response to outside factors. For example, if we were to look at the market for Beanie Babies before and after they became a popular fad, we would see a shift outwards from the initial demand curve over time. The reason for this is that as people began to like Beanie Babies, their preferences changed, and they began to want Beanie Babies enough that they would pay much more for each Beanie Baby than they would have previously. We can see this new preference for Beanie Babies in the outward shift of the demand curve: for every price, buyers will buy more Beanie Babies than they would have before the fad.

Figure %: Shift in the Demand for Beanie Babies

It is only through a shift in either the supply or the demand curve that the market equilibrium will change. Why is this? If neither curve shifts, and we move along one of the curves, the market will naturally shift back to equilibrium. For example, if we look at a market in equilibrium, and a store tries to move up its supply curve by selling goods at a higher price, the result will be that no one will buy the goods, since they are less expensive at the store's competitors. The store will have to either go out of business, or move its prices back down to equilibrium.

What happens if both curves shift? Will we end up at the same equilibrium
point? In this model, it is not possible to reach the same equilibrium: either
the price or the quantity can be the same as the previous equilibrium, but not
both, unless the curves shift back to their original positions. To illustrate
why this is true, consider the graph below. The initial equilibrium, between
supply curve 1 and demand curve 1, has price p* and quantity q*. If supply
shifts to supply curve 2, both equilibrium price and quantity change. It is now
possible to change back to our original price by shifting the demand curve to
position 2 *or* it is possible to revert to our original quantity by
shifting the demand curve to position 3. Note that we cannot reach the original
equilibrium point unless we move the curves back to their original points.

Figure %: Shift in Supply and Demand

We have worked with supply and demand equations separately, but they can also be combined to find market equilibrium. We have already established that at equilibrium, there is one price, and one quantity, on which both the buyers and the sellers agree. Graphically, we see that as a single intersection of two curves. Mathematically, we will see it as the result of setting the two equations equal in order to find equilibrium price and quantity.

If we are looking at the market for cans of paint, for instance, and we know that the supply equation is as follows:

QS = -5 + 2PAnd the demand equation is:

QD = 10 - PThen to find the equilibrium point, we set the two equations equal. Notice that quantity is on the left-hand side of both equations. Because quantity supplied is equal to quantity demanded at equilibrium, we can set the right-hand sides of the two equations equal.

QS = QDAt equilibrium, paint will cost $5 a can. To find out the equilibrium quantity, we can just plug the equilibrium price into either equation and solve for Q.

-5 + 2P = 10 - P

3P = 15

P = 5

Q* = QS

QS = -5 + 2(5)

QS = Q* = 5 cans