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How is equilibrium price determined?
Updated 7/8/2018 Jacob Reed What is equilibrium? 
How does the market move toward equilibrium? If the market price is above equilibrium, quantity supplied will be greater than quantity demanded; creating a surplus. When that occurs, market forces push the price downward toward equilibrium (increasing Qd and decreasing Qs) until the surplus is eliminated. If the market price is below equilibrium, quantity supplied will be less than quantity demanded; creating a shortage. When that occurs, market forces pull the price upward toward equilibrium (decreasing Qd and increasing Qs) until the shortage is eliminated.
How do shifts in supply and demand change equilibrium? Shifts in supply or demand curves move the equilibrium price and quantity. If demand increases, equilibrium price and quantity both increase. If demand decreases, equilibrium price and quantity both decrease. If supply increases, equilibrium price decreases, and quantity increases. If supply decreases, equilibrium price increases and equilibrium quantity decreases. How do double shifts impact price and quantity? When supply and demand both shift, either price or quantity will be indeterminate. When supply and demand move in the same direction, price is indeterminate. That is because an increase in supply decrease price while an increase in demand will increase price. Since the price axis moves in both directions, the net effect is based on which shift is stronger. Since that cannot be known, the price will be indeterminate. Since both shifts increase equilibrium quantity, the quantity will definitely increase. Similarly, when supply and demand move in opposite directions, quantity is indeterminate because one shift will increase quantity and the other will decrease quantity. The key to figuring out the impact of double shifts is to graph out both shifts and see what happens to the equilibrium price and quantity with each shift. If the shifts conflict, that axis is indeterminate. Read this section to learn how demand and supply interact with one another to determine prices and quantities that may or may not be optimal. Attempt the "Try It" problem. Take a moment to read through the stated learning outcomes for this chapter of the text, which you can find at the beginning of each section. These outcomes should be your goals as you read
through the chapter. As we have seen, when either the demand or the supply curve shifts, the results are unambiguous; that is, we know what will happen to both equilibrium price and equilibrium quantity, so long as we know whether demand or supply increased or decreased. However, in practice, several events may occur at around the same time that cause both the
demand and supply curves to shift. To figure out what happens to equilibrium price and equilibrium quantity, we must know not only in which direction the demand and supply curves have shifted but also the relative amount by which each curve shifts. Of course, the demand and supply curves could shift in the same direction or in opposite directions, depending on the specific events causing them to shift. For example, all three panels of Figure 3.11 "Simultaneous Decreases in Demand and
Supply" show a decrease in demand for coffee (caused perhaps by a decrease in the price of a substitute good, such as tea) and a simultaneous decrease in the supply of coffee (caused perhaps by bad weather). Since reductions in demand and supply, considered separately, each cause the equilibrium quantity to fall, the impact of both curves shifting simultaneously to the left means that the new equilibrium quantity of coffee is less than the old equilibrium quantity. The effect on the
equilibrium price, though, is ambiguous. Whether the equilibrium price is higher, lower, or unchanged depends on the extent to which each curve shifts. Figure 3.11 Simultaneous Decreases in Demand and Supply Both the demand and the supply of coffee decrease. Since decreases in demand and supply, considered separately, each cause equilibrium quantity to fall, the impact of both decreasing simultaneously means that a new equilibrium quantity of coffee must be less than the old equilibrium quantity. In Panel (a), the demand curve shifts farther to the left than does the supply curve, so equilibrium price falls. In Panel (b), the supply curve shifts farther to the left than does the demand curve, so the equilibrium price rises. In Panel (c), both curves shift to the left by the same amount, so equilibrium price stays the same. If the demand curve shifts farther to the left than does the supply curve, as shown in Panel (a) of Figure 3.11 "Simultaneous Decreases in Demand and Supply", then the equilibrium price will be lower than it was before the curves shifted. In this case the new equilibrium price falls from $6 per pound to $5 per pound. If the shift to the left of the supply curve is greater than that of the demand curve, the equilibrium price will be higher than it was before, as shown in Panel (b). In this case, the new equilibrium price rises to $7 per pound. In Panel (c), since both curves shift to the left by the same amount, equilibrium price does not change; it remains $6 per pound. Regardless of the scenario, changes in equilibrium price and equilibrium quantity resulting from two different events need to be considered separately. If both events cause equilibrium price or quantity to move in the same direction, then clearly price or quantity can be expected to move in that direction. If one event causes price or quantity to rise while the other causes it to fall, the extent by which each curve shifts is critical to figuring out what happens. Figure 3.12 "Simultaneous Shifts in Demand and Supply" summarizes what may happen to equilibrium price and quantity when demand and supply both shift. Figure 3.12 Simultaneous Shifts in Demand and Supply If simultaneous shifts in demand and supply cause equilibrium price or quantity to move in the same direction, then equilibrium price or quantity clearly moves in that direction. If the shift in one of the curves causes equilibrium price or quantity to rise while the shift in the other curve causes equilibrium price or quantity to fall, then the relative amount by which each curve shifts is critical to figuring out what happens to that variable. As demand and supply curves shift, prices adjust to maintain a balance between the quantity of a good demanded and the quantity supplied. If prices did not adjust, this balance could not be maintained. Notice that the demand and supply curves that we have examined in this chapter have all been drawn as linear. This simplification of the real world makes the graphs a bit easier to read without sacrificing the essential point: whether the curves are linear or nonlinear, demand curves are downward sloping and supply curves are generally upward sloping. As circumstances that shift the demand curve or the supply curve change, we can analyze what will happen to price and what will happen to quantity. What happens to equilibrium prices when demand and or supply curves shift?Shifts in supply or demand curves move the equilibrium price and quantity. If demand increases, equilibrium price and quantity both increase. If demand decreases, equilibrium price and quantity both decrease. If supply increases, equilibrium price decreases, and quantity increases.
What happens when both supply and demand curves shift?If the increase in both demand and supply is exactly equal, there occurs a proportionate shift in the demand and supply curve. Consequently, the equilibrium price remains the same. However, the equilibrium quantity rises. In such a case, the right shift of the demand curve is more relative to that of the supply curve.
What would happen to the equilibrium price and quantity if both the demand curve and supply curve shifted to the left?Since reductions in demand and supply, considered separately, each cause the equilibrium quantity to fall, the impact of both curves shifting simultaneously to the left means that the new equilibrium quantity of coffee is less than the old equilibrium quantity.
How are the equilibrium price and quantity affected when both demand and supply curves shift in the same direction?(iii) When the decrease in demand is less than the increase in supply, then the equilibrium price will fall but the quantity will rise. See Fig. (c).
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What happens to equilibrium price and quantity when both supply and demand curves shift?
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