3.5 Demand, Supply, and Efficiency

Market Equilibrium and Efficiency

At market equilibrium, supply meets demand in a way that maximizes total benefits to society. Understanding why this point is special, and what happens when it's disrupted, is central to microeconomics.

Surplus Interaction in Market Equilibrium

Consumer surplus is the difference between the maximum price a consumer is willing to pay and the price they actually pay. On a graph, it's the triangular area below the demand curve and above the equilibrium price. If you'd pay $8 for a coffee but only have to pay $5, your consumer surplus on that purchase is $3.

Producer surplus works the other way. It's the difference between the price a producer receives and the minimum price they'd be willing to accept. Graphically, it's the triangular area above the supply curve and below the equilibrium price. A seller willing to accept $3 who sells at $5 earns $2 in producer surplus.

Social surplus (also called total surplus or economic surplus) is the sum of consumer and producer surplus:

$\text{Social Surplus} = \text{Consumer Surplus} + \text{Producer Surplus}$

Social surplus is maximized at market equilibrium, where the demand and supply curves intersect. This is what economists mean by allocative efficiency: resources go to the uses that society values most. Any quantity above or below $Q_e$ would shrink total surplus.

Economic Impacts of Price Controls

Price controls are government-imposed limits on prices. They push the market away from equilibrium, and the result is always a loss of efficiency.

Price Ceilings

A price ceiling is a legally mandated maximum price, set below the equilibrium price. Rent control is the classic example.

• Because the price is held artificially low, quantity demanded rises while quantity supplied falls. The result is a shortage.
• Producer surplus shrinks because sellers receive a lower price. Consumer surplus may increase for those who can still buy the good, but some consumers who would have bought at equilibrium are now shut out.
• The transactions that no longer happen create deadweight loss, shown as a triangle between the supply and demand curves from the controlled quantity to $Q_e$.

Price Floors

A price floor is a legally mandated minimum price, set above the equilibrium price. The minimum wage is a common example.

• Because the price is held artificially high, quantity supplied rises while quantity demanded falls. The result is a surplus (in the case of labor markets, this surplus shows up as unemployment among low-skilled workers).
• Consumer surplus shrinks because buyers pay a higher price. Producer surplus may increase for those who can still sell, but some producers lose out.
• Again, the lost transactions create deadweight loss.

Deadweight Loss

Deadweight loss is the reduction in social surplus that occurs when the market moves away from equilibrium. The key thing to understand: this lost surplus doesn't transfer to anyone. It simply disappears. No consumer gets it, no producer gets it, and the government doesn't collect it. It represents transactions that would have benefited both buyers and sellers but no longer take place.

Deadweight loss can be caused by price controls, taxes, subsidies, or any other factor that distorts the equilibrium quantity.

Demand and Supply in Equilibrium

Demand describes the relationship between a good's price and the quantity consumers are willing and able to purchase. The demand curve slopes downward: as price rises, quantity demanded falls.

Supply describes the relationship between a good's price and the quantity producers are willing and able to sell. The supply curve slopes upward: as price rises, quantity supplied increases.

Market equilibrium occurs at the intersection of these two curves:

• The equilibrium price ($P_e$) is the price at which quantity demanded equals quantity supplied.
• The equilibrium quantity ($Q_e$) is the amount bought and sold at that price.

At any price above $P_e$, there's a surplus (excess supply), which pushes the price down. At any price below $P_e$, there's a shortage (excess demand), which pushes the price up. The market naturally gravitates toward equilibrium through these pressures.

Shifts in Demand and Supply

1. Changes in factors other than the good's own price cause the entire curve to shift. For demand, these include consumer income, tastes, prices of related goods, expectations, and the number of buyers. For supply, these include input costs, technology, expectations, and the number of sellers.
2. When a curve shifts, the market moves to a new equilibrium with a different $P_e$ and $Q_e$.
3. How much price and quantity change depends on the elasticity of demand and supply, which measures how responsive quantity is to a change in price.

Scarcity and Economic Decision-Making

These foundational concepts underpin everything about why markets exist and how equilibrium works:

• Scarcity is the basic economic problem: resources are limited, but human wants are not. Markets are one mechanism for deciding how to allocate scarce resources.
• Opportunity cost is the value of the next best alternative you give up when making a choice. Every transaction at equilibrium reflects buyers and sellers weighing opportunity costs.
• Marginal analysis means evaluating the additional cost and additional benefit of one more unit. Efficient outcomes occur when the marginal benefit of the last unit consumed equals its marginal cost, which is exactly what happens at $Q_e$.