Demand and Supply
Demand and the Law of Demand
Demand describes the quantity of a good or service that buyers are willing and able to purchase at each possible price. Both halves of that definition matter: wanting something is not enough to create demand unless a buyer also has the means to pay for it, and having money is not enough unless a buyer also wants the good. The price a buyer pays for one unit of a good is simply its price, and the total number of units buyers would purchase at that price is the quantity demanded. A consistent pattern shows up across nearly every market: as the price of a good rises, the quantity demanded falls, and as the price falls, the quantity demanded rises. Economists call this inverse relationship the law of demand. Picture the market for folding umbrellas in a rainy coastal city. At a low price, shoppers who might not have bothered buying one at all now find it worthwhile, so quantity demanded is high. As the price climbs, some buyers decide their old umbrella will do for another season, or that a plastic poncho is an adequate substitute, so quantity demanded falls. Plotting price against quantity demanded produces a downward-sloping demand curve, and this curve assumes that every other factor affecting demand besides price is held constant, an assumption economists call ceteris paribus.
What Shifts a Demand Curve
A change in price moves buyers along a fixed demand curve, but a change in any other factor that affects buyers’ willingness or ability to purchase shifts the entire curve to a new position. Income is one such factor: as incomes rise, demand for most goods, called normal goods, increases at every price, shifting the curve to the right, while demand for a smaller category called inferior goods, typically goods people buy mainly because better alternatives are unaffordable, actually falls as income rises. The prices of related goods matter as well. If the price of bus tickets falls sharply, demand for umbrella deliveries by private car might fall too, since the two are loosely substitutable ways of avoiding getting wet, while a fall in the price of umbrellas themselves might raise demand for rain boots, if buyers tend to purchase the two together as complements. Tastes and preferences shift demand directly: a public health campaign warning about mold on damp clothing could increase demand for umbrellas at every price. The number of buyers in a market matters too, since a market with more potential buyers has higher demand at every price than the same market with fewer buyers. Finally, expectations about the future can shift demand today; if shoppers expect umbrella prices to rise sharply next month because of a new import tariff, some will buy now rather than wait, increasing today’s demand even though today’s price has not changed at all.
Supply and the Law of Supply and What Shifts It
Supply describes the quantity of a good that sellers are willing and able to produce and offer for sale at each possible price. The quantity supplied at a given price is the amount sellers are ready to sell at that specific price, and the relationship between price and quantity supplied traces out a supply curve. Nearly all supply curves slope upward from left to right, reflecting the law of supply: as price rises, quantity supplied rises, and as price falls, quantity supplied falls, because a higher price makes production more profitable at the margin and draws additional sellers or additional output into the market. Just as with demand, factors other than price can shift the entire supply curve rather than simply moving sellers along it. The cost of inputs is central: if the price of waterproof fabric used to make umbrellas rises, producing each umbrella becomes more expensive, and sellers will supply fewer umbrellas at every price, shifting supply to the left; a fall in fabric costs has the opposite effect, shifting supply to the right. New production technology that lowers the cost of manufacturing, government taxes or regulations that raise costs, and government subsidies that lower costs all shift the supply curve in predictable directions, as does a change in the number of firms producing the good, since more producers competing in a market generally means a greater quantity supplied at every price.
Market Equilibrium and How Prices Adjust
Placing the demand curve and the supply curve for the same good on one graph reveals a single point where they cross, called the equilibrium. At the equilibrium price, the quantity that buyers wish to purchase exactly equals the quantity that sellers wish to offer, so there is no leftover pressure pushing the price up or down; this shared quantity is the equilibrium quantity. Away from equilibrium, the market sends clear signals that push the price back toward balance. Suppose the price of umbrellas sits above the equilibrium level. At that higher price, quantity supplied exceeds quantity demanded, producing a surplus of unsold umbrellas piling up in stores; sellers respond by cutting prices to clear their shelves, and the price falls back toward equilibrium. Now suppose the price sits below equilibrium instead. Quantity demanded then exceeds quantity supplied, producing a shortage, with eager buyers unable to find umbrellas on the shelves; sellers notice they can raise prices without losing many customers, and the price rises back toward equilibrium. This self-correcting tendency is why economists describe markets as having a built-in mechanism for finding balance, even without any single person or agency deliberately setting the price.
Price Ceilings and Price Floors
Governments sometimes intervene directly in a market by legally capping or supporting a price rather than letting supply and demand set it freely. A price ceiling is a legal maximum price, typically imposed because policymakers worry that the market price of some necessity, such as housing, has climbed too high for many residents to afford. Imagine a city called Millbrook imposes a ceiling on apartment rents below the market equilibrium level. At that capped price, quantity demanded for apartments exceeds quantity supplied, producing a persistent shortage: fewer apartments actually get built or rented out than would occur at the market price, existing landlords have less incentive to maintain their units well, and some renters who would have been able to find housing at the market price instead go without. A price floor works in the opposite direction, setting a legal minimum price, often used to support producers of a good whose price tends to swing widely, such as an agricultural crop like kaffa berries. If a government sets a floor above the market equilibrium price for kaffa berries, farmers respond by growing more than buyers wish to purchase at that price, and the government or another buyer typically must step in to purchase the resulting surplus, or the excess simply goes unsold. Both kinds of price controls illustrate a broader lesson: overriding the price that supply and demand would otherwise produce does not eliminate the underlying scarcity or surplus, it only changes who bears the resulting shortage or oversupply and how it shows up.
Quick Revision Summary
Demand is the quantity buyers are willing and able to purchase at each price, and the law of demand states that quantity demanded falls as price rises, all else held constant. A demand curve shifts, rather than simply moving along its existing path, when income, the price of related goods, tastes, the number of buyers, or expectations about the future change. Supply is the quantity sellers are willing and able to offer at each price, and the law of supply states that quantity supplied rises as price rises; the supply curve shifts when input costs, technology, taxes, subsidies, or the number of producers change. Market equilibrium occurs where the demand curve and supply curve intersect, and prices above or below that level create a surplus or shortage that pushes the market back toward balance. A price ceiling sets a legal maximum that, when placed below equilibrium, produces a persistent shortage, while a price floor sets a legal minimum that, when placed above equilibrium, produces a persistent surplus, and both illustrate that price controls redistribute the effects of scarcity or oversupply rather than eliminating them.
