Difficulty: Introductory | Prerequisites: Basic understanding of graphs (price on y-axis, quantity on x-axis)
Tags: market equilibrium, supply and demand, equilibrium price, equilibrium quantity, shortage, surplus, consumer surplus, producer surplus, total surplus, demand shifters, supply shifters, complements, substitutes, microeconomics
Supply and demand is the workhorse model of microeconomics. Nearly every topic that follows in the course, from taxes to trade policy to market structure, is built on top of this framework. This set of notes covers how markets reach equilibrium, what happens when they are pushed away from it by external events, and how economists measure the welfare that markets generate. If you can draw, shift, and interpret supply and demand diagrams fluently, you are well positioned for the rest of the course.
Markets settle at the price where the quantity buyers want equals the quantity sellers offer. That price is the equilibrium. When outside forces shift demand or supply, the equilibrium moves. Consumer surplus and producer surplus measure the gains that buyers and sellers enjoy from trading at the market price; together they form total surplus, which is maximised at equilibrium.
Market equilibrium
The point where the quantity demanded by buyers equals the quantity supplied by sellers. At this price, the market clears with no leftover goods and no unmet demand.
Equilibrium price
The price at which quantity demanded equals quantity supplied. Think of it as the price the market "settles on" when left alone.
Equilibrium quantity
The amount of a good bought and sold at the equilibrium price.
Surplus (excess supply)
The situation that arises when the price is above equilibrium: sellers want to sell more than buyers want to buy. Unsold goods accumulate, pushing the price downward.
Shortage (excess demand)
The situation that arises when the price is below equilibrium: buyers want to buy more than sellers are willing to sell. Unmet demand pushes the price upward.
Consumer surplus
The difference between what consumers are willing to pay and what they actually pay. On a graph, it is the area below the demand curve and above the market price. In simple terms, it is the "bonus" buyers feel they got from a good deal.
Producer surplus
The difference between the market price and the minimum price at which sellers would have been willing to sell. On a graph, it is the area above the supply curve and below the market price. In simple terms, it is the extra profit sellers earn above their bare minimum.
Total surplus
Consumer surplus plus producer surplus. It represents the total welfare generated by market transactions. At equilibrium, total surplus is at its maximum.
At the equilibrium price, every unit that buyers want is matched by a unit that sellers offer. The market clears.
Example: if the equilibrium price of a smoothie is $10, then at that price exactly 50 smoothies are demanded and 50 are supplied.
If the price were above $10, sellers would offer more than buyers want (surplus), and competition among sellers would push the price down.
If the price were below $10, buyers would want more than sellers offer (shortage), and competition among buyers would push the price up.
External factors can move the entire demand curve left or right:
Rightward shift (increase in demand): Equilibrium price rises, equilibrium quantity rises.
Leftward shift (decrease in demand): Equilibrium price falls, equilibrium quantity falls.
Common demand shifters include changes in income, tastes, the price of related goods (substitutes and complements), expectations, and the number of buyers.
External factors can also move the entire supply curve:
Rightward shift (increase in supply): Equilibrium price falls, equilibrium quantity rises.
Leftward shift (decrease in supply): Equilibrium price rises, equilibrium quantity falls.
Common supply shifters include changes in input costs, technology, expectations, the number of sellers, and the prices of related goods in production.
Complements in consumption: Goods used together. A rise in the price of rice reduces demand for beans (because people buy less rice, they also buy fewer beans to go with it).
Complements in production: Goods produced together as by-products of the same process. A fall in the price of kerosene can decrease the supply of gasoline if producers cut back on the refining process that yields both.
Substitutes in consumption: Goods that serve similar purposes. A rise in the price of Coke increases demand for Pepsi.
Consumer surplus is the triangle (or area) below the demand curve and above the price line.
Producer surplus is the triangle (or area) above the supply curve and below the price line.
At equilibrium, the combined area is as large as it can be. Any deviation from the equilibrium price shrinks total surplus.
Surplus area (for linear curves):
Consumer surplus = 0.5 x base x height, where the base is the equilibrium quantity and the height is the distance from the equilibrium price to the y-intercept of the demand curve.
Producer surplus = 0.5 x base x height, where the base is the equilibrium quantity and the height is the distance from the y-intercept of the supply curve to the equilibrium price.
These are triangle-area calculations. If the curves are not linear, you would integrate, but introductory courses generally stick to straight lines.
When a frost destroys orange crops, supply shifts left: orange prices rise and quantity sold falls. When a viral social-media trend makes a new drink popular, demand shifts right: the price and quantity both rise. The surplus framework explains why both buyers and sellers benefit from voluntary exchange, and why economists worry about policies that move markets away from equilibrium.
Students often confuse a change in quantity demanded (movement along the curve caused by a price change) with a change in demand (a shift of the entire curve caused by an external factor). These are different things.
A surplus in economics means excess supply (too much on the shelf), not "extra benefit." Do not mix this up with consumer surplus, which is about welfare.
Equilibrium does not mean "fair." It is the price that clears the market, whether or not that price feels reasonable.
⚠️ Expect graph-based questions: you will be given a supply-and-demand diagram and asked what happens to price and quantity after a specified shift.
⚠️ Know the direction of both price and quantity for each of the four basic shifts (demand up, demand down, supply up, supply down).
⚠️ Be able to calculate consumer and producer surplus from a graph with linear curves. The triangle-area formula comes up regularly.
⚠️ Questions may describe a real-world event and ask which curve shifts and in which direction. Practise mapping news-style scenarios to the model.
True or false: At equilibrium, every buyer who wants the good at the market price can get it.
Fill in the blank: A shortage puts ______ pressure on price.
True or false: An increase in supply raises the equilibrium price.
Fill in the blank: Consumer surplus is the area below the ______ curve and above the ______.
True or false: Total surplus is maximised at any price, not just the equilibrium price.
Answers: 1. True. 2. Upward. 3. False (it lowers the equilibrium price). 4. Demand; market price. 5. False (it is maximised only at the equilibrium price).
Q: The equilibrium price of a smoothie is $10 and 50 are sold. If the price is set at $12, what happens?
A: A surplus arises. At $12, quantity supplied exceeds quantity demanded, leaving unsold smoothies and creating downward pressure on price.
Q: A new study shows that drinking green tea improves memory. What happens to the green tea market?
A: Demand for green tea increases (rightward shift). Equilibrium price and quantity both rise.
Q: If consumer surplus is the triangle below demand and above price, what does a fall in the market price do to consumer surplus (all else equal)?
A: It increases consumer surplus, because the triangle between the demand curve and the lower price line is larger.
Q: A rise in the price of rice reduces demand for beans. What is the relationship between rice and beans?
A: They are complements in consumption. Consumers tend to buy them together, so a price increase in one reduces demand for the other.
Q: What is the difference between a surplus (excess supply) and producer surplus?
A: A surplus (excess supply) is unsold goods that pile up when the price is above equilibrium. Producer surplus is the welfare gain sellers receive from selling at a price above their minimum acceptable price. They share a word but measure completely different things.
Supply and demand is the foundation for price controls (ceilings and floors), which are covered in Part 4 of these notes. It also connects to elasticity (how sensitive quantity is to price changes), taxation and deadweight loss, and international trade analysis later in the course.
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