Difficulty: Introductory to Intermediate | Prerequisites: Supply and Demand notes
This set of notes covers how supply and demand interact to determine a market price, what happens when the market is out of equilibrium, how to measure the gains from trade using consumer and producer surplus, and what happens when the government intervenes with price ceilings or floors. These concepts are heavily tested and rely on everything from the previous two sets of notes.
Market equilibrium is the price where the quantity buyers want to purchase equals the quantity sellers want to sell. When the market is out of equilibrium, surpluses push prices down and shortages push prices up until balance is restored. Consumer and producer surplus measure the gains from trade in a market, and government price controls (ceilings and floors) can reduce total surplus by preventing the market from reaching equilibrium.
Market equilibrium
The price at which the quantity demanded of a good or service equals the quantity supplied. At this price, the amount buyers want to purchase matches the amount sellers want to sell, and there is no pressure for the price to change.
Equilibrium price
The specific price at which quantity demanded equals quantity supplied. Also called the market-clearing price.
Surplus (excess supply)
A situation where the quantity supplied exceeds the quantity demanded at the current price. This occurs when the price is above the equilibrium price.
Think of it as: goods are piling up on shelves because the price is too high for buyers. Sellers will lower prices to clear the excess.
Shortage (excess demand)
A situation where the quantity demanded exceeds the quantity supplied at the current price. This occurs when the price is below the equilibrium price.
Think of it as: people are queueing up but there is not enough to go around. Sellers will raise prices.
Consumer surplus (CS)
The difference between a consumer's maximum willingness to pay for a good and the price they actually pay, summed across all units purchased. On a graph, it is the triangle between the demand curve and the equilibrium price.
In simple terms, this means: the total "bonus" consumers get from paying less than they would have been willing to pay.
Producer surplus (PS)
The difference between the price a seller receives and their minimum willingness to accept (their cost), summed across all units sold. On a graph, it is the triangle between the equilibrium price and the supply curve.
In simple terms, this means: the total "bonus" sellers get from receiving more than the minimum they would have accepted.
Total surplus (TS)
Consumer surplus plus producer surplus. TS = CS + PS. This measures the total gains from trade in a market.
Price ceiling
A maximum allowable price set by the government. Only has an effect if set below the equilibrium price, in which case it creates a shortage.
Examples: rent control, prescription drug price caps, public university tuition caps.
Price floor
A minimum allowable price set by the government. Only has an effect if set above the equilibrium price, in which case it creates a surplus.
Examples: minimum wage (creates a surplus of labour, which is unemployment).
At the equilibrium price, quantity demanded = quantity supplied. There is no surplus or shortage.
If the price is above equilibrium, there is a surplus. Sellers cannot sell all their goods, so they lower prices, and the market moves back toward equilibrium.
If the price is below equilibrium, there is a shortage. Buyers cannot get all they want, so sellers raise prices, and the market moves back toward equilibrium.
Equilibrium price: $40
At $60 (above equilibrium): quantity supplied is 180, quantity demanded is 60. Surplus of 120. Only 60 units are bought and sold. Price falls back toward $40.
At $30 (below equilibrium): quantity supplied is 45, quantity demanded is 105. Shortage of 60. Only 45 units are bought and sold. Price rises back toward $40.
When an event occurs, work through these three steps:
Does this event affect consumers (demand) or sellers (supply)?
In which direction? Do consumers want to buy more or less? Do sellers want to sell more or less?
Draw it out. Shift the relevant curve, and read off the new equilibrium price and quantity.
Key patterns:
Demand increases (shifts right): equilibrium price rises, equilibrium quantity rises.
Demand decreases (shifts left): equilibrium price falls, equilibrium quantity falls.
Supply increases (shifts right): equilibrium price falls, equilibrium quantity rises.
Supply decreases (shifts left): equilibrium price rises, equilibrium quantity falls.
Both shift simultaneously: the effect on one variable (price or quantity) is ambiguous and depends on which shift is larger.
Consumer surplus (CS) is the area below the demand curve and above the market price. Producer surplus (PS) is the area above the supply curve and below the market price. Total surplus (TS) = CS + PS.
For linear supply and demand curves, these areas are triangles. The formula for a triangle is:
CS or PS = 1/2 x base x height
Where the base is the equilibrium quantity and the height is the vertical distance between the curve's intercept and the equilibrium price.
At equilibrium: CS = 1/2(60)(30 - 10) = $600. PS = 1/2(60)(10 - 5) = $150. TS = $750.
At a price of $8 (below equilibrium, a shortage of 18 units): only 48 units sold. CS = 1/2(48)(30 - 14) + 48(14 - 8) = $384. PS = 1/2(48)(8 - 5) = $72. TS = $744. Total surplus is lower than at equilibrium.
The key takeaway: total surplus is maximised at the equilibrium price. Any deviation from equilibrium reduces total surplus.
Price ceiling (maximum price)
Only binding (has an effect) if set below the equilibrium price.
Creates a shortage: quantity demanded exceeds quantity supplied at the ceiling price.
The quantity traded equals the quantity supplied (the short side of the market).
Examples: rent control, drug price caps, tuition caps.
Price floor (minimum price)
Only binding (has an effect) if set above the equilibrium price.
Creates a surplus: quantity supplied exceeds quantity demanded at the floor price.
The quantity traded equals the quantity demanded (the short side of the market).
The most important example: the minimum wage. A price floor on labour creates a surplus of labour, which is unemployment.
Production possibilities curve: shows maximum combinations of goods an economy can produce. About production capacity, not willingness to buy.
Supply curve: shows willingness of producers to sell at different prices.
Demand curve: shows willingness of consumers to buy at different prices.
Marginal cost curve: shows the cost of producing additional units. Not about consumer willingness.
Rent control in cities like New York and San Francisco is a price ceiling. It keeps rents below equilibrium, which creates a shortage of available housing units: more people want to rent at that price than landlords are willing to supply. Minimum wage laws are the textbook example of a price floor: when set above the equilibrium wage, they create a surplus of labour (more people want to work at that wage than employers want to hire), which manifests as unemployment.
Students often think a price ceiling set above the equilibrium price will cause a shortage. It does not. A ceiling above the equilibrium price is non-binding, which means it has no effect on the market.
Students often think a price floor set below the equilibrium price will cause a surplus. It does not. A floor below the equilibrium price is non-binding.
Students sometimes calculate the quantity traded at a controlled price using the demand side or the supply side incorrectly. The quantity traded is always the lesser of quantity demanded and quantity supplied at that price (the short side of the market).
Students mix up the surplus/shortage that triggers equilibrium adjustment with the consumer/producer surplus concept. A surplus (goods piling up) is about unsold inventory. Consumer surplus is about the gap between willingness to pay and the actual price.
⚠️ Know when price ceilings and price floors are binding: ceiling below equilibrium = binding (shortage), floor above equilibrium = binding (surplus). This is one of the most commonly missed questions.
⚠️ Be able to calculate consumer surplus, producer surplus, and total surplus from a supply and demand diagram using the triangle formula.
⚠️ Understand that total surplus is maximised at equilibrium. Any deviation (price controls, taxes) reduces it.
⚠️ The 3-step method for predicting equilibrium changes (who is affected, which direction, draw it) will appear in multiple exam questions. Practise it until it is automatic.
True or False: A surplus occurs when the price is below the equilibrium price.
False. A surplus occurs when the price is above the equilibrium price (quantity supplied exceeds quantity demanded).
Fill in the blank: Total surplus = ______ + ______.
Consumer surplus + producer surplus.
True or False: A price ceiling set above the equilibrium price causes a shortage.
False. A price ceiling above equilibrium is non-binding and has no effect.
Fill in the blank: When a price floor is set above equilibrium in the labour market, the resulting surplus of labour is called ______.
Unemployment.
True or False: Total surplus is maximised at the equilibrium price.
True.
Q: The equilibrium price of bread rolls is $40. If the government sets a price ceiling of $30, what happens?
A: The ceiling is below the equilibrium price, so it is binding. At $30, the quantity demanded exceeds the quantity supplied, creating a shortage. The quantity traded will equal the quantity supplied at $30 (the short side of the market).
Q: At equilibrium, 60 units are sold. The demand curve intercept is $30 and the supply curve intercept is $5. The equilibrium price is $10. Calculate CS, PS, and TS.
A: CS = 1/2(60)(30 - 10) = $600. PS = 1/2(60)(10 - 5) = $150. TS = CS + PS = $750.
Q: Supply decreases (shifts left). What happens to the equilibrium price and quantity?
A: The equilibrium price rises and the equilibrium quantity falls.
Q: Demand increases and supply decreases simultaneously. What can you say about the equilibrium price and quantity?
A: The equilibrium price definitely rises (both shifts push the price up). The equilibrium quantity is ambiguous: the demand increase pushes quantity up, but the supply decrease pushes quantity down. The net effect depends on which shift is larger.
Q: The minimum wage is set above the equilibrium wage. What type of price control is this, and what is the result?
A: This is a price floor. Because it is set above equilibrium, it is binding. It creates a surplus of labour (more people want to work at that wage than employers want to hire), which is unemployment.
Consumer and producer surplus will reappear when you study the effects of taxes and subsidies later in the course, where you will calculate the deadweight loss created by a tax. The concept of equilibrium is foundational: it returns in aggregate supply and demand (macroeconomic equilibrium), money markets, and international trade models. Price controls connect to debates about housing policy, labour markets, and healthcare economics.
Related Terms / Search Tags
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