Difficulty: Introductory | Prerequisites: Chapter 1 (cost-benefit framework, marginal analysis, reservation price).
Chapter 2 takes the individual decision-making tools from Chapter 1 and places them into a market setting. You are now looking at how buyers and sellers interact, how prices emerge from that interaction, and what happens when the government intervenes with price controls.
The demand curve and supply curve are the two most important diagrams in introductory economics. Understanding how they shift (and why a movement along a curve is different from a shift of the whole curve) is essential for nearly every topic that follows in this course.
If Chapter 1 was about how one person decides, Chapter 2 is about what happens when millions of those decisions collide in a market.
Markets reach equilibrium where supply meets demand. When price is too low, there is a shortage; when too high, a surplus. Government price controls (ceilings and floors) prevent equilibrium and create predictable side effects. The concepts of buyer's surplus, seller's surplus, and total surplus measure who gains from trade and by how much.
Market
The market for any good consists of all buyers or sellers of that good.
In simple terms, this means: a market is not a physical place. It is every person willing to buy or sell a particular product.
Demand curve
A schedule or graph showing the quantity of a good that buyers wish to buy at each price.
Think of it as the answer to: "At this price, how much do buyers want?"
Supply curve
A graph or schedule showing the quantity of a good that sellers wish to sell at each price.
Think of it as the answer to: "At this price, how much are sellers willing to offer?"
Change in demand
A shift of the entire demand curve.
In simple terms, this means: something other than price (income, preferences, the price of related goods) has changed, causing buyers to want more or less at every price.
Change in the quantity demanded
A movement along the demand curve that occurs in response to a change in price.
Think of it as sliding up or down the existing curve when price moves. The curve itself stays put.
Change in supply
A shift of the entire supply curve.
In simple terms, this means: something other than price (input costs, technology, number of sellers) has changed, causing sellers to offer more or less at every price.
Change in the quantity supplied
A movement along the supply curve that occurs in response to a change in price.
Complements
Two goods are complements in consumption if an increase in the price of one causes a leftward shift in the demand curve for the other (or if a decrease causes a rightward shift).
Think of it as goods that go together: peanut butter and jelly, printers and ink cartridges.
Substitutes
Two goods are substitutes in consumption if an increase in the price of one causes a rightward shift in the demand curve for the other (or if a decrease causes a leftward shift).
Think of it as goods that replace each other: Coca-Cola and Pepsi, butter and margarine.
Normal good
A good whose demand curve shifts rightward when the incomes of buyers increase and leftward when the incomes of buyers decrease.
In simple terms, this means: when people earn more, they buy more of it. Most goods are normal goods.
Inferior good
A good whose demand curve shifts leftward when the incomes of buyers increase and rightward when the incomes of buyers decrease.
Think of it as something you buy less of when you can afford better: instant noodles, bus rides.
Income effect
The change in the quantity demanded of a good that results because a change in the price of a good changes the buyer's purchasing power.
In simple terms, this means: when a good gets cheaper, your money goes further, so you can buy more of everything.
Substitution effect
The change in the quantity demanded of a good that results because buyers switch to or from substitutes when the price of the good changes.
Think of it as the switching behaviour: the good got more expensive, so you buy something else instead.
Equilibrium
A balanced or unchanging situation in which all forces at work within a system are cancelled by others.
Market equilibrium
Occurs in a market when all buyers and sellers are satisfied with their respective quantities at the market price.
Equilibrium price and equilibrium quantity
The price and quantity at the intersection of the supply and demand curves for the good.
In simple terms, this means: the one price at which the amount buyers want to buy equals the amount sellers want to sell.
Excess demand (or shortage)
The amount by which quantity demanded exceeds quantity supplied when the price of a good lies below the equilibrium price.
Excess supply (or surplus)
The amount by which quantity supplied exceeds quantity demanded when the price of the good exceeds the equilibrium price.
Buyer's surplus
The difference between the buyer's reservation price and the price he or she actually pays.
Think of it as the "deal" the buyer gets: you were willing to pay $20 but only paid $12, so your surplus is $8.
Seller's surplus
The difference between the price received by the seller and his or her reservation price.
Think of it as the seller's "bonus": you would have accepted $5 but got $12, so your surplus is $7.
Seller's reservation price
The smallest dollar amount for which a seller would be willing to sell an additional unit, generally equal to marginal cost.
Total surplus
The difference between the buyer's reservation price and the seller's reservation price.
In simple terms, this means: the total gain from a transaction, shared between buyer and seller.
Cash on the table
An economic metaphor for unexploited gains from exchange.
Think of it as money left on the table because a mutually beneficial trade is not happening.
Efficiency (or economic efficiency)
A condition that occurs when all goods and services are produced and consumed at their respective socially optimal levels.
Socially optimal quantity
The quantity of a good that results in the maximum possible economic surplus from producing and consuming the good.
Price ceiling
A maximum allowable price, specified by law.
In simple terms, this means: the government says sellers cannot charge more than a set amount. If set below equilibrium, it creates a shortage.
Price floor
A minimum allowable price, specified by law.
In simple terms, this means: the government says the price cannot fall below a set amount. If set above equilibrium, it creates a surplus.
Rent control
A type of price ceiling which specifies the maximum rent that can be charged on a particular home or apartment.
Minimum wage
A type of price floor which specifies the minimum wage that can be paid for a particular job.
The demand curve slopes downward: as price rises, quantity demanded falls.
Two forces drive this: the substitution effect (buyers switch to cheaper alternatives) and the income effect (a higher price reduces the buyer's real purchasing power).
A change in price causes a movement along the curve. A change in anything else (income, preferences, prices of related goods, expectations, number of buyers) shifts the whole curve.
Complements and substitutes matter for shifts. If the price of a complement rises, demand for the other good falls (shifts left). If the price of a substitute rises, demand for the other good increases (shifts right).
Normal goods see demand rise with income. Inferior goods see demand fall with income.
The supply curve slopes upward: as price rises, quantity supplied increases.
The seller's reservation price is the minimum a seller will accept, typically equal to marginal cost.
A change in price causes a movement along the supply curve. A change in input prices, technology, expectations, or the number of sellers shifts the whole curve.
Equilibrium is where the supply curve and demand curve intersect. At this price, quantity demanded equals quantity supplied.
Below equilibrium price, there is excess demand (a shortage): more people want the good than sellers are willing to supply. This puts upward pressure on the price.
Above equilibrium price, there is excess supply (a surplus): sellers offer more than buyers want. This puts downward pressure on the price.
Markets tend toward equilibrium naturally through these pressures.
Buyer's surplus = reservation price minus price paid. Seller's surplus = price received minus reservation price.
Total surplus = buyer's surplus + seller's surplus = buyer's reservation price minus seller's reservation price.
A market is efficient when total surplus is maximised, which happens at the socially optimal quantity.
"Cash on the table" means there are trades that would benefit both parties but are not happening. An efficient market leaves no cash on the table.
A price ceiling set below equilibrium creates a shortage. Buyers want more than sellers will supply at the capped price. Rent control is the classic example.
A price floor set above equilibrium creates a surplus. Sellers offer more than buyers want at the mandated price. Minimum wage is the classic example.
Price controls set on the "wrong" side of equilibrium (a ceiling above equilibrium, a floor below) have no effect because the market already clears at a price within the legal range.
Both types of binding price controls reduce total surplus compared to the free-market outcome.
Students often confuse a "change in demand" (a shift of the whole curve) with a "change in quantity demanded" (a movement along the curve). Price changes cause movements. Everything else causes shifts. The same distinction applies on the supply side.
Students sometimes think a surplus means buyers benefit and a shortage means sellers benefit. A surplus means sellers cannot sell all they want. A shortage means buyers cannot buy all they want. Neither situation is "good" for the side that appears to have more.
Students often assume price ceilings and price floors always have an effect. They only bind, and only cause distortions, when the ceiling is below equilibrium or the floor is above equilibrium.
Students frequently mix up normal and inferior goods. "Inferior" does not mean "bad quality." It means demand falls when income rises, because people switch to something they prefer now that they can afford it.
⚠️ The shift-vs-movement distinction is tested relentlessly. Any question that says "the price of X rises" and asks what happens to the demand for Y is testing whether you know how to shift the right curve in the right direction.
⚠️ Surplus calculations are a near-certainty on exams. Know how to compute buyer's surplus, seller's surplus, and total surplus from reservation prices and market prices.
⚠️ Price control questions will ask you to identify whether a ceiling or floor is binding (does it sit on the right side of equilibrium?) and what the resulting shortage or surplus looks like.
⚠️ Be prepared to classify goods as normal or inferior given a scenario about income changes and demand shifts. The terminology is precise and exams expect you to use it correctly.
True or false: If the price of coffee rises and the demand for tea increases, coffee and tea are complements.
False. They are substitutes. A rise in the price of one increases demand for the other.
Fill in the blank: When quantity demanded exceeds quantity supplied, there is ________.
Excess demand (a shortage).
True or false: A price ceiling set above the equilibrium price will cause a shortage.
False. A ceiling above equilibrium is non-binding and has no effect.
Fill in the blank: The difference between the buyer's reservation price and the price actually paid is called ________.
Buyer's surplus.
True or false: An inferior good is one that is low quality.
False. "Inferior" means demand decreases when income rises, not that the good is poor quality.
Q: The price of petrol rises sharply. What happens to the demand for public transport, and why?
A: The demand for public transport increases (the demand curve shifts rightward). Petrol-powered cars and public transport are substitutes. When the price of one rises, consumers switch to the other.
Q: A buyer's reservation price for a textbook is $80. She buys it for $55. What is her buyer's surplus?
A: $80 minus $55 = $25.
Q: The government imposes a price ceiling of $8 on a good whose equilibrium price is $10. What results?
A: The ceiling is binding because it sits below the equilibrium price. At $8, quantity demanded exceeds quantity supplied, creating a shortage (excess demand).
Q: Incomes rise in a city. Demand for instant noodles falls while demand for restaurant meals rises. Classify each good.
A: Instant noodles are an inferior good (demand falls as income rises). Restaurant meals are a normal good (demand rises as income rises).
Q: At a price of $5, quantity demanded is 200 units and quantity supplied is 200 units. Is this an equilibrium? What if the price were $3 and quantity demanded were 300 while quantity supplied were 100?
A: At $5, yes, this is an equilibrium because quantity demanded equals quantity supplied. At $3, there is excess demand of 200 units (300 minus 100), so $3 is below equilibrium and the price will tend to rise.
Q: A new technology reduces the cost of producing electric cars. What happens to the supply curve and the equilibrium price?
A: The supply curve shifts rightward (producers are willing to supply more at every price). The equilibrium price falls and the equilibrium quantity increases.
This connects to Chapter 1 because the buyer's reservation price, marginal cost, and surplus concepts from Chapter 1 are now applied in a market context to build the demand and supply framework.
Supply and demand analysis extends into later chapters on elasticity, where you will measure how sensitive quantity demanded and supplied are to price changes. The welfare analysis here (total surplus, efficiency) lays the groundwork for evaluating taxes, trade, and market failures.
Price controls reappear in policy discussions throughout the course. Rent control and minimum wage are two of the most commonly debated applications.
supply and demand, demand curve, supply curve, market equilibrium, equilibrium price, equilibrium quantity, shortage, surplus, excess demand, excess supply, buyer's surplus, seller's surplus, total surplus, consumer surplus, producer surplus, complements, substitutes, normal good, inferior good, income effect, substitution effect, price ceiling, price floor, rent control, minimum wage, cash on the table, economic efficiency, socially optimal quantity, reservation price, ECON 2314, intro to microeconomics, University of Texas, microecon chapter 2, econ key terms, supply and demand study notes