Source: Besanko & Braeutigam, Microeconomics 5th ed.
Difficulty: Introductory | Prerequisites: Basic understanding of what markets are, familiarity with graphing price vs. quantity.
Supply and demand analysis is the backbone of microeconomics, and this chapter is where most intermediate courses build their analytical foundation. You need to understand how markets reach equilibrium, what shifts supply or demand curves (and which direction), and how to measure responsiveness through elasticity. If you skipped introductory micro or it was a while ago, revisit what "quantity demanded" and "quantity supplied" mean before diving in. Everything from consumer theory to welfare analysis later in the course rests on the mechanics covered here.
Markets settle at a price where quantity demanded equals quantity supplied. External events shift either curve, changing the equilibrium price and quantity. How much quantity responds to a price change is measured by price elasticity of demand, and the choke price is the price at which demand drops to zero.
Demand curve
A graph showing the quantity of a good consumers are willing and able to buy at each price, holding all other factors constant. Think of it as the answer to: "If the price were £X, how many units would people want?"
Supply curve
A graph showing the quantity of a good producers are willing and able to sell at each price, holding all other factors constant. In simple terms, it tells you how much firms will put on the market at any given price.
Equilibrium price and quantity
The price and quantity where the demand curve and supply curve intersect, so that quantity demanded equals quantity supplied. This is the market's natural resting point: no surplus, no shortage.
Substitute goods
Goods that can replace each other in consumption. When the price of one rises, demand for the other increases. Think of tea and coffee: if tea gets expensive, some people switch to coffee.
Complement goods
Goods that are consumed together. When the price of one rises, demand for the other falls. Coffee and coffee cups are complements: if cups become wildly expensive, the overall cost of drinking coffee rises.
Choke price
The price at which quantity demanded falls to zero. Set Q = 0 in the demand equation and solve for P. In simple terms, it is the price so high that nobody buys the product at all.
Shift vs. movement along a curve
A change in the good's own price causes a movement along the existing curve. A change in anything else (income, tastes, prices of related goods, input costs, technology) shifts the entire curve to a new position.
Price of substitutes rises: Demand for the good shifts right (increases). If the price of tea doubles, some tea drinkers switch to coffee, so coffee demand increases.
Price of complements rises: Demand for the good shifts left (decreases). If Styrofoam coffee cups become 300% more expensive, the total cost of drinking coffee rises, and some consumers cut back.
Consumer tastes and preferences change: Negative information shifts demand left. A study linking caffeine to cancer would reduce demand for coffee.
Income changes: For normal goods, higher income shifts demand right. For inferior goods, higher income shifts demand left.
Input costs rise: Supply shifts left (decreases). If raw materials become scarcer or more expensive, producers supply less at every price.
Natural disasters or weather events: A frost that kills half the Colombian coffee crop destroys supply, shifting the supply curve left. This raises the equilibrium price and lowers the equilibrium quantity.
Technology improvements: Better production technology shifts supply right.
(a) Price of tea doubles: Tea is a substitute for coffee. Tea becomes relatively more expensive, so consumers shift towards coffee. Demand for coffee shifts right. Equilibrium price and quantity of coffee both rise.
(b) Caffeine linked to cancer: Consumer preferences change negatively. Demand for coffee shifts left. Equilibrium price and quantity both fall.
(c) Frost kills half the Colombian crop: Supply shock. Supply of coffee shifts left. Equilibrium price rises and equilibrium quantity falls.
(d) Price of Styrofoam cups up 300%: Cups are a complement to coffee. Higher cup prices reduce the overall demand for coffee. Demand shifts left. Equilibrium price and quantity both fall.
The choke price is a specific concept for linear demand curves. It is the price at which quantity demanded hits zero, i.e. the y-intercept when you plot the demand curve with price on the vertical axis.
Start with the demand equation in the form Q = a - bP.
Set Q = 0.
Solve for P.
Given Q = 50 - 100P:
Set Q = 0: 0 = 50 - 100P
Rearrange: 100P = 50
Solve: P = 50/100 = 0.50
The choke price is P = 0.50. At any price above £0.50, quantity demanded would be negative, which has no economic meaning, so demand is simply zero.
Linear demand curve (general form)
Q = a - bP, where a is the quantity intercept and b is the slope (in absolute value).
Choke price
Set Q = 0, solve: P = a/b.
Equilibrium condition
Set quantity demanded equal to quantity supplied: Qd = Qs. Solve for P, then substitute back to find Q.
Diagram notes for curve shifts
A rightward shift of the demand curve: draw the new demand curve (D2) to the right of the original (D1), with the same supply curve. The new equilibrium sits at a higher price and higher quantity.
A leftward shift of the supply curve: draw the new supply curve (S2) to the left of the original (S1), with the same demand curve. The new equilibrium sits at a higher price and lower quantity.
Supply and demand curve shifts explain everyday price changes. A frost in Brazil (the world's largest coffee producer) regularly causes coffee futures prices to spike, exactly as the supply-shift model predicts. The substitution effect is visible whenever ride-hailing surge pricing kicks in and more people turn to public transport or taxis instead.
Students often confuse a shift of the curve with a movement along the curve. A change in the good's own price moves you along the existing curve. Only a change in something other than the good's own price shifts the curve.
A complement's price going up does not shift the supply curve. It shifts the demand curve. The complement relationship is about consumption, not production.
Students sometimes think a leftward supply shift means quantity and price both fall. In fact, a leftward supply shift raises the equilibrium price while lowering the equilibrium quantity.
The choke price is not the equilibrium price. It is the maximum price at which any unit would be demanded, i.e. the y-intercept of the demand curve.
⚠️ Expect questions that give you a real-world event and ask you to identify which curve shifts and in which direction. This is the most commonly tested format for this chapter.
⚠️ Choke price is a quick calculation question. If you can set Q = 0 and solve for P, it is free marks.
⚠️ You will likely be asked to distinguish between a shift and a movement along the curve. Know the difference cold.
True or False: If the price of a substitute good falls, demand for the original good increases. (False, demand decreases.)
True or False: A frost that destroys supply shifts the supply curve to the left. (True.)
Fill in the blank: The price at which quantity demanded equals zero is called the ______. (Choke price.)
True or False: An increase in the price of a complement shifts the demand curve for the related good to the right. (False, it shifts left.)
Fill in the blank: For Q = 50 - 100P, the choke price is ______. (P = 0.50.)
Q: A new health study finds that drinking coffee reduces the risk of heart disease. What happens to the equilibrium price and quantity of coffee?
A: Demand for coffee shifts right (increase in consumer preference). Equilibrium price rises. Equilibrium quantity rises.
Q: Suppose a new coffee-growing technique allows farmers to produce twice as much coffee per acre. Illustrate the effect on the coffee market.
A: Supply shifts right. The equilibrium price falls and the equilibrium quantity rises.
Q: Given the demand equation Q = 80 - 200P, what is the choke price?
A: Set Q = 0: 0 = 80 - 200P, so P = 80/200 = 0.40. The choke price is $0.40.
Q: The government places a ban on importing tea. What happens in the coffee market?
A: Tea is a substitute for coffee. Banning tea imports reduces tea supply, raising tea prices. Consumers switch to coffee. Coffee demand shifts right. Equilibrium price and quantity of coffee both rise.
This connects directly to consumer and producer surplus (Chapter 3 in most texts), because surplus is the area between the price line and the supply or demand curve, and curve shifts change those areas. The concept of substitutes and complements reappears in consumer theory when you study indifference curves and cross-price effects. Choke price links forward to welfare analysis and taxation, where the demand curve's intercepts determine deadweight loss geometry.
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