Difficulty: Introductory to Intermediate | Prerequisites: Ch. 1 foundations (scarcity, opportunity cost, PPF)
Tags: comparative advantage, absolute advantage, specialisation, trade, law of demand, law of supply, demand curve, supply curve, market equilibrium, shortage, surplus, ceteris paribus, demand shifters, supply shifters, ECO 101
This chunk of the course moves from "choices have costs" to "how markets actually work." Comparative advantage explains why trade makes both parties better off, even when one is better at producing everything. Then demand and supply give you the mechanics of markets: how buyers and sellers interact, how prices are set, and what happens when the market is out of balance.
If you have not yet covered scarcity, opportunity cost, and the PPF, go back to Ch. 1 first. Everything here builds on those ideas.
Producers should specialise in the good where their opportunity cost is lowest (comparative advantage), then trade. In a market, demand slopes down (higher price, less buying) and supply slopes up (higher price, more selling). Where the two curves cross is equilibrium. If the price is too high, a surplus forms; too low, a shortage forms. The market pushes prices back toward equilibrium.
Absolute advantage
The ability to produce more of a good than another producer using the same amount of resources.
In simple terms, this means who is faster or more productive in absolute numbers.
Comparative advantage
The ability to produce a good at a lower opportunity cost than another producer. This is the basis for specialisation and trade.
Think of it as: who gives up less to make this thing? That is who should make it.
Specialisation
Concentrating production on the good for which a producer has the lowest opportunity cost, then trading for everything else.
Law of demand
As the price of a good increases, the quantity demanded decreases, all else equal. This inverse relationship gives the demand curve its downward slope.
Law of supply
As the price of a good increases, the quantity supplied increases, all else equal. This direct relationship gives the supply curve its upward slope.
Demand function
Qd = f(P). Quantity demanded is a function of price. The relationship is inverse.
Supply function
Qs = f(P). Quantity supplied is a function of price. The relationship is direct.
Ceteris paribus
Latin for "all else held constant." Economists use this assumption to isolate the effect of one variable (usually price) on quantity demanded or supplied.
Substitution effect
When the price of a good rises, consumers switch to cheaper alternatives. One of the two reasons the demand curve slopes downward.
Income effect
When the price of a good falls, consumers' purchasing power increases, so they can buy more. The other reason the demand curve slopes downward.
Market equilibrium
The point where quantity demanded equals quantity supplied (Qd = Qs). At this price (P*) and quantity (Q*), there is no pressure for the price to change.
Surplus (excess supply)
Occurs when the market price is above equilibrium (P > P*). Quantity supplied exceeds quantity demanded. Market pressure pushes the price down.
Shortage (excess demand)
Occurs when the market price is below equilibrium (P < P*). Quantity demanded exceeds quantity supplied. Market pressure pushes the price up.
Absolute advantage tells you who produces more with the same resources. Comparative advantage tells you who gives up less to produce it. Trade decisions are based on comparative advantage.
Trade is mutually beneficial when each producer specialises in the good where their opportunity cost is lowest, then exchanges.
Voluntary trade is a win-win: both parties end up with more than they could produce alone.
How to apply the model (three steps)
Compute the opportunity cost of each good for each producer.
Identify who has the lower opportunity cost for each good.
That producer should specialise in that good.
Worked example
Country | Wheat (tons) | Cloth (units) | OC of 1 Wheat | OC of 1 Cloth |
|---|---|---|---|---|
A | 10 | 20 | 2 Cloth | 0.5 Wheat |
B | 5 | 15 | 3 Cloth | 0.33 Wheat |
Country A has the comparative advantage in wheat (lower OC: 2 cloth vs. 3 cloth). Country B has the comparative advantage in cloth (lower OC: 0.33 wheat vs. 0.5 wheat). Each should specialise accordingly and trade.
The broader impact of trade
Specialisation increases global output and tends to lower prices.
Trade shifts a country's consumption possibilities beyond its own PPF.
Real-world trade is affected by tariffs, transport costs, and policy, but the core logic holds.
As price increases, quantity demanded decreases (inverse relationship).
The demand curve slopes downward from left to right.
Two forces explain the downward slope:
Substitution effect: when price rises, buyers switch to cheaper alternatives.
Income effect: when price falls, buyers' real purchasing power increases, so they buy more.
Demand shifters (factors that shift the entire curve)
A change in price causes a movement along the demand curve. A change in any of the following shifts the whole curve left or right:
Income (normal goods: more income shifts demand right; inferior goods: more income shifts demand left)
Tastes and preferences
Expectations of future prices (if buyers expect prices to rise, demand shifts right now)
Number of buyers in the market
Prices of related goods (substitutes and complements)
As price increases, quantity supplied increases (direct relationship).
The supply curve slopes upward from left to right.
Higher prices mean more profit, so sellers supply more. Lower prices mean less profit, so sellers supply less.
Supply shifters (factors that shift the entire curve)
A change in price causes a movement along the supply curve. A change in any of the following shifts the whole curve:
Input costs (higher costs shift supply left; lower costs shift supply right)
Technology (improvements shift supply right)
Expectations of future prices
Number of sellers in the market
Shifts vs. movements: the critical distinction
Movement along the curve: caused by a change in the good's own price. You slide along the existing curve.
Shift of the curve: caused by a change in any non-price factor (a demand shifter or a supply shifter). The entire curve moves left or right.
Equilibrium is where quantity demanded equals quantity supplied (Qd = Qs). At this point, the market clears.
On a graph, equilibrium is the intersection of the demand and supply curves.
The equilibrium price is written as P* and the equilibrium quantity as Q*.
Solving for equilibrium algebraically
Set the demand function equal to the supply function and solve for P.
Example: Qd = 100 - 2P and Qs = 20 + 3P.
100 - 2P = 20 + 3P, so 80 = 5P, so P* = 16. Then Q* = 100 - 2(16) = 68.
Shortages and surpluses
When the market price is below equilibrium (P < P*), quantity demanded exceeds quantity supplied. This is a shortage (excess demand). Market pressure pushes the price up.
When the market price is above equilibrium (P > P*), quantity supplied exceeds quantity demanded. This is a surplus (excess supply). Market pressure pushes the price down.
In a free market, these pressures move the price back toward equilibrium.
Opportunity cost formula (for comparative advantage)
Opportunity Cost of Good A = Output of Good B / Output of Good A
Compare across producers. The one with the lower opportunity cost has the comparative advantage for that good.
Demand and supply functions
Demand: Qd = f(P), typically a downward-sloping linear function such as Qd = a - bP.
Supply: Qs = f(P), typically an upward-sloping linear function such as Qs = c + dP.
Finding equilibrium
Set Qd = Qs, solve for P* (equilibrium price), then plug P* back into either function to find Q* (equilibrium quantity).
Supply and demand graph (key features to label)
Price on the vertical axis, quantity on the horizontal axis.
Demand curve: downward-sloping.
Supply curve: upward-sloping.
Equilibrium: the point where the two curves intersect (P*, Q*).
Surplus area: above P*, between the two curves.
Shortage area: below P*, between the two curves.
Nations trade because resources are unevenly distributed. No country can efficiently produce everything it needs, so each specialises where its opportunity cost is lowest.
Adam Smith's "invisible hand" connects to comparative advantage: when people specialise to earn a living, they unintentionally contribute to the well-being of society by creating value others want to buy.
Everyday price changes (petrol, rent, groceries) are explained by shifts in demand or supply, or both. If you can identify which curve shifted and in which direction, you can predict what happens to the price and quantity.
Students frequently confuse absolute advantage with comparative advantage. A country can have the absolute advantage in both goods and still not have the comparative advantage in both. Comparative advantage is always about opportunity cost, not total output.
Students mix up shifts and movements. A price change causes a movement along the curve. A change in income, tastes, input costs, or technology shifts the entire curve. This distinction is heavily tested.
Students sometimes think equilibrium means the market is "good" or "fair." Equilibrium is simply the point where Qd = Qs. Whether it is desirable is a separate question (one that comes up in the welfare and price-controls chapters).
Students occasionally forget that surplus and shortage are defined relative to the equilibrium price, not relative to some arbitrary level.
Expect comparative advantage problems: you will be given a table of outputs and asked to compute opportunity costs, identify who has the comparative advantage, and state who should specialise in what.
Know how to solve for equilibrium algebraically (set Qd = Qs, solve for P*, plug back in for Q*).
Be able to draw and label a supply-and-demand graph: curves, equilibrium, surplus, and shortage.
The shifts vs. movements distinction is one of the most commonly tested concepts. If the exam asks "what happens when income rises," that is a shift of the demand curve, not a movement along it.
Know the demand shifters (income, tastes, expectations, number of buyers, prices of related goods) and the supply shifters (input costs, technology, expectations, number of sellers) by heart.
True or false: The producer with the absolute advantage in a good always has the comparative advantage in that good. (False. Comparative advantage depends on opportunity cost, not total output.)
Fill in the blank: A movement along the demand curve is caused by a change in ______. (The good's own price.)
True or false: If the market price is above equilibrium, there is a shortage. (False. Price above equilibrium causes a surplus.)
Fill in the blank: The demand curve slopes downward because of the ______ effect and the ______ effect. (Substitution; income.)
True or false: An improvement in technology shifts the supply curve to the right. (True.)
Q: Country X can produce 20 tonnes of rice or 10 tonnes of steel. Country Y can produce 15 tonnes of rice or 5 tonnes of steel. Which country has the comparative advantage in steel?
A: Country X's opportunity cost of 1 tonne of steel = 20/10 = 2 tonnes of rice. Country Y's opportunity cost of 1 tonne of steel = 15/5 = 3 tonnes of rice. Country X has the lower opportunity cost, so Country X has the comparative advantage in steel.
Q: The demand for umbrellas increases on a rainy day. Is this a shift of the demand curve or a movement along it?
A: A shift of the demand curve to the right. The weather (a taste/preference factor) changed, not the price of umbrellas.
Q: Given Qd = 200 - 4P and Qs = 40 + 2P, find the equilibrium price and quantity.
A: Set Qd = Qs: 200 - 4P = 40 + 2P, so 160 = 6P, so P* = 26.67. Q* = 200 - 4(26.67) = 93.33. (On an exam, these numbers would typically come out to round figures.)
Q: At a price of $10, quantity demanded is 500 and quantity supplied is 300. Is there a surplus or a shortage, and what pressure does this create?
A: A shortage (excess demand), because quantity demanded exceeds quantity supplied. Market pressure pushes the price upward toward equilibrium.
Q: Why does the supply curve slope upward?
A: Higher prices mean higher profit margins, which motivate sellers to supply more. Lower prices reduce profitability, so sellers supply less. The direct relationship between price and quantity supplied gives the curve its upward slope.
Comparative advantage builds directly on the PPF and opportunity cost from Ch. 1. Without understanding opportunity cost, the comparative advantage formula makes no sense.
The demand and supply framework is the backbone of the rest of the course. Elasticity (Ch. 2.7 onwards) measures how responsive demand and supply are to price changes. Surplus and welfare (Ch. 2.10 onwards) measure the benefits buyers and sellers get from trading at equilibrium.
Price controls (Ch. 2.12 onwards) are government interventions that push the price away from equilibrium, creating the shortages and surpluses you have just learned to identify.
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