Difficulty: Introductory | Prerequisites: Part 1 notes (Foundational Concepts, PPF, Opportunity Cost)
This unit explains why countries and individuals trade, and how markets find a price that balances what buyers want with what sellers offer. Comparative advantage is the reason trade makes both sides better off, even when one side is better at producing everything. Supply and demand is the central model of the entire course: nearly every later topic (elasticity, market structures, government intervention) builds on it. You need to be comfortable with the PPF and opportunity cost from Part 1 before this will make full sense.
Trade works because each party specialises in what they can produce at the lowest opportunity cost (comparative advantage), then exchanges. Supply and demand curves determine the market price and quantity. When something other than price changes, the whole curve shifts; when only price changes, you move along the existing curve.
Absolute Advantage
The ability to produce more of a good than another party using the same amount of resources.
In simple terms, this means: who is faster or more productive in raw output terms.
Comparative Advantage
The ability to produce a good at a lower opportunity cost than another party. This is the basis for mutually beneficial trade.
Think of it as: who gives up less to make this thing.
Specialisation
When each person, firm or country focuses on producing the good for which they have a comparative advantage, then trades for the rest.
Terms of Trade
The rate at which goods are exchanged between trading parties. For both sides to benefit, the terms of trade must fall between their respective opportunity costs.
Think of it as: the agreed swap rate that makes the deal worthwhile for everyone.
Demand Shifters
Factors other than price that change the entire demand curve: income, consumer preferences, prices of related goods (substitutes and complements), number of buyers, expectations about future prices, and taxes or subsidies.
Supply Shifters
Factors other than price that change the entire supply curve: input costs, technology, number of sellers, expectations, prices of related goods in production, state of nature (weather, disasters), and taxes or subsidies.
Equilibrium Price and Quantity
The price and quantity where the demand curve and supply curve intersect. At this point, the quantity buyers want to purchase equals the quantity sellers want to sell.
In simple terms, this means: the market clears, with no leftover goods and no unmet demand.
Surplus (Excess Supply)
Occurs when the market price is above equilibrium. Quantity supplied exceeds quantity demanded. Sellers cannot sell everything they produce at that price.
Shortage (Excess Demand)
Occurs when the market price is below equilibrium. Quantity demanded exceeds quantity supplied. Buyers cannot get as much as they want at that price.
Absolute advantage is about raw productivity. If Country A can produce 10 cars per day and Country B can produce 6, Country A has the absolute advantage in cars.
Comparative advantage is about opportunity cost. Even if Country A is better at making both cars and wheat, it should specialise in whichever good it produces at the lowest opportunity cost.
A party can have the absolute advantage in everything but cannot have the comparative advantage in everything. Comparative advantage is always relative.
Trade based on comparative advantage makes both parties better off, even when one is more productive across the board.
Each party specialises in the good where it has the comparative advantage, then trades for the other good.
The terms of trade (the exchange rate between the two goods) must fall between the two parties' opportunity costs. If it does not, at least one party is worse off trading than producing on its own.
Example: if Country A's opportunity cost of 1 car is 3 tonnes of wheat, and Country B's is 5 tonnes, the terms of trade must be somewhere between 3 and 5 tonnes of wheat per car for both to gain.
Demand shifters (factors that move the entire demand curve):
Income (normal goods: demand rises with income; inferior goods: demand falls with income)
Consumer preferences and tastes
Price of related goods (substitutes: price of one rises, demand for the other rises; complements: price of one rises, demand for the other falls)
Number of buyers in the market
Expectations about future prices
Taxes or subsidies on buyers
Supply shifters (factors that move the entire supply curve):
Input costs (wages, raw materials, energy)
Technology improvements (shift supply right)
Number of sellers in the market
Expectations about future prices
Prices of related goods in production
State of nature (weather, natural disasters)
Taxes or subsidies on producers
A shift means the entire curve moves to a new position. This happens when a non-price factor changes. A rightward shift in demand means consumers want more at every price.
A movement along the curve happens when the price of the good itself changes. Price goes up, quantity demanded moves up along the supply curve and down along the demand curve.
The distinction is heavily tested. If the question says "the price of the good changed," that is a movement. If it says "income rose" or "technology improved," that is a shift.
Opportunity cost:
\text{Opportunity Cost of Good A} = \frac{\text{Amount of Good B given up}}{\text{Amount of Good A gained}}Surplus and shortage:
\text{Surplus} = Q_{\text{supply}} - Q_{\text{demand}}\text{Shortage} = Q_{\text{demand}} - Q_{\text{supply}}Market supply and demand: Add all individual supply (or demand) quantities at each price to get the market curve.
Equilibrium: Found at the intersection of the market supply and market demand curves. Read the price off the vertical axis and the quantity off the horizontal axis.
Comparative advantage explains why countries like Bangladesh export textiles while importing electronics. Bangladesh has a lower opportunity cost of producing clothing relative to high-tech goods, so it specialises there and trades.
Supply and demand is the model behind every market price you encounter. When a drought destroys wheat crops, supply shifts left, and the price of bread rises. When a new phone launches and millions want it, demand shifts right, and the price stays high until supply catches up.
Students often confuse absolute advantage with comparative advantage. A country can have the absolute advantage in both goods but will still have the comparative advantage in only one. The exam tests this distinction relentlessly.
Students frequently mix up shifts and movements. A change in the price of the good itself causes a movement along the curve, not a shift. A shift is caused by something other than the good's own price.
Many students think a surplus means "extra profit." It does not. A surplus means unsold goods are piling up because the price is too high.
Students sometimes forget that demand shifters and supply shifters are different lists. Income affects demand; input costs affect supply. Mixing them up leads to wrong answers on curve-shifting questions.
⚠️ Comparative advantage problems appear on nearly every AP Micro exam. You will be given a table and asked to calculate opportunity costs, determine who has the comparative advantage, and identify the acceptable terms of trade.
⚠️ Shift-the-curve questions are the most common supply-and-demand format. The exam gives you a scenario and asks which curve shifts, in which direction, and what happens to equilibrium price and quantity.
⚠️ Know the difference between a change in demand (shift) and a change in quantity demanded (movement). Using the wrong phrase costs marks on free-response questions.
⚠️ Surplus and shortage calculations are straightforward but require reading the graph carefully. Make sure you are reading the correct quantities at the given price.
True or False: A country with the absolute advantage in both goods also has the comparative advantage in both. (False. A country can only have the comparative advantage in one of the two goods.)
Fill in the blank: Terms of trade must fall between the two parties' ______ for both to benefit. (Opportunity costs.)
True or False: A rise in consumer income shifts the supply curve to the right. (False. Income is a demand shifter, not a supply shifter.)
True or False: If the price of a good falls and quantity demanded increases, that is a movement along the demand curve, not a shift. (True.)
Fill in the blank: Surplus = Q_supply minus ______ . (Q_demand.)
Q: Country X can produce 20 cars or 40 tonnes of rice. Country Y can produce 10 cars or 50 tonnes of rice. Which country has the comparative advantage in cars?
A: Country X. Its opportunity cost of 1 car is 2 tonnes of rice (40/20). Country Y's opportunity cost of 1 car is 5 tonnes of rice (50/10). Country X gives up less rice per car.
Q: What is the acceptable range for the terms of trade (in tonnes of rice per car) between Country X and Country Y?
A: Between 2 and 5 tonnes of rice per car. Below 2, Country X is worse off trading. Above 5, Country Y is worse off.
Q: A news report says "consumer confidence has risen sharply." Which curve shifts, and in which direction?
A: The demand curve shifts to the right. Higher consumer confidence increases willingness to spend at every price level.
Q: The price of steel (an input) increases. What happens to the supply curve for cars?
A: The supply curve shifts to the left. Higher input costs make it more expensive to produce each unit, reducing the quantity sellers are willing to supply at every price.
Q: At a price of £8, quantity demanded is 200 and quantity supplied is 350. Is this a surplus or a shortage, and what is its magnitude?
A: A surplus of 150 units (350 supplied minus 200 demanded). The price is above equilibrium.
Comparative advantage and the PPF are tightly linked. The slope of each country's PPF gives the opportunity cost, which is exactly what you use to determine comparative advantage.
Supply and demand connects forward to elasticity (Part 3 of these notes). Elasticity measures how sensitive quantity demanded or supplied is to a change in price, income or the price of another good.
Shifters reappear in every government-intervention topic: taxes shift the supply curve, subsidies shift it the other way, price floors and ceilings create surpluses or shortages.
Absolute advantage, comparative advantage, specialisation, specialization, terms of trade, opportunity cost of trade, gains from trade, demand shifters, supply shifters, determinants of demand, determinants of supply, shift vs movement, change in demand vs change in quantity demanded, equilibrium price, equilibrium quantity, market clearing price, surplus, shortage, excess supply, excess demand, normal good, inferior good, substitute good, complement good, AP Micro Unit 1, AP Micro Unit 2, supply and demand basics