Difficulty: Introductory | Prerequisites: None
This module lays the groundwork for the entire microeconomics course. It introduces scarcity as the central problem in economics and explains how markets, through the interaction of buyers and sellers, allocate scarce resources. You will learn how supply and demand determine prices and quantities, and how to distinguish between movements along a curve and shifts of a curve. If you are joining the course late, start here: every later module builds on these definitions and the supply-and-demand framework.
Resources are scarce, so societies need a way to allocate them. Markets do this through prices, which are set by the interaction of supply (sellers) and demand (buyers). When supply equals demand, the market is in equilibrium and there is no pressure for the price to change.
Scarcity
There is not enough of a resource to satisfy everyone who wants it. Because scarcity exists, societies need some mechanism to decide who gets what.
In simple terms, this means there is always a gap between what people want and what is available.
Demand
How much of a good consumers will buy at each possible price. Represented as Q_D = f(P).
Think of it as the entire schedule of "if the price were X, buyers would want Y units."
Supply
How much output firms are willing to sell at each possible price. Represented as Q_S = h(P).
Think of it as the seller's side of the same schedule: "if the price were X, we would produce Y units."
Equilibrium
A state in which there is no tendency for change. In a market, this occurs where quantity demanded equals quantity supplied.
In simple terms, the market has "settled" and neither buyers nor sellers have reason to adjust their behaviour.
Opportunity cost
The value of the next best foregone opportunity. Every choice has one, because choosing one option means giving up another.
Think of it as "what you gave up to do this instead."
Shortage
Quantity demanded is greater than quantity supplied at a given price. The price is too low to clear the market.
In simple terms, more people want the good at that price than sellers are willing to provide.
Surplus
Quantity supplied is greater than quantity demanded at a given price. The price is too high to clear the market.
In simple terms, sellers have stock left over because not enough buyers want the good at that price.
Prices
Rationing devices formed by the interaction of buyers and sellers. Prices signal where resources should flow.
Think of prices as the traffic lights of an economy, directing goods toward whoever values them most.
Scarcity is the foundational problem: wants exceed available resources.
Because of scarcity, every society needs an allocation mechanism to decide who gets what.
Markets are one such mechanism. Prices do the rationing.
Demand describes the relationship between price and the quantity consumers wish to buy.
Shift of the demand curve = a change in demand itself (the whole curve moves left or right). Caused by changes in income, tastes, prices of related goods, expectations, or number of buyers.
Movement along the demand curve = a change in quantity demanded (you slide along the existing curve). Caused only by a change in the good's own price.
This distinction is one of the most frequently tested points in introductory economics.
Supply describes the relationship between price and the quantity firms are willing to sell.
Shift of the supply curve = a change in supply itself (the whole curve moves). Caused by changes in input costs, technology, expectations, or number of sellers.
Movement along the supply curve = a change in quantity supplied. Caused only by a change in the good's own price.
Equilibrium occurs where the demand curve and the supply curve intersect.
At equilibrium, quantity demanded equals quantity supplied and there is no tendency for the price to change.
If the price is below equilibrium, a shortage results (Q_D > Q_S), which pushes the price up.
If the price is above equilibrium, a surplus results (Q_S > Q_D), which pushes the price down.
The market self-corrects toward equilibrium through these pressures.
Q_D = f(P)Quantity demanded is a function of price. Note: in the standard supply-and-demand diagram, price is on the vertical axis and quantity on the horizontal, so the graph appears "flipped" relative to the mathematical convention (where the independent variable sits on the horizontal axis).
Q_S = h(P)Quantity supplied is also a function of price, with the same graphing convention.
Equilibrium condition:
The market clears where Q_D = Q_S. At that price, there is neither a shortage nor a surplus.
Variables to know:
P = price
Q = market output (quantity)
D = demand
S = supply
Concert tickets that sell out in minutes are a textbook shortage: at the listed price, quantity demanded far exceeds quantity supplied. Scalpers (resellers) move the price toward equilibrium by charging what the market will bear.
Opportunity cost is the reason "there's no such thing as a free lunch." Even when you don't pay money, you give up time or another option. Governments weigh opportunity costs when deciding how to allocate a budget.
Students often confuse a "change in demand" (shift of the curve) with a "change in quantity demanded" (movement along the curve). These are different concepts with different causes. If only the good's own price changed, it is a movement. If something else changed (income, tastes, related prices), it is a shift.
Students sometimes think equilibrium means the market outcome is "fair" or "good." It does not. Equilibrium simply means no tendency for change, with no value judgement attached.
Opportunity cost is not the same as the monetary price. It is the value of the next best alternative you gave up, which may include non-monetary factors like time or enjoyment.
A surplus does not mean "extra goods that nobody wants." It means that at the current price, some goods go unsold because the price is above equilibrium.
The shift-versus-movement distinction appears on nearly every introductory exam. Be precise with language: "demand increased" means the curve shifted right, not that consumers bought more because the price fell.
Questions about opportunity cost often present a scenario with multiple options and ask you to identify the cost of the chosen one. The answer is always the single next best alternative, not the sum of everything else you gave up.
Shortage and surplus questions typically give you a price and ask whether Q_D or Q_S is larger. Draw the diagram if you are unsure.
True or False: A decrease in the price of a good causes the demand curve to shift to the right.
Fill in the blank: The value of the next best foregone opportunity is called ______.
True or False: At equilibrium, quantity demanded equals quantity supplied.
Fill in the blank: When quantity demanded exceeds quantity supplied at a given price, the result is a ______.
True or False: A change in consumer income causes a movement along the demand curve.
Answers: 1. False (it causes a movement along the curve, not a shift). 2. Opportunity cost. 3. True. 4. Shortage. 5. False (it causes a shift of the demand curve).
Q: Suppose the price of coffee rises. Using the correct terminology, describe what happens in the market for coffee and in the market for tea (a substitute).
A: In the coffee market, the higher price causes a decrease in quantity demanded (movement along the demand curve). In the tea market, because tea is a substitute for coffee, the demand for tea increases (the demand curve for tea shifts to the right), leading to a higher equilibrium price and quantity of tea.
Q: A new technology reduces the cost of producing smartphones. What happens to the supply curve, the equilibrium price, and the equilibrium quantity?
A: The supply curve shifts to the right (supply increases). The equilibrium price falls, and the equilibrium quantity rises.
Q: You have a free ticket to a film, but you could instead spend the evening tutoring for 30 dollars. What is the opportunity cost of going to the film?
A: The opportunity cost is the 30 dollars you would have earned tutoring (the value of the next best alternative you gave up).
Q: If the government sets a price below the equilibrium price, what results?
A: A shortage results, because at the lower price quantity demanded exceeds quantity supplied.
This connects to Module 2 (Government Intervention in Markets) because price controls, such as price ceilings and floors, create shortages or surpluses by preventing the market from reaching equilibrium.
The concept of opportunity cost reappears in Module 3 (Firms, Productions, and Costs), where it is used to distinguish economic profit from accounting profit.
Supply and demand analysis is the foundation for every market structure studied later in the course, from perfect competition to monopoly.
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