Difficulty: Introductory | Prerequisites: None. This is foundational material for any microeconomics course.
Demand, supply, and market equilibrium form the backbone of microeconomics. Every other topic in this course, from elasticity to welfare analysis, builds on the mechanics covered here. If you can read a supply-and-demand diagram and explain what happens when a curve shifts, you have the single most useful skill in introductory economics. You should already be comfortable with basic graphing (plotting points, reading slopes) before working through these notes.
Buyers want more when prices fall (demand). Sellers offer more when prices rise (supply). The market settles at the price where quantity demanded equals quantity supplied, and any shock to income, costs, or preferences shifts one of the curves and moves the equilibrium to a new price and quantity.
Competitive market
A market with many small buyers and sellers, none of whom can individually influence the market price. Think of it as: everyone is a "price taker," not a price maker.
Market demand function
The total quantity of a good all consumers are willing to buy at each price, holding other factors (income, preferences, prices of related goods) constant. In simple terms, it answers: "How much will the whole market buy at this price?"
Demand curve
The graphical representation of the demand function, with price on the vertical axis and quantity on the horizontal axis. It slopes downward, reflecting the law of demand.
Law of demand
All else equal, a higher price leads to a lower quantity demanded, and vice versa. Think of it as: when something gets more expensive, people buy less of it.
Substitution effect
The portion of the change in quantity demanded that results from consumers switching to or from alternative goods when a price changes.
Income effect
The portion of the change in quantity demanded that results from a price change altering consumers' real purchasing power.
Movement along the demand curve
A change in quantity demanded caused solely by a change in the good's own price. The curve itself does not move.
Shift of the demand curve
A change in the position of the entire curve, caused by a change in a factor other than the good's own price (e.g. income, tastes, prices of related goods, expectations).
Market supply function
The total quantity of a good all producers are willing to sell at each price, holding other factors (input costs, technology) constant.
Supply curve
The graphical representation of the supply function. It generally slopes upward, reflecting the law of supply.
Law of supply
All else equal, a higher price leads to a higher quantity supplied. In simple terms, higher prices make production more profitable, so firms produce more.
Movement along the supply curve
A change in quantity supplied caused solely by a change in the good's own price.
Shift of the supply curve
A change in the position of the entire supply curve, caused by a change in an external factor such as input prices, technology, or government policy.
Market equilibrium
The price and quantity at which the demand and supply curves intersect. At this point, quantity demanded equals quantity supplied and there is no pressure for the price to change. Think of it as: the market's "resting point."
Excess demand (shortage)
A situation where quantity demanded exceeds quantity supplied at the current price. The resulting competition among buyers pushes the price upward.
Excess supply (surplus)
A situation where quantity supplied exceeds quantity demanded at the current price. The resulting competition among sellers pushes the price downward.
A competitive market has many buyers and sellers, each too small to affect the price on their own.
Every participant is a "price taker": they accept the going market price as given.
Because no one can manipulate the price, resources get allocated based on supply and demand forces, which leads to efficient outcomes.
The market demand function aggregates every individual consumer's willingness to buy at each price.
When plotted on a graph (price on the y-axis, quantity on the x-axis), it produces the demand curve.
The curve slopes downward: higher price, lower quantity demanded.
Ceteris paribus (all else equal), price and quantity demanded move in opposite directions.
Two forces drive this:
Substitution effect: a price rise makes the good relatively more expensive than alternatives, so consumers switch.
Income effect: a price rise reduces what consumers can afford overall, so they buy less.
Movement along the curve: caused only 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 other demand driver. The entire curve relocates.
Rightward shift (increase in demand): consumers want more at every price. Triggers include rising income (for normal goods), a rise in the price of a substitute, a fall in the price of a complement, favourable changes in tastes, or population growth.
Leftward shift (decrease in demand): the reverse of the above.
The market supply function aggregates every producer's willingness to sell at each price.
The supply curve plots this relationship and generally slopes upward.
Ceteris paribus, price and quantity supplied move in the same direction.
Higher prices make production more profitable, giving producers an incentive to increase output.
Movement along the curve: caused only by a change in the good's own price.
Shift of the curve: caused by changes in input prices, technology, government policy, or other external factors.
Rightward shift (increase in supply): producers offer more at every price. Common triggers include falling input costs, technological improvements, or favourable regulation.
Leftward shift (decrease in supply): the reverse.
Equilibrium occurs where the demand curve and supply curve intersect.
At the equilibrium price, quantity demanded equals quantity supplied; there is no pressure for the price to move.
If the price is above equilibrium, excess supply (surplus) pushes it down. If the price is below equilibrium, excess demand (shortage) pushes it up. The market self-corrects toward equilibrium.
Increase in demand (rightward shift): equilibrium price rises, equilibrium quantity rises.
Decrease in demand (leftward shift): equilibrium price falls, equilibrium quantity falls.
Increase in supply (rightward shift): equilibrium price falls, equilibrium quantity rises.
Decrease in supply (leftward shift): equilibrium price rises, equilibrium quantity falls.
When both curves shift at the same time, the direction of one variable (price or quantity) becomes ambiguous and depends on the relative size of the shifts.
Equilibrium condition
Qd = Qs
At the equilibrium price (P*), the quantity consumers demand equals the quantity producers supply.
Standard demand-and-supply diagram
Price (P) on the vertical axis, Quantity (Q) on the horizontal axis. The demand curve slopes downward from left to right. The supply curve slopes upward from left to right. Their intersection marks the equilibrium point (P*, Q*).
The demand-and-supply framework explains everyday pricing. When a drought cuts wheat harvests, the supply curve for bread shifts left, and bread prices rise at the supermarket. When a new smartphone launches and everyone wants one, the demand curve shifts right, and retailers can charge a premium until supply catches up. These are the same mechanics you see in the diagrams, just playing out in real markets.
Students often confuse a "change in demand" (a shift of the curve) with a "change in quantity demanded" (a movement along the curve). They are different things. A price change causes a movement; a change in income, tastes, or related-good prices causes a shift.
The same confusion applies on the supply side: a "change in supply" (shift) is not the same as a "change in quantity supplied" (movement along).
Students sometimes think equilibrium means the market is "good" or "fair." It does not. It simply means there is no tendency for the price to change given the current curves. Equilibrium can exist at a price that many people find unaffordable.
A common error on exams is shifting the wrong curve. If the shock is about costs or technology, it hits supply. If it is about income, tastes, or the price of a related good from the buyer's perspective, it hits demand.
⚠️ You will almost certainly be asked to identify whether a given event causes a shift of a curve or a movement along it. Practise classifying shocks by the factor they affect.
⚠️ Expect questions where two curves shift simultaneously and you must determine the effect on equilibrium price and quantity. Remember that one variable will be determinate and the other ambiguous unless you know the relative magnitudes.
⚠️ Diagram questions often ask you to label the old and new equilibrium. Always mark P*, Q*, the direction of the shift, and the new intersection clearly.
⚠️ "Explain using a supply-and-demand diagram" is a standard exam instruction. If you skip the diagram, you lose marks even if your written reasoning is correct.
Q: True or false: A fall in the price of butter, a complement to bread, will shift the demand curve for bread to the right.
A: True. A cheaper complement increases demand for the paired good.
Q: True or false: If the government imposes a new tax on producers, the demand curve shifts left.
A: False. A tax on producers raises their costs, shifting the supply curve left. The demand curve is unaffected.
Q: Fill in the blank: At a price above equilibrium, there is excess ______, which puts ______ pressure on the price.
A: Excess supply (surplus); downward.
Q: True or false: A movement along the demand curve can be caused by a change in consumer income.
A: False. A change in income shifts the demand curve. Only a change in the good's own price causes a movement along the curve.
Q: Fill in the blank: When both demand and supply increase simultaneously, equilibrium quantity ______, but the effect on equilibrium price is ______.
A: Rises; ambiguous (it depends on the relative size of the two shifts).
Q: Suppose a new study shows that coffee causes health benefits. Using a supply-and-demand diagram, explain what happens to the equilibrium price and quantity of coffee.
A: The study increases consumer preference for coffee, shifting the demand curve to the right. With supply unchanged, the new equilibrium has a higher price and a higher quantity.
Q: A technological breakthrough lowers the cost of producing electric vehicles. What happens to the equilibrium price and quantity in the EV market?
A: Lower production costs shift the supply curve to the right. Equilibrium price falls and equilibrium quantity rises.
Q: The price of steel rises sharply. Explain the effect on the market for cars.
A: Steel is an input in car production. Higher steel prices raise production costs, shifting the supply curve for cars to the left. Equilibrium price of cars rises, and equilibrium quantity falls.
Q: Distinguish between a change in demand and a change in quantity demanded.
A: A change in demand is a shift of the entire demand curve, caused by a factor other than the good's own price (income, preferences, prices of related goods). A change in quantity demanded is a movement along the existing curve, caused solely by a change in the good's own price.
Q: At a price below equilibrium, what mechanism drives the market back toward equilibrium?
A: Below equilibrium, quantity demanded exceeds quantity supplied, creating a shortage. Buyers compete for the limited supply, bidding the price upward. As price rises, quantity demanded falls and quantity supplied rises until the two are equal again at equilibrium.
This material feeds directly into price elasticity of demand (covered in the companion study notes), which asks how much quantity demanded responds to a price change. Elasticity is built on the same demand curve mechanics covered here.
The equilibrium framework reappears in consumer and producer surplus analysis, where the areas between the curves and the equilibrium price measure the gains from trade. It also underpins government intervention topics such as price ceilings, price floors, taxes, and subsidies, all of which create deviations from the free-market equilibrium described in these notes.
Supply and demand, competitive market, price taker, market demand function, demand curve, law of demand, substitution effect, income effect, demand shifters, supply shifters, movement along the curve, shift of the curve, market supply function, supply curve, law of supply, market equilibrium, equilibrium price, equilibrium quantity, excess demand, shortage, excess supply, surplus, simultaneous shifts, ceteris paribus, normal goods, inferior goods, complements, substitutes, input prices, technology shock