Jupiter2U School
Chapter 07

Supply & Demand

Middle School

If economics had one master diagram, this is it. Two curves — one for buyers, one for sellers — cross at a single point, and that crossing determines both the price and the quantity of almost everything traded in a market. Master this and you can reason about oil, housing, coffee and wages alike.

At a glance
Core ideaPrice and quantity settle wherever the supply and demand curves cross.
Key termEquilibrium — the price where Qd = Qs, with no shortage or surplus.
You can…Shift the correct curve the correct way and read the new crossing.
Watch outA change in the good's own price moves along a curve; anything else shifts it.
Theory

Demand, supply and equilibrium

The law of demand: other things equal, as the price of a good rises, the quantity demanded falls. The demand curve slopes downward. Two reasons: the substitution effect (buyers switch to now-cheaper alternatives) and the income effect (a higher price makes buyers poorer in real terms).

The law of supply: other things equal, as price rises, the quantity supplied rises. The supply curve slopes upward, because higher prices cover higher marginal costs and reward extra output.

Crucially, distinguish a movement along a curve (caused only by a change in the good's own price) from a shift of the whole curve (caused by anything else). Demand shifters — remember PIRATES: Price of related goods (substitutes/complements), Income, Rates (interest), Advertising/tastes, Tastes, Expectations, Size of population. Supply shifters: costs of inputs, technology, taxes/subsidies, number of firms, weather (for agriculture).

Equilibrium: Quantity demanded (Qd) = Quantity supplied (Qs) → the market-clearing price P*
Explanation

How the market finds its balance

At the equilibrium price, the plans of buyers and sellers are consistent — everyone who wants to trade at that price can. Away from it, pressure builds:

  • If price is above equilibrium, Qs > Qd — a surplus. Unsold stock forces sellers to cut prices.
  • If price is below equilibrium, Qd > Qs — a shortage. Frustrated buyers bid the price up.

The price keeps adjusting until the surplus or shortage vanishes. This self-correcting tendency is Adam Smith's "invisible hand" in miniature.

S D₁ D₂ P*₁ Q*₁ P*₂ Q*₂ Price Quantity
An increase in demand (D₁→D₂) raises both equilibrium price and quantity, moving along the fixed supply curve.

Reasoning about shifts is the core skill. Rule: shift the correct curve in the correct direction, then read the new crossing. Example: a heatwave raises demand for ice cream → demand shifts right → price ↑ and quantity ↑. A frost destroys the orange crop → supply of orange juice shifts left → price ↑ and quantity ↓.

Practical

Worked example: solving for equilibrium algebraically

A market has demand and supply schedules (P in $, Q in thousands):

Demand: Qd = 120 − 4P Supply: Qs = 20 + 6P
Step 1 — set Qd = Qs120 − 4P = 20 + 6P
Step 2 — solve for P*120 − 20 = 6P + 4P → 100 = 10P → P* = $10.
Step 3 — find Q*Substitute: Qd = 120 − 4(10) = 80 thousand units (check: Qs = 20 + 6(10) = 80 ✓).
Step 4 — test disequilibriumAt a price ceiling of $6: Qd = 120 − 24 = 96; Qs = 20 + 36 = 56. Shortage = 96 − 56 = 40 thousand units.
Step 5 — a demand shockIncomes rise, adding 30 to demand at every price: Qd = 150 − 4P. New equilibrium: 150 − 4P = 20 + 6P → 130 = 10P → P* = $13, Q* = 98. Both price and quantity rose, as the shift rule predicts.
Q&A
Q1Both demand and supply increase. What happens to price and quantity?

Quantity definitely rises (both shifts push it up). Price is indeterminate — it depends on which shift is larger. If demand rises more than supply, price rises; if supply rises more, price falls; if they are equal, price is unchanged. Whenever both curves move, one of the two outcomes is always ambiguous without the magnitudes.

Q2A tabloid says, "Prices rose because demand rose, and demand rose because prices were expected to rise." Is that circular?

No — it is a real feedback loop. Expectations are a genuine demand shifter. If buyers expect higher future prices (say, of housing), they buy now, shifting current demand right and raising today's price. That realised increase can reinforce the expectation. This is how speculative bubbles inflate, and it is distinct from the ordinary law of demand.

Q3Why doesn't a fall in price shift the demand curve?

The demand curve already describes quantity demanded at every possible price — price is the variable on the axis. A change in the good's own price therefore moves you along the existing curve, not to a new one. Only a change in the other determinants (income, tastes, related-good prices, etc.) shifts the whole curve.

Q4Coffee and sugar are complements. The price of coffee triples. What happens in the sugar market?

Higher coffee prices reduce the quantity of coffee bought, so people also want less sugar to go with it. The demand for sugar shifts left → the equilibrium price and quantity of sugar both fall. Note nothing happened in the sugar supply: the shock arrived through the complementary-good linkage on the demand side.

Q5Are equilibrium prices "fair"?

That is a normative question the model does not answer. Equilibrium is efficient in the narrow sense that all mutually beneficial trades occur, but it can leave outcomes many would call unfair (e.g. essential medicine priced beyond the poor). Economics separates the positive claim ("this is the market-clearing price") from the normative one ("this price ought to be allowed").

← Prev: Scarcity & Choice ↑ Top Next: Elasticity →
Concept mind map

How the ideas connect

Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.

Demand slopes downSupply slopes upEquilibrium priceSurplusShortageShifts vsmovementsNon-price factorsSupply & Demand
Infographic

The process, step by step

Step 1Price too highQuantity supplied exceeds quantity demanded, creating a surplus of unsold goods.
Step 2Sellers cut priceTo clear stock, sellers lower the price; buyers demand more as price falls.
Step 3Price too lowQuantity demanded exceeds quantity supplied, creating a shortage.
Step 4Price bid upBuyers compete for scarce goods, pushing price up; sellers offer more.
Step 5EquilibriumPrice settles where quantity demanded equals quantity supplied; the market clears.
Solved examples

Worked problems, step by step

Follow each solution line by line, then try to reproduce it on paper before moving on.

Example 1Demand Qd = 100 - 2P and supply Qs = 20 + 2P. Find the equilibrium price and quantity.

  1. Set Qd = Qs: 100 - 2P = 20 + 2P.
  2. Solve: 80 = 4P, so P = 20.
  3. Quantity: Qs = 20 + 2(20) = 60.

Example 2At P = $30 with Qd = 100 - 2P and Qs = 20 + 2P, is there a surplus or shortage, and how big?

  1. Qd = 100 - 60 = 40; Qs = 20 + 60 = 80.
  2. Qs (80) exceeds Qd (40).
  3. Surplus = 80 - 40 = 40 units.
Practice problem set

Now you try

Work each one out first, then tap to reveal the worked answer.

1Why does the demand curve slope downward?
As price falls, goods are cheaper relative to others (substitution) and buyers can afford more (income effect), so quantity demanded rises.
2What is the difference between a shift of demand and a movement along it?
A price change causes a movement along the curve; a non-price factor (income, tastes) shifts the whole curve.
3A frost destroys half the coffee crop. What happens in the coffee market?
Supply shifts left, causing a shortage at the old price, so price rises and quantity traded falls.
4Incomes rise and coffee is a normal good. Show the effect.
Demand shifts right, raising both the equilibrium price and quantity.
5What does market equilibrium mean?
The price at which quantity demanded equals quantity supplied, so there is no surplus or shortage.
6Name two factors that shift supply.
Input costs and technology -- lower costs or better tech shift supply right.