Demand, supply and market equilibrium (SACE Stage 2 Economics)
“Use demand and supply to explain how market equilibrium is determined and how it changes.”
Demand slopes downward (lower prices, more bought) and supply slopes upward (higher prices, more sold). The market settles at the equilibrium price and quantity where the two curves cross. A shift in demand or supply moves equilibrium to a new price and quantity.
Jump to a section
What this dot point is asking
You must explain the laws of demand and supply, find equilibrium where they intersect, distinguish movements along a curve from shifts of a curve, and predict how shifts change price and quantity.
Demand
The quantity of a good consumers are willing and able to buy at each possible price over a period of time, all else held constant.
The law of demand states that as price falls, quantity demanded rises, and as price rises, quantity demanded falls. The demand curve therefore slopes downward from left to right. Two reasons: lower prices make a good cheaper relative to substitutes, and they raise consumers' real purchasing power.
A change in the good's own price causes a movement along the demand curve (a change in quantity demanded). A change in any other factor shifts the whole curve (a change in demand). Demand shifters include:
- income (for normal goods, higher income shifts demand right);
- prices of substitutes and complements;
- consumer tastes and preferences;
- population and expectations of future prices.
Supply
The quantity of a good producers are willing and able to offer for sale at each possible price over a period of time, all else held constant.
The law of supply states that as price rises, quantity supplied rises, because higher prices make production more profitable and cover the higher costs of expanding output. The supply curve slopes upward from left to right.
A change in the good's own price causes a movement along the supply curve. Other factors shift the curve. Supply shifters include:
- costs of production (wages, raw materials, energy);
- technology and productivity;
- number of producers;
- taxes and subsidies;
- weather, for agricultural goods.
Market equilibrium
Equilibrium is where the demand and supply curves intersect. At this point the quantity demanded equals the quantity supplied, and there is no pressure for price to change.
- Above equilibrium price there is a surplus (excess supply): sellers cut prices to clear stock, pushing price down.
- Below equilibrium price there is a shortage (excess demand): buyers bid prices up, pushing price up.
These forces drive the market back to equilibrium.
At the equilibrium price the market clears: every buyer willing to pay that price finds a seller, and every seller willing to accept it finds a buyer. There is no surplus and no shortage, so price is stable until a curve shifts.
Shifts and the four basic outcomes
When a curve shifts, equilibrium moves. The four single-shift results are:
- Demand rises (shifts right): price up, quantity up.
- Demand falls (shifts left): price down, quantity down.
- Supply rises (shifts right): price down, quantity up.
- Supply falls (shifts left): price up, quantity down.
A frost hits the coffee crop
A frost destroys part of the coffee harvest, raising production costs and reducing output. Supply shifts left.
At the old price there is now a shortage, so price rises. As price rises, quantity demanded contracts along the demand curve. The new equilibrium has a higher price and a lower quantity. Note the demand curve does not move - only the quantity demanded changes as a movement along it.
When both curves shift
If demand and supply both shift, one of price or quantity will be predictable and the other ambiguous, depending on the relative size of the shifts. For example, if demand rises and supply rises, quantity definitely rises, but price could rise, fall or stay the same.
Do not confuse a "change in quantity demanded" (a movement along the curve, caused only by the good's own price) with a "change in demand" (a shift of the whole curve, caused by other factors). Writing that a price rise "decreases demand" is wrong; a price rise decreases the quantity demanded but the demand curve itself does not move.
In data response and graphical analysis tasks, label axes (price, quantity), the original and new curves, and both equilibrium points. Marks are awarded for a correctly labelled diagram, not just the written explanation.
Exam-style questions
Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
2019 SACE Stage 2-style2 marksNew technology will make driverless vehicles more reliable and lead to substantially lower production costs. Explain why the price of driverless vehicles is expected to decrease significantly by 2030. Complete the demand and supply diagram to support your answer.Show worked answer →
Two marks: one for the supply analysis, one for a correct diagram.
Explanation (1 mark). Lower production costs reduce the cost of supplying each unit, so producers are willing to supply more at every price. This shifts the supply curve to the right (S to S1). With demand unchanged, the new equilibrium occurs at a lower price and higher quantity, so the price of driverless vehicles falls significantly.
Diagram (1 mark). Draw a standard demand (D) and supply (S) diagram with price on the vertical axis and quantity on the horizontal axis. Show the supply curve shifting right to S1, the equilibrium moving from E1 to E2, price falling from P1 to P2 and quantity rising from Q1 to Q2.
Markers reward the rightward supply shift caused specifically by lower costs, not a demand shift.
2022 SACE Stage 2-style2 marksAt the start of the COVID-19 pandemic there was a large increase in demand for medical-grade face masks, while supply was reduced by staffing shortages. Explain the likely impact of these changes in demand and supply on the market equilibrium for medical-grade face masks, and complete a demand and supply diagram to support your answer.Show worked answer →
Two marks: the equilibrium outcome plus the supporting diagram.
Explanation (1 mark). Demand increases (curve shifts right) while supply decreases (curve shifts left). Both shifts unambiguously raise the equilibrium price. The effect on quantity is ambiguous because the two shifts pull it in opposite directions; the net change depends on the relative size of each shift. The clear result is a higher equilibrium price.
Diagram (1 mark). Show D shifting right to D1 and S shifting left to S1. Mark the new equilibrium at a higher price (Pe to Pe1). The equilibrium quantity may rise, fall or stay roughly the same depending on the relative magnitude of the shifts.
Markers reward recognising the rise in price and the ambiguous quantity, not assuming quantity must rise.
2023 SACE Stage 2-style2 marksPoor weather reduced the supply of processing potatoes, which are used to make frozen fries. Explain how the price mechanism would impact both producers and consumers to return the market for frozen fries to equilibrium.Show worked answer →
Two marks: explain the rationing and signalling/incentive role of price for both sides.
Initial disequilibrium. The fall in supply of processing potatoes raises the cost and reduces the supply of frozen fries, creating a shortage at the old price as quantity demanded exceeds quantity supplied.
How price restores equilibrium. The shortage pushes the price up. The higher price rations the good among consumers: it discourages some consumption so quantity demanded contracts. At the same time the higher price acts as a signal and incentive for producers to supply more (expand output where they can). Quantity demanded falls and quantity supplied rises until they are equal again at the new, higher equilibrium price.
Markers want both the rationing effect on consumers and the incentive effect on producers driving the market back to equilibrium.
Practise this
Sources & how we know this
- Stage 2 Economics subject outline (2025 onwards) — SACE Board
- Stage 2 Economics: external assessment — SACE Board