Supply: core idea

· Supply = the quantity of a good or service that producers are willing and able to offer for sale at different prices over a given time period.
· Law of supply: there is a positive relationship between price and quantity supplied — as price rises, quantity supplied rises; as price falls, quantity supplied falls.
· This gives an upward-sloping supply curve.
· Always remember ceteris paribus: the law only holds when other factors affecting supply are unchanged.
· In exam answers, define supply carefully: do not confuse supply with quantity supplied.

Why the supply curve slopes upward

· As firms increase output in the short run, they often face diminishing marginal returns.
· Diminishing marginal returns means that adding more of a variable factor (for example, labour) to a fixed factor (for example, machinery or factory space) eventually adds less and less extra output.
· Because extra output becomes harder to produce, marginal costs rise.
· Increasing marginal costs help explain why firms require a higher price to supply more output.
· Chain to memorise: diminishing marginal returns → rising marginal cost → upward-sloping supply curve.

Supply curve, supply schedule, and market supply

· A supply schedule is a table showing the quantity supplied at different prices.
· A supply curve is the graph of that relationship.
· An individual producer’s supply shows how much one firm is willing and able to sell at each price.
· Market supply is the sum of all individual firms’ supplies in the market.
· So, market supply is found by adding individual quantities supplied horizontally at each price.
· In diagrams, always label axes correctly: Price on the vertical axis, Quantity on the horizontal axis.

Non-price determinants of supply

· Costs of factors of production (FOPs): if wages, rent, raw material, or energy costs rise, supply decreases; if costs fall, supply increases.
· Prices of related goods:
· Competitive supply: if another product becomes more profitable, firms may switch resources away, so supply of the original good falls.
· Joint supply: if goods are produced together, an increase in production of one may increase supply of the other.
· Indirect taxes and subsidies: taxes decrease supply because they raise costs; subsidies increase supply because they lower costs.
· Future price expectations: if producers expect higher prices in the future, they may hold back supply now; if they expect lower future prices, they may increase supply now.
· Technology: improved technology usually increases supply by reducing costs or raising productivity.
· Number of firms: more firms in the market means greater market supply; fewer firms means less market supply.

Movement along the supply curve vs shift of the supply curve

· A movement along the supply curve is caused only by a change in the good’s own price.
· Extension in supply = a movement up/right along the curve caused by a rise in price.
· Contraction in supply = a movement down/left along the curve caused by a fall in price.
· A shift of the supply curve is caused by a non-price determinant changing.
· Increase in supply = curve shifts right.
· Decrease in supply = curve shifts left.
· Exam rule: if price changes, move along the curve; if costs, taxes, subsidies, technology, expectations, related goods, or number of firms change, the curve shifts.

This graphic shows standard supply and demand curves meeting at equilibrium, with S for supply and D for demand. It is helpful for seeing supply in the wider market context and for practising curve labels clearly. Source

How to write and interpret supply diagrams in exams

· Draw a clear upward-sloping supply curve and label it S.
· If supply increases, shift S to the right and label the new curve S1 or S2 clearly.
· If supply decreases, shift S to the left.
· If the question says price changes, do not shift the curve — show a movement along it.
· In explanation, always identify the cause, the direction of change, and whether it is a movement or a shift.
· Strong exam wording: “A fall in production costs increases supply, so the supply curve shifts right.”