Topic 1.5 Supply: explain the law of supply, distinguish a change in quantity supplied from a change in supply, and identify the determinants that shift the supply curve.

What this topic is asking

Topic 1.5 introduces the supply side of a market. The College Board wants you to state the law of supply, to draw and read a supply curve, to distinguish a change in quantity supplied (a movement along the curve) from a change in supply (a shift of the whole curve), and to identify the determinants that shift supply. It mirrors the demand topic, and the shift-versus-movement distinction is just as important here.

The law of supply

The law of supply holds because a higher price makes production more profitable, encouraging existing producers to make more and new producers to enter, while also covering the rising marginal cost of producing extra units. As price climbs, supplying more becomes worthwhile.

Movement along versus shift of supply

As with demand, the central distinction is between a movement and a shift:

If the price of wheat rises, you move up the existing wheat supply curve. If a cheaper fertilizer lowers costs so producers supply more at every price, the whole wheat supply curve shifts right. Treating an own-price change as a shift is a classic error the exam punishes.

The determinants of supply

The factors that shift the supply curve can be remembered with the prompt "ROTTEN" (Resource/input prices, Other goods' prices, Technology, Taxes and subsidies, Expectations, Number of sellers):

• Resource (input) prices: cheaper inputs lower costs and shift supply right; costlier inputs shift it left.
• Prices of other goods producers could make: if a firm can profitably switch to a different product, the supply of the original falls.
• Technology: cost-reducing technology shifts supply right.
• Taxes and subsidies: a tax on producers shifts supply left (raises costs); a subsidy shifts it right (lowers costs).
• Expectations: if producers expect higher future prices, they may withhold supply now, shifting current supply left.
• Number of sellers: more sellers shift supply right.

Putting supply to work

Supply is the second half of every market model, so the same discipline applies as with demand: when something changes, ask first whether it is the good's own price (a movement along the curve) or a determinant (a shift of the whole curve). Almost every supply shifter can be understood through its effect on production cost, which is a useful unifying lens for the exam. A new technology, a cheaper input, or a subsidy all reduce the cost of producing each unit, so firms are willing to supply more at every price, and the curve moves right; a new tax, a costlier input, or a regulation raises cost and moves the curve left. Naming the determinant, stating the direction, and labelling the graph with S1 and S2 is what earns free-response points. The supply curve also has a quiet logic worth remembering: it is built up from the rising marginal cost of producing extra units, which is why it slopes upward, and why a higher price is needed to coax out more output. This connects supply to the firm's cost structure and, in later units, to the upward-sloping short-run aggregate supply curve, where higher price levels likewise call forth more output. Mastering the determinants and the shift-versus-movement distinction here means you can analyze any market change quickly and correctly, which is exactly what the equilibrium topic asks you to do next.

Try this

Q1. State the law of supply and explain why it holds. [2 points]

• Cue. As price rises, quantity supplied rises (ceteris paribus); this holds because a higher price makes production more profitable and covers the rising marginal cost of extra units, drawing out more output.

Q2. Identify whether a subsidy to producers shifts supply left or right, and explain. [2 points]

• Cue. Right; a subsidy lowers production cost, so producers supply more at every price.