Why are firms willing to supply more at higher prices, and what shifts the whole supply curve?
State the law of supply, distinguish a movement along from a shift of the supply curve, and identify the determinants of supply
A focused answer to the H2 Economics learning outcome on supply. The law of supply and why the curve slopes upward, the movement-versus-shift distinction, and the determinants that shift supply.
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What this dot point is asking
SEAB wants you to state the law of supply, explain why the supply curve slopes upward, distinguish a movement along the curve from a shift of the whole curve, and list the determinants that shift supply. As with demand, the key discipline is that a change in the good's own price moves you along the curve, while a change in anything else shifts it.
The answer
The law of supply
Supply here means the quantity producers are willing and able to offer for sale at each price over a period of time.
Why supply slopes upward
Two related reasons explain the positive relationship:
- Profit incentive. A higher price raises the profit on each unit, so existing firms expand output and new firms are drawn into the market.
- Rising marginal cost. As firms produce more in the short run, marginal cost typically rises (diminishing returns to the variable factor). Producers will only supply the extra, higher-cost units if the price rises to cover them.
Movement along versus shift of the curve
The same disciplined distinction as demand applies:
- A change in the good's own price causes a movement along the supply curve, called a change in quantity supplied.
- A change in any other determinant shifts the whole curve, called a change in supply. A rightward shift is an increase in supply; a leftward shift is a decrease.
Determinants that shift supply
The main supply shifters are:
- Costs of production. A rise in the price of inputs (wages, raw materials, energy) raises costs and shifts supply left; a fall shifts it right.
- Technology. Improved technology lowers unit costs and shifts supply right.
- Taxes and subsidies. An indirect tax on producers shifts supply left (it acts like a cost); a subsidy shifts it right.
- Number of firms. More firms in the market increase supply.
- Prices of related goods in production. If a firm can switch to producing a more profitable alternative, supply of this good falls.
- Expectations and shocks. Weather, disasters or expected future prices can shift supply, especially for agricultural goods.
Examples in context
Example 1. Energy costs in Singapore. Singapore generates most of its electricity from imported natural gas, so a rise in global gas prices raises generation costs and shifts the electricity supply curve left, pushing tariffs up. This is a cost-driven supply shift, not a movement along the curve.
Example 2. Technology in electronics. Continual improvements in semiconductor manufacturing lower the unit cost of chips, shifting the supply curve for electronics rightward over time. This is why, holding demand constant, the prices of many electronic goods fall as the technology matures.
Try this
Q1. State the law of supply. [2 marks]
- Cue. Other things equal, as the price of a good rises its quantity supplied rises, and as price falls quantity supplied falls.
Q2. Explain how an indirect tax on producers affects the supply curve. [2 marks]
- Cue. The tax acts like an extra cost per unit, so it shifts the whole supply curve leftward (a decrease in supply), reducing the quantity supplied at each price.
Q3. Explain why improved technology shifts supply rather than moving along the curve. [3 marks]
- Cue. Technology is a non-price determinant; it lowers unit costs at every price, so it shifts the entire supply curve right, whereas only the good's own price causes a movement along it.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksExplain the law of supply, and distinguish between a movement along the supply curve and a shift of the supply curve.Show worked answer →
A 10 mark question rewards the law, a reason for the upward slope, and a precise movement-versus-shift distinction.
- The law
- Other things equal, as the price of a good rises its quantity supplied rises, and as price falls quantity supplied falls. The supply curve slopes upward.
- Why
- A higher price makes production more profitable, so existing firms expand output and new firms enter. As output rises, marginal cost typically rises, so a higher price is needed to justify the extra units.
- Movement versus shift
- A change in the good's own price causes a movement along the curve (a change in quantity supplied). A change in any other determinant (costs of production, technology, taxes and subsidies) shifts the whole curve (a change in supply).
Markers reward the ceteris paribus law, the profit and marginal-cost reasoning for the slope, and a clean separation of movement (own price) from shift (other determinants).
Original8 marksExplain three factors that would shift the supply curve for electricity to the left, raising its price.Show worked answer →
An 8 mark question rewards three correct supply shifters with a clear mechanism for each.
- Higher input costs
- A rise in the price of natural gas raises the cost of generating electricity, reducing supply at every price and shifting the curve left.
- A new tax on producers
- A carbon tax on power stations raises their costs per unit, shifting supply left.
- A reduction in capacity
- The closure of a major plant after a fault reduces the quantity that can be supplied at each price, shifting the curve left.
Markers reward three distinct supply shifters, the leftward direction, and the link from each to higher production cost or lower capacity, hence a higher equilibrium price.
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