Why are firms willing to offer more of a good when its price rises?
Define supply and the law of supply, and explain why the supply curve slopes upward
A clear O-Level answer on supply and the law of supply. What supply means, why quantity supplied rises with price, the profit and rising-cost reasons for the upward slope, and how the supply curve is drawn.
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What this dot point is asking
The syllabus wants you to define supply and the law of supply, and to explain why the supply curve slopes upward. The big idea is that sellers, like buyers, respond to price, but in the opposite direction: the higher the price, the more producers are willing and able to offer for sale.
The answer
What supply means
As with demand, the words willing and able matter: a firm must both want to sell at that price and actually be able to produce the good. Supply is always measured over a period, such as per week or per year.
The law of supply
This is a direct (positive) relationship, the opposite of demand. Other things being equal means costs of production and other determinants are held constant so we can see the effect of price alone.
The supply curve
Plotting price on the vertical axis and quantity supplied on the horizontal axis, the law of supply gives a curve that slopes upward from left to right. At a low price, little is supplied; at a high price, much more is supplied.
Why the curve slopes upward
There are two reasons a higher price raises the quantity supplied:
- The profit incentive. A higher price, with costs unchanged, means more profit on each unit. Existing firms produce more, and new firms are tempted to enter the market, so quantity supplied rises.
- Covering rising costs. As a firm produces more, the cost of each extra unit tends to rise (for example, paying overtime or using less suitable resources). Firms will only make these dearer extra units if the price is high enough to cover the higher cost.
Both reasons point the same way: a higher price leads to a higher quantity supplied, so the curve slopes up.
Examples in context
Example 1. Higher fish prices and fishing effort. If the market price of fish rises, fishing firms are willing to send out more boats and spend longer at sea, raising the quantity of fish supplied, because the higher price both raises profit and covers the extra cost of the additional fishing trips.
Example 2. New sellers entering a popular market. When the price and profit in a market rise, such as for a popular food trend in Singapore, new stalls and firms enter to share in the profit. This entry of new producers is part of why quantity supplied rises with price.
Try this
Cue. Define supply. The quantity of a good or service that producers are willing and able to offer for sale at each price over a given period of time.
Cue. State the law of supply. Other things being equal, as price rises the quantity supplied rises, and as price falls the quantity supplied falls; price and quantity supplied move in the same direction.
Cue. Explain one reason the supply curve slopes upward. A higher price means more profit on each unit with costs unchanged, so existing firms produce more and new firms enter the market, raising the quantity supplied.
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.
Original4 marksDefine supply and state the law of supply.Show worked answer →
A 4 mark question rewards a clear definition of supply and a correct statement of the law.
Supply. Supply is the quantity of a good or service that producers are willing and able to offer for sale at each price over a given period of time.
Law of supply. The law of supply states that, other things being equal, as the price of a good rises the quantity supplied rises, and as the price falls the quantity supplied falls. Price and quantity supplied move in the same direction.
Markers reward the willing-and-able wording for supply and the direct (same-direction) relationship between price and quantity supplied, with the other-things-equal condition.
Original5 marksExplain why the supply curve for a good slopes upward from left to right.Show worked answer →
A 5 mark question rewards the profit incentive and the rising-cost reasons for the upward slope.
The supply curve slopes upward because a higher price raises the quantity producers are willing to supply, for two reasons.
Profit incentive. A higher price, with costs unchanged, means a larger profit on each unit sold. This encourages existing firms to produce more and attracts new firms into the market, raising quantity supplied.
Covering rising costs. As firms expand output, the cost of producing each extra unit tends to rise (for example, paying overtime or using less efficient resources). Firms will only produce these extra, more costly units if the price is high enough to cover the higher cost.
Together these mean a higher price leads to a higher quantity supplied, giving an upward-sloping curve.
Markers reward both reasons, the profit incentive and the need for a higher price to cover rising costs, and the link to the upward slope.
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