Why do buyers want more of a good when its price falls?
Define demand and the law of demand, and explain why the demand curve slopes downward
A clear O-Level answer on demand and the law of demand. What effective demand means, why quantity demanded rises as price falls, the income and substitution reasons behind it, and how the demand curve is drawn.
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What this dot point is asking
The syllabus wants you to define demand and the law of demand, and to explain why the demand curve slopes downward. The big idea is that buyers respond to price: the lower the price, the more they are willing and able to buy, and this simple relationship is the foundation of everything that follows in demand and supply.
The answer
What demand means
The phrase willing and able is important. This is effective demand: you must both want the good and have the money to pay for it. A person who would love a car but cannot afford one is not part of the demand for cars, because their want is not backed by ability to pay.
The law of demand
The phrase other things being equal (sometimes written as ceteris paribus) means we hold everything else, such as income and the prices of other goods, constant so that we can focus on the effect of price alone.
The demand curve
If we plot price on the vertical axis and quantity demanded on the horizontal axis, the law of demand gives a curve that slopes downward from left to right. At a high price, quantity demanded is low; at a low price, quantity demanded is high. Each point on the curve answers the question, how much would buyers purchase at this price?
Why the curve slopes downward
There are two reasons a lower price raises the quantity demanded:
- The income effect. When a good's price falls, your money buys more, so your real income (purchasing power) rises. You can afford to buy more of the good.
- The substitution effect. When a good's price falls, it becomes cheaper compared with other goods. Buyers switch toward it and away from now-relatively-dearer substitutes, so they buy more.
Both effects push in the same direction: a lower price leads to a higher quantity demanded, which is why the curve slopes down.
Examples in context
Example 1. A bubble tea price cut. When a bubble tea chain in Singapore runs a one-dollar-off promotion, the lower price raises the quantity demanded: students can afford more cups and the drink becomes cheaper relative to coffee. The queues that form on promotion days are the law of demand in action.
Example 2. MRT fares and ridership. If train fares were lowered, the quantity of trips demanded would rise, as travel becomes cheaper relative to taxis and buses and commuters' money goes further. This is why fare changes are studied carefully before they are made.
Try this
Cue. Define demand. The quantity of a good or service that consumers are willing and able to buy at each price over a given period of time.
Cue. State the law of demand. Other things being equal, as price rises the quantity demanded falls, and as price falls the quantity demanded rises; price and quantity demanded move in opposite directions.
Cue. Explain the substitution effect of a fall in the price of tea. When tea becomes cheaper, it is now cheaper relative to coffee and other drinks, so consumers switch toward tea, raising the quantity of tea demanded.
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 demand and explain what is meant by the law of demand.Show worked answer →
A 4 mark question rewards a precise definition of demand and a correct statement of the law.
Demand. Demand is the quantity of a good or service that consumers are willing and able to buy at each price over a period of time. It is effective demand, meaning the consumer must both want the good and have the money to pay for it.
Law of demand. The law of demand states that, other things being equal, as the price of a good falls, the quantity demanded rises, and as the price rises, the quantity demanded falls. There is an inverse relationship between price and quantity demanded.
Markers reward the willing-and-able wording for demand and the inverse price-quantity relationship for the law, with the other-things-equal condition stated.
Original5 marksExplain why the demand curve for a normal good slopes downward from left to right.Show worked answer →
A 5 mark question rewards the two standard reasons, the income effect and the substitution effect, explained simply.
The demand curve slopes downward because a fall in price raises the quantity demanded for two reasons.
Income effect. When the price of a good falls, a consumer's money can now buy more. Their real income, or purchasing power, has risen, so they can afford to buy more of the good.
Substitution effect. When the price of a good falls, it becomes cheaper relative to other goods that could be bought instead. Consumers switch toward the now-cheaper good and away from substitutes, so they buy more of it.
Together these mean a lower price leads to a higher quantity demanded, giving a downward-sloping curve.
Markers reward both effects named and explained, and the conclusion that they produce the inverse relationship and so the downward slope.
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