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NotesEconomics HLTopic 2.1The law of demand
Back to Economics HL Topics
2.1.16 min read

The law of demand (Economics HL)

IB Economics · Unit 2

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Contents

  1. 1What demand means
  2. 2The law of demand and the demand curve
  3. 3Why it slopes down: income and substitution effects
  4. 4Diminishing marginal utility
  5. 5Exam-style question

Demand in economics means wanting a good and being able to pay for it, at a given price, over a set time. Wanting on its own is not enough.

The points to remember

  • Willing: the buyer wants it.
  • Able: the buyer can pay for it.
  • At each price: demand is linked to a price.
  • Over a period: per week, month or year.

Example: a €40 concert ticket

1

Aiko

Loves the band and has €40. Willing and able: she is part of demand.

2

Ben

Loves the band but has €15. Willing, not able: not part of demand.

3

Carla

Has €200 but no interest in the band. Able, not willing: not part of demand.

Individual demand is one buyer's demand. Market demand adds every buyer's quantity at each price: if 2 000 fans each want one ticket at €40, market demand at €40 is 2 000 tickets.

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The law of demand: as the price of a good falls, the quantity demanded rises, and as the price rises, it falls, ceteris paribus.

The canteen's price list

Price of a sliceSlices sold per day
€630
€542
€460
€390
€2150

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The points to remember

  • Price and quantity demanded move in opposite directions.
  • Price on the vertical axis, quantity on the horizontal.
  • A price change moves buyers along the curve; it changes quantity demanded.

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A price cut makes buyers take more for two separate reasons. Each one has a name.

A slice of pizza falls from €4 to €3

1

Income effect

Your real income rises: €12 now buys 4 slices instead of 3, so you can afford more.

2

Substitution effect

Pizza is now cheaper compared with a sandwich, so you swap some sandwiches for pizza.

3

Together

Both push the same way: quantity demanded rises. A price rise works in reverse.

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The points to remember

  • Income effect: a lower price raises real income, so buyers can afford more.
  • Substitution effect: the good is now relatively cheaper than its substitutes, so buyers switch to it.
  • Both effects raise quantity demanded when the price falls.

Utility is the satisfaction a good gives you. Marginal utility is the extra satisfaction from one more unit.

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The points to remember

  • Each extra unit gives less extra satisfaction than the one before.
  • So a buyer will pay less for each extra unit.
  • So a lower price is needed to sell more: demand slopes down.
Remember it as: Hungry for the first slice, full by the fifth.

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Paper 2 tests this micro with define the term demand, for 2 marks. The income and substitution effects and diminishing marginal utility have not been set on their own yet: they are the reasons a Paper 1 essay gives for the downward slope.

IB-style questionDefine[2 marks]

Demand for electric bikes in the city has doubled in five years.

Define the term demand.

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Drawing a demand curve in the exam

  • Label the axes P and Q, or Price and Quantity.
  • Label the curve D.
  • A title is not needed.
  • In a 4-mark diagram question, a wrong label caps the answer at 3 marks.
IB-style questionExplain[10 marks]

Explain why the demand curve for a good slopes downward.

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the law of demand. [2 marks]

Related Economics HL Topics

Continue learning with these related topics from the same unit:

2.1.2Determinants of demand2.1.3Substitutes and complements2.1.4Movements vs shifts of demand2.10.1Asymmetric information and market failure
View all Economics HL topics
Improve your exam techniqueCommand terms, paper structure, and mark-scheme tips for Economics HL
Previous1.2.6Economics now: real behaviour and the circular economyNextDeterminants of demand2.1.2

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