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Demand

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Ghi chú bài học

What demand is

  • Demand describes willingness and ability to buy at alternative prices over a stated period. A want without resources is not effective demand. Quantity demanded refers to the amount at one price; distinguish it from the complete demand relationship.
  • Specify product, market, units and time. Demand measures intended purchases under given conditions, not necessarily actual sales if supply is constrained. Changes in several determinants can occur together, so observed sales do not identify a single cause.

Read a demand claim precisely

Read a demand claim preciselyQuestionWhat to specifyWhich product?This good, not its substituteWhich price?Own price or related-good priceWhich quantity?Units and periodWhich conditions?Other determinants held constantWhichrelationship?Quantity at one price or wholecurve

The law of demand

  • In the standard downward-sloping model, a lower own price increases quantity demanded, other demand determinants unchanged. With fictional P = 10 − 0.1Q, P6 corresponds to Q40 and P4 to Q60 items per week. The change is movement down the same curve, called extension.
  • A higher own price creates a contraction up that curve. Draw price on the vertical axis and quantity per period on the horizontal. The curve holds income, tastes, related prices and the relevant buyer group constant; it is not simply a line joining every observed market outcome.

Movement along fictional demand

Movement along fictional demandO2468101220406080100Items demanded per weekPrice per item (£)A: price £6B: price £4

Reasons

  • A lower relative price encourages substitution towards a good rather than alternatives. Separately, with money income unchanged, a price fall changes real purchasing power. For a normal good this income effect supports more consumption, reinforcing the substitution effect.
  • For an inferior good the income effect can oppose substitution; inferior status does not itself mean demand must slope upward. Diminishing marginal utility can provide intuition about willingness to pay for additional units, but is not a claim that all consumers value each unit identically.

Two channels from a lower own price

Two channels from a lower own priceChannelMeaningRelative-pricesubstitutionThis good is cheaper againstalternativesReal purchasing powerSame money income can buy moreNormal goodIncome effect supports greaterconsumptionInferior goodIncome effect can opposesubstitution

Movement along vs shift

  • This good’s own-price change, with other demand determinants unchanged, moves along the curve. A change in a determinant such as income, preferences or a related good’s price can shift it: right means more quantity at each price, left less.
  • Fictional D1: P = 10 − 0.1Q and D2: P = 12 − 0.1Q give Q40 and Q60 respectively at P6. Compare at the same price to identify the shift. Without supply information, a demand shift alone does not identify the new equilibrium price or actual sales.

A fictional rightward demand shift

A fictional rightward demand shiftO246810121420406080100120Items demanded per weekPrice per item (£)D2D1D1 at £6D2 at £6

Factors shifting demand

  • Demand shifters include income, preferences, effective advertising, prices of substitutes and complements, expectations and the number or composition of buyers. State direction and mechanism; warmer weather can change demand for a seasonal product, but season and sales correlation alone is not proof.
  • Normal-good demand rises with income and inferior-good demand falls, other determinants unchanged, over the relevant range. Inferior is not a quality label. A product’s income relationship can differ across buyers and ranges, so use evidence or explicit assumptions rather than brand stereotypes.

Income response is not a quality label

Income response is not a quality labelCaseEffect of higher incomeNormal good over thisrangeDemand rises, other factorsunchangedInferior good over thisrangeDemand falls, other factorsunchangedA brand name aloneInsufficient to identify theresponseDifferent buyers orincome rangeClassification may differ

Substitutes and complements

  • Substitutes are alternatives for a purpose: if coffee and tea are substitutes for these buyers, a higher coffee price can increase tea demand at a given tea price. This shifts tea demand rather than moving along it because tea’s own price has not changed.
  • Complements are used together in the stated setting. Dearer petrol can reduce demand for petrol cars; cheaper consoles can raise demand for compatible games. Compatibility, existing ownership and efficiency matter: more car purchases do not necessarily add to total petrol use.

Trace the related-good effect

Trace the related-good effectChanged priceDemand for this othergoodCoffee dearer; tea a substituteTea demand may risePetrol dearer; petrol carscomplementCar demand may fallConsoles cheaper; gamescompatibleGames demand may riseTea’s own price changes aloneMovement along teademand

Individual and market demand

  • Market quantity demanded adds all buyers’ quantities at the same price and period. At £4 per item, A24 plus B12 equals market demand of 36 per week. At £2, A32 plus B16 gives 48; at £6, A16 plus B8 gives 24.
  • This is horizontal summation: do not add prices or combine quantities measured at different prices. A changed number or composition of buyers may change the market curve even when each existing buyer’s preferences remain unchanged.

Fictional weekly market demand

Fictional weekly market demandPrice / individual quantitiesMarket quantity£2: A32 plus B1648 items per week£4: A24 plus B1236 items per week£6: A16 plus B824 items per week

Slide

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Câu hỏi luyện tập

Xem trước miễn phí — 8 trên 34 câu hỏi. Đăng ký để xem tất cả.

  1. 1.What is demand?

    Easy
    • Aquantities consumers are willing and able to buy at alternative prices in a stated period
    • Bthe amount produced
    • Cthe price
    • Dprofit

  2. 2.In the standard demand model used in this lesson, what is the curve’s slope?

    Easy
    • Aupward sloping
    • Bhorizontal
    • Cvertical
    • Ddownward sloping

  3. 3.Along a given downward-sloping demand curve, what happens to quantity demanded when the good’s own price falls, other determinants unchanged?

    Easy
    • Ait stays the same
    • Bit vanishes
    • Cit rises
    • Dit falls

  4. 4.What causes a movement along this product’s demand curve, other demand determinants unchanged?

    Easy
    • Aa change in advertising
    • Ba change in the price of the good
    • Ca change in income
    • Da change in tastes

  5. 5.What causes a shift of the demand curve?

    Easy
    • Aa change in buyers’ tastes for this good
    • Ba change in this good’s own price alone
    • Cno change in any determinant
    • Donly relabelling the diagram

  6. 6.What does an increase in demand look like?

    Easy
    • Aa shift to the left
    • Ba movement down
    • Ca movement up
    • Da shift of the curve to the right

  7. 7.What does a decrease in demand look like?

    Easy
    • Aa movement up
    • Ba movement down
    • Ca shift of the curve to the left
    • Da shift to the right

  8. 8.What is a substitute good?

    Easy
    • Aa necessity
    • Bone that can replace another
    • Cone used with another
    • Da luxury

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