Law Demand and Law Supply

Demand and the Law of Demand

  • Context: Part 1 of Chapter 3 slides introduces the law of demand, the law of supply, and the important distinction between changes in quantity demanded vs changes in the entire demand relationship. It also sets up the idea of market equilibrium (equilibrium price and equilibrium quantity) and previews how equilibrium responds to changes in demand or supply.
  • Key theme: The material is taught with the ceteris paribus assumption (all other variables that affect demand or supply are held constant).
  • Core questions to answer:
    • How much of something are consumers willing and able to purchase? (Willingness vs income/ability to purchase)
    • What is price? The amount paid for the actual item.
    • What is quantity demanded? The amount buyers want to purchase at a particular price.
    • How do demand and supply respond to price changes, holding other factors constant?
  • Distinctions introduced early:
    • Change in quantity demanded vs change in demand (the entire demand relationship).
    • Change in quantity supplied vs change in supply (the entire supply relationship).
    • These distinctions will be clarified with the concept of movements along curves vs shifts of curves.
  • Definitions and key ideas (Demand side):
    • Demand: How much of something buyers are willing and able to purchase at various prices, all else equal.
    • Quantity demanded: The amount buyers want to purchase at a given price.
    • Law of Demand: When other factors are held constant, price and quantity demanded move in opposite directions. If price rises, quantity demanded falls; if price falls, quantity demanded rises.
    • Formal intuition: There is a negative relationship between price and quantity demanded, holding all else constant.
  • Demand schedule vs demand curve:
    • Demand schedule: A table showing price and the corresponding quantity demanded.
    • Demand curve: A graphical representation of the demand schedule; it shows the relationship between price (vertical axis) and quantity demanded (horizontal axis).
    • Axes convention: Always place quantity on the horizontal axis and price on the vertical axis.
  • Gasoline demand example (illustrative data):
    • Prices tested: from P = 1.00 to P = 2.20, in increments of 0.20.
    • At P = 1.00, Qd = 800{,}000{,}000 gallons.
    • At P = 1.20, Qd = 700{,}000{,}000 gallons.
    • At P = 1.40, Qd falls again (exact figure not stated beyond that it decreases as price rises).
    • Observation: As price rises, quantity demanded decreases; as price falls, quantity demanded increases (inverse relationship).
    • Interpretation: The law of demand holds under the assumption that other determinants of demand remain constant.
  • Construction of the demand curve from data:
    • Plot the data points from the demand schedule and connect them to form the demand curve.
    • The resulting demand curve slopes downward, illustrating the law of demand.
  • Variations in the slope of the demand curve:
    • The steepness or flatness of the demand curve can vary:
    • A very steep demand curve (buyers are not very responsive to price changes) means a large price change results in a small change in quantity demanded.
    • A very flat demand curve (buyers are highly responsive to price changes) means a small price change leads to a large change in quantity demanded.
    • Important note: The law of demand states only the direction (downward slope) of the relationship, not the elasticity (how responsive quantity is to price).
  • Transition to supply (introduces the other side of the market):
    • Supply: At various prices, how much the producer(s) are willing to supply.
    • Quantity supplied: The amount producers are willing to sell at the listed price.
    • Law of Supply: With other factors held constant, there is a positive relationship between price and quantity supplied. As price rises, quantity supplied rises; as price falls, quantity supplied falls.
  • Supply schedule vs supply curve:
    • Supply schedule: A table showing price and the corresponding quantity supplied.
    • Supply curve: Graph of the supply schedule with price on the vertical axis and quantity supplied on the horizontal axis.
  • Gasoline supply example (illustrative data):
    • Prices tested: P = 1.00 to P = 2.20, increments of 0.20.
    • At P = 1.00, Qs = 500{,}000{,}000 gallons.
    • At P = 1.20, Qs = 550{,}000{,}000 gallons.
    • At P = 1.40, Qs = 600{,}000{,}000 gallons.
    • Observations: As price increases, quantity supplied increases; this is a positive relationship, consistent with the law of supply.
    • Note on increments: The increases in quantity supplied between price levels are not necessarily equal (e.g., increases of 50,000,000; then 50,000,000; then 40,000,000, etc.), illustrating that the slope of the supply curve can vary.
  • Variation in the slope of the supply curve:
    • A flat (elastic) supply curve indicates high sensitivity to price changes (small price change → large change in quantity supplied).
    • A steep (inelastic) supply curve indicates low sensitivity to price changes (large price change → small change in quantity supplied).
  • Graphical interpretation:
    • The busy textbook diagram simply places the table data on the graph to yield the upward-sloping supply curve.
    • The shape of the curve (steep vs flat) reflects the elasticity of supply.
  • Summary of key ideas introduced in this video (slides 4–11):
    • Definition and interpretation of demand and quantity demanded under ceteris paribus.
    • The downward-sloping demand curve and its elasticity consideration.
    • Definition and interpretation of supply and quantity supplied under ceteris paribus.
    • The upward-sloping supply curve and its elasticity considerations.
    • The concept that curves provide a graphical representation of the underlying schedules.
  • Preview of upcoming topics (as stated):
    • Market equilibrium: equilibrium price and equilibrium quantity.
    • How equilibrium responds to shifts in demand (rise/fall in demand) and shifts in supply (rise/fall in supply).
    • Distinctions between movements along curves versus shifts of curves in response to non-price determinants.
  • Note on presentation style:
    • The lecturer explicitly labels slide numbers and emphasizes that the coverage aligns with the homework and exam scope, including material from chapters 1 and 2.
    • The material presented uses concrete numerical examples to illustrate the relationships between price, quantity demanded, and quantity supplied.
  • Quick formal expressions to remember:
    • Demand and price relationship (holding other factors constant):
      rac{ ext{d}Q_d}{ ext{d}P} < 0 ext{ (ceteris paribus)}
    • Supply and price relationship (holding other factors constant):
      rac{ ext{d}Q_s}{ ext{d}P} > 0 ext{ (ceteris paribus)}
    • Demand curve: downward-sloping graph of price vs. quantity demanded.
    • Supply curve: upward-sloping graph of price vs. quantity supplied.
  • Market equilibrium (to be covered in depth in the next part):
    • Equilibrium price: P^ such that Qd(P^) = Qs(P^*)
    • Equilibrium quantity: Q^* = Qd(P^) = Qs(P^)
    • Changes in equilibrium:
    • A rise in demand (demand curve shifts right) raises both equilibrium price and quantity (ceteris paribus).
    • A fall in demand (demand curve shifts left) lowers both equilibrium price and quantity (ceteris paribus).
    • A rise in supply (supply curve shifts right) lowers equilibrium price and raises equilibrium quantity (ceteris paribus).
    • A fall in supply (supply curve shifts left) raises equilibrium price and lowers equilibrium quantity (ceteris paribus).
  • Final note:
    • This is just the first video covering slides 1–11; equilibrium analysis and the effects of shifts will be laid out in subsequent materials.

Demand and the Law of Demand (Glossary and Key Concepts)

  • Demand: the relationship between price and the quantity demanded, assuming other factors constant.
  • Quantity demanded: the specific amount buyers are willing and able to purchase at a given price.
  • Law of Demand: negative relationship between price and quantity demanded, holding all else constant; higher price → lower quantity demanded; lower price → higher quantity demanded.
  • Demand schedule: a table listing price versus quantity demanded.
  • Demand curve: a graph of the demand schedule with quantity demanded on the horizontal axis and price on the vertical axis; slope is downward.
  • Elasticity vs slope distinction:
    • Slope describes the steepness of the curve.
    • Elasticity measures responsiveness; a flat curve implies higher elasticity; a steep curve implies lower elasticity.
  • Movement along vs shift of the demand curve:
    • Movement along the demand curve corresponds to a change in quantity demanded due to a price change alone.
    • A shift of the demand curve occurs when a non-price determinant (income, prices of related goods, tastes, expectations, number of buyers) changes.

Demand Data (Gasoline Example) – Numerical Details

  • Price points tested: $$P \