Demand and Supply - Comprehensive Study Notes

Demand

  • Markets and the demand framework
    • Markets are where buyers and sellers interact for goods and services and set the prices we pay or receive.
    • Demand is the relationship between price and quantity demanded, holding other factors constant.
  • The Law of Demand
    • Quantity demanded is inversely related to price, holding other factors constant.
    • If price rises, quantity demanded falls; if price falls, quantity demanded rises.
    • Symbolically: rac{ ext{d}Q_d}{ ext{d}P} < 0\text{ (ceteris paribus)}
    • Alternative representation: P↑⇒Q<em>d↓ ;  P↓⇒Q</em>d↑P \uparrow \Rightarrow Q<em>d \downarrow\,;\; P \downarrow \Rightarrow Q</em>d \uparrow
  • The Demand Schedule and the Demand Curve
    • Demand schedule: A table relating prices to quantity demanded over a specified time period, with time dimension and constant-quality units.
    • Demand curve: A graphical representation of the demand schedule; a downward-sloping line showing the inverse relationship between price and quantity demanded, all else equal.
    • Time dimension and constant-quality units are essential to interpret the schedule/curve correctly.
  • Individual vs Market Demand
    • Individual demand: Demand of a single consumer for a good. Shown in figures like Panel (a) of Figure 3-1.
    • Market demand: The total demand of all consumers in the market for a good. It is the horizontal summation of all individual demand curves at each price.
    • Market demand at a given price: Q<em>dmarket(P)=∑</em>iQd,i(P)Q<em>d^{market}(P) = \sum</em>i Q_{d,i}(P) where the sum is over all buyers in the market.
  • Horizontal summation of demand curves (illustrative)
    • When combining two buyers, the market demand at each price is the sum of each buyer’s quantity demanded at that price.
    • Example conceptually: Buyer 1 and Buyer 2 together yield a larger total quantity demanded than either alone.
  • The Demand Schedule/Curve for a real-world good (example)
    • Market demand schedules/curves can be shown for specific goods (e.g., Flash Memory Pen Drives) to illustrate how price changes affect total demand.
  • What happens if non-price determinants change (shifts in the demand curve)
    • If a factor other than price changes, the entire demand curve shifts left or right.
    • Example: Government policy giving notebook computers to every student shifts demand for notebooks.
    • If such a factor increases demand, the demand curve shifts right; if it decreases demand, the curve shifts left.
  • Determinants of Demand (Ceteris Paribus Conditions)
    • Ceteris-Paribus: Determinants that are held constant along a demand curve; changes in these factors shift the curve.
    • Major determinants include:
    • Income: Normal goods/services vs Inferior goods/services
    • Tastes and preferences
    • Prices of related goods/services: Substitutes and Complements
    • Population
    • Expected future prices
  • Change in Demand vs Change in Quantity Demanded
    • Change in Demand: A shift of the entire demand curve caused by a non-price determinant changing.
    • Change in Quantity Demanded: Movement along the demand curve caused by a change in price.
  • Important examples to illustrate shifts (3-13, 3-14)
    • If the government gives every student a notebook computer, the demand for notebooks increases (demand curve shifts right).
    • If universities prohibit notebook computers, demand decreases (demand curve shifts left).

Supply

  • What is supply?
    • Supply is the schedule showing the relationship between price and quantity supplied for a specified time period, holding other factors constant.
    • It reflects the amount firms are willing to sell at alternative prices.
  • The Law of Supply
    • Quantity supplied is directly related to price, holding other factors constant.
    • If price rises, quantity supplied rises; if price falls, quantity supplied falls.
    • Symbolically: \frac{\text{d}Q_s}{\text{d}P} > 0\text{ (ceteris paribus)}
  • The Supply Schedule and the Supply Curve
    • Supply schedule: A table relating prices to quantity supplied at each price.
    • Supply curve: A graphical representation of the supply schedule; a positively sloped line showing a direct relationship between price and quantity supplied, all else equal.
    • Time dimension and constant-quality units are essential to interpret correctly.
  • The Individual Producer's Supply Schedule/Curve (illustrative)
    • Figures show how a single producer’s willingness to supply changes with price (Panel (a) and (b) in Figure 3-6).
  • Horizontal summation of supply curves (two suppliers)
    • Market supply is the horizontal sum of individual suppliers’ quantities at each price.
  • The Market Supply Schedule and Curve (example)
    • Market supply combines multiple firms; Panel (a) shows the market supply schedule and Panel (b) shows the market supply curve for Flash Memory Pen Drives.
  • What happens if costs change (shifts in the supply curve) (3-27, 3-28)
    • A decrease in production costs shifts the supply curve to the right (more supply at each price).
    • An increase in production costs shifts the supply curve to the left (less supply at each price).
  • Determinants of Supply (Ceteris-Paribus Conditions)
    • Determinants include:
    • Price of input (costs)
    • Technological change
    • Taxes or subsidies
    • Number of firms in the market
    • Expected future prices
  • Change in Supply vs Change in Quantity Supplied
    • Change in Supply: A shift of the entire supply curve due to a determinant other than price.
    • Change in Quantity Supplied: Movement along the supply curve due to a price change.

Putting Demand and Supply Together

  • Equilibrium (Market-Clearing) concept
    • Equilibrium occurs where quantity supplied equals quantity demanded at a given price.
    • Equilibrium price (market-clearing price) is the price at which Qd = Qs.
    • Graphically, it is where the demand curve and the supply curve intersect.
    • Mathematical representation: at equilibrium price $P^$, Q<em>d(P</em>)=Q</em>s(P∗)Q<em>d(P^</em>) = Q</em>s(P^*)
  • Shortages and Surpluses
    • Shortage: Occurs when quantity demanded exceeds quantity supplied at a given price; Q<em>d>Q</em>s.Q<em>d > Q</em>s. Exists at any price below the market-clearing price.
    • Surplus: Occurs when quantity supplied exceeds quantity demanded at a given price; Q<em>d<Q</em>s.Q<em>d < Q</em>s. Exists at any price above the market-clearing price.
  • Practical notes for analysis
    • Draw S and D curves, identify equilibrium, and observe how price movements eliminate shortages or surpluses.
    • High market price tends to move toward eliminating a surplus; low market price tends to eliminate a shortage.
  • Tips for practice (3-33)
    • Use S/D curves to illustrate equilibrium and how it responds to shocks.
    • Create examples of high prices (market price moves down) and low prices (market price moves up) to illustrate shortages and surpluses.
  • Additional notes
    • The notes repeatedly emphasize the distinction between shifts (due to determinants) and movements along curves (due to price changes).
    • All numerical references, formulas, and diagrams in the source use constant-quality units and time dimensions to maintain consistency across schedules and curves.

Key Formulas and Concepts (summary)

  • Demand elasticity intuition (notationally):
    • \frac{\partial Q_d}{\partial P} < 0
  • Supply intuition:
    • \frac{\partial Q_s}{\partial P} > 0
  • Market demand: Q<em>dmarket(P)=∑</em>iQd,i(P)Q<em>d^{market}(P) = \sum</em>i Q_{d,i}(P)
  • Equilibrium condition: Q<em>d(P<em>)=Q</em>s(P</em>)Q<em>d(P^<em>) = Q</em>s(P^</em>)
  • Shortage condition: Q<em>d>Q</em>sQ<em>d > Q</em>s
  • Surplus condition: Q<em>d<Q</em>sQ<em>d < Q</em>s
  • Determinants of Demand (ceteris paribus): income (normal/inferior), tastes, substitutes, complements, population, expected future prices, etc.
  • Determinants of Supply (ceteris paribus): input prices, technology, taxes/subsidies, number of firms, expected future prices, etc.