Demand and Supply: Comprehensive Study Notes (Lecture on Equilibrium, Shifts, and Equations)

Demand and Supply: Comprehensive Study Notes

  • Market price is determined by the interaction of demand and supply; we start with the demand side (buyers) and then the supply side (sellers/firms).
  • Demand vs. Quantity Demanded:
    • Demand refers to the entire demand curve (the relationship between price and the quantity buyers are willing to purchase, holding other factors constant).
    • Quantity demanded is a single point on the demand curve at a given price.
  • Supply vs. Quantity Supplied:
    • Supply refers to the entire supply curve (the relationship between price and the quantity sellers are willing to offer for sale, holding other factors constant).
    • Quantity supplied is a single point on the supply curve at a given price.
  • The paribus (ceteris paribus) assumption: when we hold everything else constant and only change the price, we move along the curve (change in price causes a movement along the curve; change in any determinant other than price shifts the entire curve).
  • Key takeaway: the curves shift when determinants change; movement along curves occurs when price changes while holding determinants constant.

Demand: movement along the curve and shifts in the curve

  • Movement along the demand curve (price changes, all else constant):
    • If the price changes from p1 to p2 to p3, quantity demanded changes correspondingly, moving up or down the same demand curve.
  • Shifts of the demand curve (price held constant, determinants change):
    • If any determinant changes (e.g., income, prices of related goods, tastes, expectations, or number of buyers) holding price constant, the entire demand curve can shift to a new position (upward/right or downward/left).
  • Important distinctions:
    • Demand refers to the whole curve; a shift means the curve itself moves.
    • Quantity demanded at each price is a point on the curve.

Income effects on demand

  • Normal goods: when income increases, at the same price you buy more; the demand curve shifts right.
  • Inferior goods: when income increases, you buy less; the demand curve shifts left.
  • Example from lecture:
    • Normal good: higher income leads to more purchases at each price (curve shifts right).
    • Inferior good: example given was fast food; as income rises, you buy less fast food, so the demand curve for fast food shifts left.
  • Exam pitfall about normal vs inferior goods:
    • The classification (normal vs inferior) applies to how quantity demanded responds to a change in income, not to a change in price.
    • A statement like "price of good X falls; consumers purchase less of it" would be false if income is not specified; inferior/normal depends on income changes, not price changes.

Price of related goods (demand determinants)

  • Substitutes in consumption (two goods that can replace each other):
    • Example: coffee and tea. If the price of coffee rises, consumers substitute tea, increasing the demand for tea (shift right of the tea demand curve).
    • Graphically: when price of one good changes, the demand curve for the other shifts.
  • Complements in consumption (two goods commonly used together):
    • Example: pancakes and maple syrup. If the price of maple syrup increases, consumption of pancakes falls and maple syrup consumption falls as well (demand for maple syrup shifts left if price of pancakes changes; but the key is that cross-price effects move related demand curves).
  • Substitutes in production (production side):
    • The concept that some inputs can be used to produce multiple different goods (alternatives in production).
    • Example: maple sap can be used to produce maple syrup or maple sugar; these are substitutes in production.
    • When the price of one production option rises or when production becomes more profitable, producers may shift output toward that option, reducing supply of the alternative product.

Taste and preferences

  • Changes in tastes lead to shifts in the demand for affected goods:
    • If tastes shift in favor of mangoes, demand for mangoes increases (curve shifts right).
    • If tastes shift away from apples, demand for apples decreases (curve shifts left).
  • Practical advice from the lecture: always draw the graph; practice by drafting the curves while solving problems rather than memorizing rules.

Expectations about future prices (demand determinant)

  • If price is expected to rise in the future, current demand increases (people buy now to avoid higher future prices); the current demand curve shifts right.
  • If price is expected to fall, current demand decreases (people wait), and the demand curve shifts left.
  • The instructor emphasized that this is important because in finance and money/ banking, expectations drive market movements.

Population (number of buyers)

  • More people in the market increases overall demand at every price; the demand curve shifts right.
  • Conversely, fewer people would shift the demand curve left.
  • This determinant is straightforward but not typically the exam focus; it illustrates how demographic changes affect demand.

The supply side: movement along the curve and shifts in the curve

  • Supply: the producers' side; how much they are willing to offer for sale at each price.
  • Movement along the supply curve (price changes, all else constant):
    • With a higher price, quantity supplied increases; the supply curve slopes upward (positive relationship between price and quantity supplied).
  • Shifts of the supply curve (price held constant, determinants change):
    • If any determinant changes (e.g., input prices, technology, number of firms, weather, expected price, prices of alternatives in production), the entire supply curve can shift.
  • Distinguishing quantity supplied vs supply curve:
    • Quantity supplied is a single point on the supply curve at a given price.
    • Supply refers to the whole curve; a shift means the seller behavior at every price changes.

Determinants of supply

  • Input prices: higher input costs reduce supply (supply curve shifts left); lower input costs increase supply (shift right).
  • Technology: better technology reduces production costs, increasing supply (shift right); worse or outdated technology reduces supply (shift left).
  • Number of firms in the market: more firms increase supply (shift right); fewer firms decrease supply (shift left).
  • Weather and other natural conditions (especially for agricultural products): favorable weather increases supply (shift right); adverse weather decreases supply (shift left).
  • Expected price (supply expectations): if producers expect higher prices in the future, they may hold back supply today (supply shifts left); if they expect lower prices, they may rush to sell now (supply shifts right).
  • Prices of alternatives in production (opportunity costs): if an input could be used to produce other products with higher returns, this affects the incentive to supply the current good.
  • The lecture emphasized the concept of substitutes in production with maple syrup and maple sugar to illustrate how one production path can reduce the supply of the other.

Substitutes in production (examples and intuition)

  • If input can be used to produce multiple products, those products are substitutes in production.
  • Maple sap can be used to produce maple syrup or maple sugar; increasing output of maple syrup can reduce the supply (and price) of maple sugar, and vice versa.
  • The effect on prices is determined by how the shift in one production path affects the other path’s supply and price at those outputs.

Reading a supply and demand graph: intuition

  • A rise in price along a fixed demand curve means movement along the demand curve; the quantity demanded changes, but the curve itself stays put.
  • A shift of the demand curve occurs when a determinant changes (income, tastes, etc.) while price is held constant.
  • A shift of the supply curve occurs when a determinant affecting producers changes (input prices, technology, etc.).
  • The intersection of the demand and supply curves determines the market-clearing (equilibrium) price and quantity.

Equilibrium, excess demand, and excess supply

  • Equilibrium (stable) price and quantity occur where the demand and supply curves intersect: Qd = Qs at price p^* and quantity q^*.
  • Excess demand (shortage) occurs when at a given price, quantity demanded exceeds quantity supplied (Qd > Qs). In such a case, buyers bid up the price, pushing toward a higher price.
  • Excess supply (surplus) occurs when at a given price, quantity supplied exceeds quantity demanded (Qs > Qd). In such a case, sellers lower prices to clear the market.
  • In the graphical explanation, you first pick a price, read off Qd and Qs at that price, and determine whether there is excess demand or excess supply. Then price moves toward the equilibrium.
  • At equilibrium, there is no excess demand or excess supply; price tends to stay constant unless an external factor shifts the curves.

Stability and movement toward equilibrium

  • Stable (equilibrium) price is the one around which prices tend to hover when nothing else changes.
  • If a single determinant shifts the curves, a new equilibrium is established at a new intersection point.

Example: a market for maple syrup (detailed walk-through)

  • Demand side: effects of income, tastes, price of related goods, etc.; Supply side: effects of input prices, technology, weather, etc.
  • Hypothetical demand and supply data presented in the lecture:
    • Demand schedule (example):
    • Price $1 → Q_d = 75{,}000
    • Price $3 → Q_d = 50{,}000
    • Price $5 → Q_d = 35{,}000
    • Supply schedule (example):
    • Price $1 → Q_s = 25{,}000
    • Price $3 → Q_s = 50{,}000
    • Price $5 → Q_s = 65{,}000
  • Equilibrium (from the equation-based example):
    • Demand equation: Q_d = 10 - rac{p}{2}
    • Supply equation: Qs=p−2Q_s = p - 2
    • Set them equal to find equilibrium: 10 - rac{p}{2} = p - 2
    • Solve: multiply by 2 → 20−p=2p−4⇒3p=24⇒p∗=820 - p = 2p - 4 \Rightarrow 3p = 24 \Rightarrow p^* = 8
    • Then equilibrium quantity: Q^* = Q_d(p^*) = 10 - rac{8}{2} = 10 - 4 = 6
  • Graphical orientation: price on the vertical axis, quantity on the horizontal axis.
  • At price 3, the intersection yields Qd = Qs = 6; this is the crossing point where the curves meet (the equilibrium under the equation example).
  • Interpretation of other prices:
    • Price $1: Qd = 75{,}000$, Qs = 25{,}000 (excess demand).
    • Price $5: Qd = 35{,}000$, Qs = 65{,}000 (excess supply).
  • The mechanism of price adjustment:
    • Excess demand → price tends to rise.
    • Excess supply → price tends to fall.
  • Equilibrium is the price where the two curves cross; at that point Qd = Qs, and there is no pressure for the price to change, given the determinants.

Equilibrium changes when determinants shift

  • When one curve shifts, a new equilibrium price and quantity emerge at the intersection with the other curve.
  • Example: If income increases, demand shifts to the right (new equilibrium price higher, new equilibrium quantity higher).
    • Illustration: old equilibrium at p = 3, q = 50,000; new demand curve leads to higher price and higher quantity.
  • If bad weather hits (shifts supply left) while demand remains unchanged, the equilibrium price rises and quantity may move in the direction determined by the new intersection.
  • When both curves shift simultaneously, the outcome depends on the relative magnitudes of the shifts:
    • Demand increase + supply decrease → price rises; quantity change is ambiguous (could rise, fall, or be unchanged depending on magnitudes).
    • Demand increase + supply decrease can result in an upward pressure on price with an indeterminate quantity.
    • In some cases, the quantity could rise, fall, or stay the same; there is no guaranteed direction for quantity unless the magnitudes are specified.
  • The instructor suggested working through three subcases for each scenario:
    • Increase in demand vs. increase in supply (and the relative magnitudes:
    • Demand increase stronger than supply increase
    • Demand increase weaker than supply increase
    • Demand increase equal to supply increase
    • Apply the same logic to other combinations (e.g., demand increase vs supply decrease).
  • The takeaway: when both curves shift, you can say with certainty that price moves in one direction, but the direction of the other variable (quantity) can be ambiguous unless the relative magnitudes are specified.

The algebra of demand and supply (note about the instructor’s plan)

  • There is a second way to analyze demand and supply using equations (demand and supply equations) in addition to schedules and graphs.
  • A demand equation expresses Qd as a function of price and other determinants; a supply equation expresses Qs as a function of price and other determinants.
  • Example given in the lecture (to be posted later):
    • Demand equation example: Q_d = 10 - rac{p}{2} (as previously used in the graph example).
    • Supply equation example: Qs=p−2Q_s = p - 2.
  • Using the equations, equilibrium is found by solving Q<em>d=Q</em>sQ<em>d = Q</em>s:
    • 10 - rac{p}{2} = p - 2.
    • The solution yields the equilibrium price and quantity: p∗=8, Q∗=6.p^* = 8, \, Q^* = 6.
  • Plotting considerations: when you plot the demand curve, you plot price on the y-axis and quantity on the x-axis; ensure to reflect the axes correctly when transferring between tables/equations and graphs.
  • The instructor promised to share the equations part later tonight or tomorrow morning.

Practical study advice and exam strategy

  • Always draw a graph when analyzing a problem: sketch the demand and supply curves, indicate the current price, and read off the corresponding quantities.
  • For multiple-choice questions or practice problems, draft the graph first and then answer based on the visual reading rather than memorizing rules.
  • Build your own study groups (three to four students) to discuss and fill gaps in notes; explaining concepts to others reinforces understanding.
  • Understand the language precisely:
    • Demand vs. quantity demanded
    • Supply vs. quantity supplied
    • Curve shift vs. movement along the curve
    • Equilibrium vs. stable price
  • True/False question strategy (conceptual): inferior vs normal goods depends on income changes, not price changes; you cannot determine whether a good is inferior or normal from a price change alone.
  • Expectation and its role in the market: price expectations can influence current demand and current supply, and thus future equilibria. This concept is foundational for more advanced topics (e.g., money, banking, and financial markets).

Key formulas and numerical examples to memorize

  • Demand function (example):
    • Q_d = 10 - rac{p}{2}
  • Supply function (example):
    • Qs=p−2Q_s = p - 2
  • Equilibrium condition: Q<em>d=Q</em>sQ<em>d = Q</em>s
  • Solve for equilibrium: from 10 - rac{p}{2} = p - 2
    • Multiply by 2: 20−p=2p−420 - p = 2p - 4
    • Solve: 3p=24⇒p∗=83p = 24 \Rightarrow p^* = 8
    • Then Q^* = Q_d(p^*) = 10 - rac{8}{2} = 6

Final reminders from the lecture

  • When you shift only one determinant (holding price constant), you shift the corresponding curve.
  • When you shift both curves, the direction of price change can often be determined, but the direction of quantity change can be ambiguous without knowing the relative magnitudes of the shifts.
  • The concept of equilibrium is central: it is the point where supply equals demand, with no inherent pressure for price to move unless a determinant changes.
  • Always practice graph-drawing as a preparation method for exams; it’s repeatedly emphasized as the key to understanding and succeeding.
  • The next course (money and banking) will rely heavily on the notion that expectations drive market behavior; this lecture’s coverage of expectations lays the groundwork for that topic.