Demand, Equilibrium, and Determinants of Demand (Lecture Notes)

Demand and Equilibrium Basics

• Demand curve characteristics can vary across products: some demand curves are flat (elastic), some are steep (inelastic), and some can be straight. Elasticity of demand affects how quantity demanded responds to price changes.

• Law of demand: as price falls, quantity demanded tends to rise; as price rises, quantity demanded tends to fall. This implies a downward-sloping demand curve: the slope is negative.

• Equilibrium concept: the market-clearing price and quantity where quantity supplied equals quantity demanded.

  • Equilibrium price and quantity:

  • At equilibrium, there is no inherent pressure for price to change because the forces of supply and demand are balanced.

• Surplus and shortage (conceptual implications of prices away from equilibrium):

  • If the price is above the equilibrium price, a surplus occurs (excess supply). This is usually a bad situation for producers because unsold goods may force price reductions over time.

  • If the price is below the equilibrium price, a shortage occurs (excess demand).

• Notation for equilibrium: the equilibrium price is commonly denoted as $P^<em>$ and the equilibrium quantity as $Q^</em>$, with the condition that at equilibrium $Q<em>d = Q</em>s$ where $Q<em>d = Q</em>d(P)$ and $Q<em>s = Q</em>s(P)$.

• The concept of ceteris paribus: when discussing demand, we often say that all other factors are held constant (holding many other factors constant) to isolate the effect of a single determinant.

Demand Curve: Movements vs Shifts

• Movement along the demand curve (price-only change):

  • When the price changes and all other determinants remain constant, quantity demanded changes along the same demand curve.

  • Example interpretation: a decrease in price leads to an increase in quantity demanded along the current demand curve.

  • Mathematical intuition: if $P$ changes, then rac{dQ_d}{dP} < 0.

• Shift of the demand curve (non-price determinants):

  • Changes in determinants cause the entire demand curve to shift to the right (increase in demand) or to the left (decrease in demand).

  • When the discussion says

  • Example noting population of hats: if hats become more desirable due to fashion, demand for hats shifts to the right.

  • Important distinction: shifting the curve is not the same as moving along it.

Factors That Shift the Demand Curve (Determinants of Demand)

• Income (normal goods):

  • If income increases, quantity demanded at each price tends to increase for normal goods, shifting the demand curve to the right.

  • If income decreases, demand shifts to the left.

  • Example: Organic apples may see higher demand at a given price when income rises.

• Taste and preferences:

  • An increase in taste or preference for a good shifts the demand curve to the right.

  • A decrease in taste or preference shifts it to the left.

• Prices of related goods:

  • Substitutes: an increase in the price of a substitute good tends to increase demand for the good in question, shifting the curve to the right. Example: butter and margarine; if the price of butter rises, demand for margarine may rise.

  • Complements: an increase in the price of a complementary good tends to decrease demand for the good in question, shifting the curve to the left.

• Expectations about future income or prices:

  • If people expect their income to rise in the future, they may buy more now, increasing current demand.

  • The expectation of higher future prices can also affect current demand depending on the context.

• Other practical considerations:

  • When the narrator mentions keeping other factors constant and discussions about access or other practical concerns, these are examples of real-world factors that can influence demand besides price.

Examples and Intuition

• Income effect example: Restaurant dining

  • If income rises, people are more likely to dine out at nicer restaurants, increasing demand for restaurant meals at each price and shifting the demand curve to the right.

• Substitutes vs complements (quick recap):

  • Substitutes: goods that can replace each other; price rise in one increases demand for the other (shift right).

  • Complements: goods that are often consumed together; price rise in one decreases demand for the other (shift left).

• Expectations example: If you expect higher income next month, you might buy more today, increasing current demand.

Clarifications on the Demand Curve Shifts vs Movements (User-facing recap)

• When demand determinants change (income, tastes, prices of related goods, expectations, etc.), the entire demand curve shifts to the left or right.

• When only the price of the good changes, quantity demanded moves along the same demand curve (no shift in the curve).

• A common illustrative phrase: “holding many other factors constant” (ceteris paribus) to emphasize that the observed change is due to a single determinant at a time.

Quick References (Formulas and Key Points)

• Equilibrium condition: $Q<em>d = Q</em>s$ at price $P = P^<em>$ and quantity $Q = Q^</em>$.

• Market pressure near equilibrium: if price deviates, market tends to move back toward equilibrium through the surplus/shortage mechanism.

• Demand slope intuition: the demand curve is downward-sloping; the negative slope indicates that quantity demanded increases as price falls. Mathematically: rac{dQ_d}{dP} < 0.