Supply and Demand: Foundations of Market Equilibrium

Supply and Demand

Introduction to Markets

Markets are institutions that bring buyers and sellers together to facilitate the exchange of goods and services. Examples include eBay auctions, stock exchanges, online flight booking platforms, and farmers markets. Despite their differences, all markets share the common function of determining prices and quantities traded.

• Market Participants: Buyers (demand side) and sellers (supply side) interact to define prices.

• Role of Prices: Prices convey information about scarcity and preferences, guiding decisions for both buyers and sellers.

• Money Price: The amount of money required to purchase a good.

• Relative Price: The ratio of the money price of one good to another, representing opportunity cost.

Competitive Markets

Competitive markets are characterized by many buyers and sellers, all trading identical products. In such markets, a single equilibrium price emerges, ensuring efficient allocation of resources.

• Market Equilibrium Price: The unique price at which quantity supplied equals quantity demanded.

The Determination of Market Prices

Historically, economists debated whether supply or demand determined prices. Alfred Marshall resolved this by likening the process to a pair of scissors: both blades (supply and demand) are necessary. Thus, market prices are determined by the interaction of supply and demand.

• Buyers' Willingness to Pay: Reflects demand.

• Sellers' Cost of Production: Reflects supply.

Demand

Law of Demand

The law of demand states that, ceteris paribus, as the price of a good increases, the quantity demanded decreases; as the price decreases, the quantity demanded increases.

• Substitution Effect: When the price of a good rises, consumers substitute it with cheaper alternatives, reducing quantity demanded.

• Income Effect: A price increase reduces consumers' purchasing power, decreasing quantity demanded.

Demand Curve and Schedule

The demand curve illustrates the relationship between the price of a good and the quantity demanded. It is derived from individual willingness-to-pay curves and represents marginal benefit.

• Individual Demand: The quantity a single buyer is willing to purchase at various prices.

• Market Demand: The sum of all individual demands in the market.

Example: Market Demand for Apples

Suppose Anna and Elsa have the following demand schedules for apples:

Price ($)	Anna	Elsa	Market
120	0	0	0
100	0	10	10
80	5	15	20
60	10	20	30
40	15	25	40
20	20	30	50

Change in Demand vs. Change in Quantity Demanded

A change in quantity demanded is caused by a change in the price of the good, moving along the demand curve. A change in demand is caused by non-price factors, shifting the entire demand curve.

• Change in Quantity Demanded: Movement along the curve due to price change.

• Change in Demand: Shift of the curve due to factors such as tastes, income, number of buyers, prices of related goods, and expectations.

Determinants of Demand

• Tastes and Preferences: Trends and health information can increase or decrease demand.

• Number of Buyers: More buyers increase market demand.

• Income: Demand for normal goods rises with income; demand for inferior goods falls.

• Prices of Related Goods: Substitutes and complements affect demand.

• Expectations: Anticipation of future prices or income changes can shift demand.

Supply

Law of Supply

The law of supply states that, ceteris paribus, as the price of a good increases, the quantity supplied increases; as the price decreases, the quantity supplied decreases.

• Individual Supply: The quantity a single seller is willing to offer at various prices.

• Market Supply: The sum of all individual supplies in the market.

Supply Curve and Schedule

The supply curve shows the relationship between the price of a good and the quantity supplied. It is also a minimum-supply-price curve, representing marginal cost.

• Horizontal Summation: Market supply is obtained by summing individual supply curves horizontally.

Change in Supply vs. Change in Quantity Supplied

A change in quantity supplied is caused by a change in the price of the good, moving along the supply curve. A change in supply is caused by non-price factors, shifting the entire supply curve.

• Change in Quantity Supplied: Movement along the curve due to price change.

• Change in Supply: Shift of the curve due to factors such as input prices, technology, number of sellers, state of nature, prices of related goods, and expectations.

Determinants of Supply

• Factor Prices: Lower input costs increase supply.

• Technology: Improved technology increases supply.

• Number of Sellers: More sellers increase market supply.

• State of Nature: Natural disasters decrease supply.

• Prices of Related Goods: Substitutes and complements in production affect supply.

• Expectations: Anticipation of future price changes can shift supply.

Market Equilibrium

Definition and Characteristics

Market equilibrium occurs when the quantity supplied equals the quantity demanded ($Q_s = Q_d$). The equilibrium price is the price at which this equality holds, and the equilibrium quantity is the corresponding output.

• Surplus: Occurs when quantity supplied exceeds quantity demanded; price tends to fall.

• Shortage: Occurs when quantity demanded exceeds quantity supplied; price tends to rise.

Example: Surplus and Shortage

Price ($)	Quantity Demanded	Quantity Supplied	Result
80	20	40	Surplus of 20 units
50	35	25	Shortage of 10 units

Adjustments to Equilibrium

Any price above or below equilibrium creates incentives for buyers and sellers to adjust their behavior, moving the market toward equilibrium.

• Price Below Equilibrium: Shortage; price rises.

• Price Above Equilibrium: Surplus; price falls.

Shifts in Demand and Supply

Changes in demand or supply shift the equilibrium price and quantity.

• Increase in Demand: Higher equilibrium price and quantity.

• Decrease in Demand: Lower equilibrium price and quantity.

• Increase in Supply: Lower equilibrium price, higher equilibrium quantity.

• Decrease in Supply: Higher equilibrium price, lower equilibrium quantity.

• Both Curves Shift: The effect on equilibrium price depends on the relative magnitude of the shifts; equilibrium quantity always rises if both increase.

Key Concepts and Definitions

• Markets: Institutions for exchange.

• Price System: Mechanism for allocating resources.

• Willingness-to-Pay: Maximum price a buyer will pay.

• Demand: Relationship between price and quantity demanded.

• Law of Demand: Inverse relationship between price and quantity demanded.

• Demand Curve: Graphical representation of demand.

• Determinants of Demand: Factors affecting demand.

• Normal Good: Demand increases with income.

• Inferior Good: Demand decreases with income.

• Substitute Goods: Goods that can replace each other.

• Complementary Goods: Goods consumed together.

• Supply: Relationship between price and quantity supplied.

• Law of Supply: Direct relationship between price and quantity supplied.

• Supply Curve: Graphical representation of supply.

• Determinants of Supply: Factors affecting supply.

• Equilibrium: Point where supply equals demand.

• Surplus: Excess supply.

• Shortage: Excess demand.

Practice Questions

• What happens to the equilibrium price and quantity of inferior goods when incomes fall?

• How does a change in the price of a substitute or complement affect demand?

• What is the effect of an increase in the number of sellers on market supply?

Formulas and Equations

• Market Equilibrium Condition: $Q_s = Q_d$

• Relative Price: $\text{Relative Price} = \frac{\text{Money Price of Good A}}{\text{Money Price of Good B}}$