Supply and Demand: Principles and Applications in Competitive Markets

Supply and Demand

Introduction

Supply and demand are foundational concepts in microeconomics, describing how prices and quantities of goods and services are determined in competitive markets. This study guide summarizes the key principles, definitions, and applications relevant to college-level microeconomics.

Competitive Markets

What is a Market?

A market is any arrangement that allows buyers and sellers to exchange goods, services, or resources. Markets can be physical (e.g., bazaars, trading floors) or virtual (e.g., online platforms like Amazon).

• Competitive Market: A market is competitive if all participants are too small to influence the aggregate outcome. No single buyer or seller can affect the market price.

• Price Mechanism: In competitive markets, prices are determined by the interaction of supply and demand, not by individual negotiation.

• Free Market vs. Competitive Market: Not all free markets are competitive; competition requires many small participants.

Example: The stock market and online retail platforms are examples of competitive markets.

Fixed-Price Mechanism

• Sellers offer products at a fixed price per unit.

• Buyers can purchase any quantity at that price, as long as supplies last.

• Unit prices are often expressed in money (e.g., dollars).

• Relative Price: The ratio of the prices of two goods, representing the opportunity cost for buyers.

Example: If apples cost $2 and bananas cost $1, the relative price of apples to bananas is 2:1.

Demand

Definition and Determinants

The quantity demanded of a good or service is the amount consumers are willing and able to buy at a given price, within a specified time frame.

• Depends on both willingness and ability to pay.

• Distinction between individual and aggregate demand.

• Demand Curve: Graphical representation of the relationship between price and quantity demanded.

• Demand Schedule: Table showing quantities demanded at different prices.

Law of Demand

• Other things being equal (ceteris paribus), as the price of a good increases, the quantity demanded decreases; as the price decreases, quantity demanded increases.

• Income Effect: Higher prices reduce purchasing power.

• Substitution Effect: Higher prices make alternatives more attractive.

Example: If the price of chocolate bars rises from $1.00 to $1.50, the quantity demanded falls from 15 million to 10 million bars per week.

Shifts in Demand

Factors that shift the entire demand curve (not just movement along the curve):

• Price of Related Goods:

  • Substitutes: Increase in the price of one increases demand for the other (inverse relation).

  • Complements: Increase in the price of one decreases demand for the other (parallel relation).

• Income or Wealth:

  • Normal Goods: Higher income increases demand.

  • Inferior Goods: Higher income decreases demand.

• Expected Future Prices

• Population

• Consumer Preferences and Tastes

Change in Quantity Demanded: Movement along the demand curve due to price change. Change in Demand: Shift of the entire demand curve due to other factors.

Supply

Definition and Determinants

The quantity supplied of a good or service is the amount sellers are willing and able to produce and sell at a given price.

• Depends on technology, resources, and willingness to sell (price vs. marginal cost).

• Supply Curve: Graphical representation of the relationship between price and quantity supplied.

• Supply Schedule: Table showing quantities supplied at different prices.

Law of Supply

• Other things being equal, as the price of a good increases, the quantity supplied increases; as the price decreases, quantity supplied decreases.

• Willingness to sell is related to marginal cost, which tends to increase with output.

Example: If the price of chocolate bars rises from $1.00 to $1.50, the quantity supplied increases from 6 million to 10 million bars per week.

Shifts in Supply

Factors that shift the entire supply curve:

• Price of Related Goods and Services:

  • Inputs: Labor, capital, components.

  • Production Substitutes: Alternative uses of factors.

• Expected Future Prices

• Number of Suppliers

• Technology

• State of Nature

Change in Quantity Supplied: Movement along the supply curve due to price change. Change in Supply: Shift of the entire supply curve due to other factors.

Competitive Equilibrium

Equilibrium and Adjustment

A market equilibrium occurs when the quantity demanded equals the quantity supplied at a particular price. This price is called the equilibrium price (P*), and the corresponding quantity is the equilibrium quantity (Q*).

• At equilibrium, there is no tendency for price or quantity to change.

• Prices adjust in response to surpluses (excess supply) or shortages (excess demand).

Surplus and Shortage

• Surplus: Quantity supplied exceeds quantity demanded at a given price; suppliers lower prices.

• Shortage: Quantity demanded exceeds quantity supplied at a given price; suppliers raise prices.

Price (dollars/bar)	Quantity Demanded (million bars/week)	Quantity Supplied (million bars/week)	Surplus/Shortage (Qs - Qd)
0.50	20	0	-20
1.00	15	5	-10
1.50	10	10	0
2.00	7	13	+6
2.50	5	15	+10

Comparative Statics

Analyzing Changes in Equilibrium

Comparative statics examines how changes in demand or supply affect equilibrium price and quantity.

• Increase in Demand: Raises both equilibrium price and quantity.

• Decrease in Supply: Raises equilibrium price, lowers equilibrium quantity.

• Increase in Both Demand and Supply: Quantity increases, price effect depends on relative magnitude.

• Increase in Demand and Decrease in Supply: Price increases, quantity effect depends on relative magnitude.

Solving for Equilibrium

Linear Demand and Supply Functions

In many cases, demand and supply are represented by linear equations:

• Demand: $P = a - bQ$

• Supply: $P = c + dQ$

At equilibrium, $Q_d = Q_s = Q^*$ and $P^* = a - bQ^* = c + dQ^*$.

Solving for Equilibrium Quantity:

$a - bQ^* = c + dQ^*$ $a - c = bQ^* + dQ^*$ $Q^* = \frac{a - c}{b + d}$

Solving for Equilibrium Price:

$P^* = a - bQ^*$ $P^* = c + dQ^*$

Example: If $P = 12 - 2Q$ (demand) and $P = 2 + Q$ (supply): Set $12 - 2Q = 2 + Q$ $12 - 2Q = 2 + Q$ $10 = 3Q$ $Q^* = \frac{10}{3} \approx 3.33$ $P^* = 12 - 2(3.33) \approx 5.33$

Summary: Understanding supply and demand, competitive equilibrium, and comparative statics is essential for analyzing market outcomes and predicting the effects of economic changes.