Supply & Demand Basics

Equilibrium

Market equilibrium is a fundamental concept in microeconomics, representing the point at which the quantity demanded equals the quantity supplied. This intersection determines the market price and quantity traded.

• Market Equilibrium: Occurs where quantity demanded equals quantity supplied.

• Graphical Representation: The intersection of the supply and demand curves.

• Surplus: When quantity supplied exceeds quantity demanded at a price above equilibrium. Example: If Qs = 150 and Qd = 100, there is a 50-unit surplus.

• Shortage: When quantity demanded exceeds quantity supplied at a price below equilibrium.

Shifts in Supply & Demand

Supply Shifts (NESTS)

Changes in supply are driven by several factors, summarized by the acronym NESTS. Each factor can shift the supply curve left (decrease) or right (increase).

• Nature: Natural events (e.g., weather, disasters) affect supply. Example: A hurricane destroying crops shifts supply left.

• Expectations: Anticipation of future prices or events can alter current supply.

• Subsidies: Government payments to producers increase supply.

• Technology: Improvements increase supply by lowering production costs.

• Sellers: The number of sellers in the market affects total supply.

Additional info: Demand shifts are typically caused by changes in income, tastes, prices of related goods, expectations, and number of buyers.

Consumer & Producer Surplus

Definitions and Market Efficiency

Surplus measures the benefit to consumers and producers from participating in the market. Total surplus is maximized at equilibrium.

• Consumer Surplus: The difference between the maximum price a consumer is willing to pay and the actual market price.

• Producer Surplus: The difference between the market price and the minimum price a supplier is willing to accept.

• Total (Economic) Surplus: The sum of consumer and producer surplus. Maximized at equilibrium.

Example: If the market price falls, consumer surplus increases as buyers pay less than their maximum willingness to pay.

Government Intervention: Taxes, Price Floors, Price Ceilings

Types and Effects

Government policies can alter market outcomes, often leading to inefficiencies such as surpluses, shortages, and deadweight loss.

• Price Ceiling: A maximum legal price (e.g., rent control). Leads to shortages.

• Price Floor: A minimum legal price (e.g., minimum wage). Leads to surpluses.

• Taxes: A per-unit tax shifts the supply curve left by the amount of the tax. Buyers pay more, sellers receive less.

• Deadweight Loss: Taxes reduce total surplus by lowering the quantity traded below the efficient equilibrium.

• Tax Revenue: Calculated as Tax × Quantity after tax. Example: $3 × 100 units = $300.

• Tax Equity:

  • Benefits Principle: Pay based on benefits received (e.g., gas tax funds roads).

  • Ability-to-Pay Principle: Higher-income individuals should pay more.

Elasticity

Price Elasticity of Demand

Elasticity measures how responsive quantity demanded or supplied is to changes in price. It is crucial for understanding consumer and producer behavior.

• Formula:

• Elastic: %ΔQ > %ΔP (luxury goods, many substitutes).

• Inelastic: %ΔQ < %ΔP (necessities).

• Graph Interpretation:

  • Perfectly elastic: horizontal line.

  • Perfectly inelastic: vertical line.

• Total Revenue Test:

  • Elastic region: price ↑ → TR ↓

  • Inelastic region: price ↑ → TR ↑

• Total Revenue:

Income Elasticity

• Positive: Normal good.

• Negative: Inferior good. Example: Income elasticity = –0.5 → inferior good.

Cross-Price Elasticity

• Positive: Substitutes.

• Negative: Complements.

Price Elasticity of Supply

• Measures: Responsiveness of quantity supplied to price changes.

• Always positive: Law of supply (as price increases, quantity supplied increases).

• Perfectly elastic supply: Horizontal line.

Quantitative Problems

Applying Formulas and Methods

Quantitative analysis is essential for solving supply and demand problems, especially when government policies are involved.

• Adjusting Supply for Taxes: Example: Original supply function ; with a $3\text{Qs} = 5(P - 3) - 15$.

• Percentage Change: Example: Price changes from $50; % change = .

• Midpoint Method (Supply Example): Price: ; Quantity: ; Elasticity = .

Summary Table: Types of Elasticity

One-Sentence Super Summary

These notes cover how markets reach equilibrium, how surpluses and shortages form, how taxes and price controls distort markets, and how elasticity explains consumer and producer responsiveness — all tied together with surplus, efficiency, and government policy.