Supply and Demand — AP Microeconomics Micro Study Guide

For: AP Microeconomics candidates sitting AP Microeconomics.

Covers: Determinants of demand and supply, market equilibrium and disequilibrium, four types of elasticity, government price controls and taxes, and consumer and producer surplus, fully aligned to the AP Microeconomics CED.

You should already know: No prior econ required.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Microeconomics style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Supply and Demand?

Supply and demand is the foundational model of microeconomics that describes how interactions between buyers (demand) and sellers (supply) determine the quantity of a good or service exchanged in a market and the price it is sold at. It forms the core of Unit 2 of the AP Microeconomics CED, and provides the analytical framework for all later topics including market failure, firm behavior, factor markets, and international trade. You may see it referred to as the market model or S-D model in exam materials, and 20-30% of your total exam score will come from questions that directly test these concepts.

2. Determinants of demand and supply

Core Definitions

Demand is the quantity of a good consumers are willing and able to buy at every possible price, holding all other factors constant (ceteris paribus). The law of demand states that price and quantity demanded are inversely related, so the demand curve slopes downward. Price changes only cause movement along the existing demand curve, not shifts of the curve itself. Supply is the quantity of a good producers are willing and able to sell at every possible price, ceteris paribus. The law of supply states that price and quantity supplied are directly related, so the supply curve slopes upward. Like demand, price changes only cause movement along the supply curve.

Demand Shifters (Determinants)

Use the mnemonic TRIBE to remember non-price factors that shift the entire demand curve:

1. Tastes and preferences of consumers
2. Prices of related goods (substitutes and complements)
3. Income of consumers
4. Number of buyers in the market
5. Expectations of future price changes

Supply Shifters (Determinants)

Use the mnemonic TRICE to remember non-price factors that shift the entire supply curve:

1. Technology used in production
2. Prices of related producer goods (alternative products a seller could make)
3. Input costs (wages, raw materials, utilities)
4. Number of sellers (competition) in the market
5. Expectations of future price changes

Worked Example

If a viral social media trend promotes reusable water bottles as a status symbol, which factor shifts demand, and what is the effect? The positive shift in consumer tastes increases demand for reusable bottles, shifting the entire demand curve to the right: quantity demanded is higher at every price point, with no movement along the original demand curve.

3. Market equilibrium and disequilibrium

Equilibrium Definition

Market equilibrium occurs at the intersection of the supply and demand curves, where quantity demanded equals quantity supplied. The equilibrium price ($P^{\ast }$) is called the market-clearing price, as there is no excess inventory or unmet consumer demand at this price. The equilibrium quantity ($Q^{\ast }$) is the total number of units exchanged in the free market. To calculate equilibrium mathematically, set the demand function ($Q_d$) equal to the supply function ($Q_s$) and solve for $P^{\ast }$, then substitute $P^{\ast }$ back into either function to find $Q^{\ast }$.

Disequilibrium

Disequilibrium occurs when the market price deviates from $P^{\ast }$, creating inherent pressure for price to adjust back to equilibrium:

1. Surplus: Occurs when price is above $P^{\ast }$, so $Q_s>Q_d$. Sellers cut prices to clear excess inventory, pushing price down to $P^{\ast }$.
2. Shortage: Occurs when price is below $P^{\ast }$, so $Q_d>Q_s$. Buyers bid up prices to access scarce goods, pushing price up to $P^{\ast }$.

Worked Example

The market for t-shirts has demand function $Q_d=400-5P$ and supply function $Q_s=100+10P$. Find equilibrium, and identify the disequilibrium if the current price is $25.

1. Set $Q_d=Q_s$: $400-5P=100+10P$ → $300=15P$ → $P^{\ast }=\$20$, $Q^{\ast }=400-5(20)=300$ units.
2. At $P=\$25$: $Q_d=400-125=275$, $Q_s=100+250=350$. There is a surplus of $350-275=75$ t-shirts, so price will fall back to $\$20$.

4. Elasticity — PED, PES, YED, XED

Elasticity is a unit-free measure of how responsive one economic variable is to a change in another, allowing you to compare sensitivity across unrelated goods. There are four elasticity types tested on the AP Micro exam:

1. Price Elasticity of Demand (PED)

Measures how responsive quantity demanded is to a change in price: $$PED=\frac{\%\Delta Q_d}{\%\Delta P}$$ Because of the law of demand, PED is always negative; we use its absolute value for classification:

• $|PED|>1$: Elastic (quantity responds more than price, e.g. luxury goods)
• $|PED|=1$: Unit elastic
• $|PED|<1$: Inelastic (quantity responds less than price, e.g. prescription drugs) Determinants of PED: number of substitutes, luxury vs necessity, share of income spent on the good, time horizon for consumers to adjust behavior.

2. Price Elasticity of Supply (PES)

Measures how responsive quantity supplied is to a change in price: $$PES=\frac{\%\Delta Q_s}{\%\Delta P}$$ PES is always positive due to the law of supply, and uses the same classification thresholds as PED. Determinants: availability of spare inputs, time to adjust production, storage capacity for the good.

3. Income Elasticity of Demand (YED)

Measures how responsive quantity demanded is to a change in consumer income ($Y$): $$YED=\frac{\%\Delta Q_d}{\%\Delta Y}$$ Sign matters for classification:

• $YED>0$: Normal good (demand rises with income). $YED>1$ = luxury, $0<YED<1$ = necessity.
• $YED<0$: Inferior good (demand falls as income rises, e.g. generic grocery products)

4. Cross-Price Elasticity of Demand (XED)

Measures how responsive quantity demanded of Good A is to a change in price of Good B: $$XED=\frac{\%\Delta Q_{d,A}}{\%\Delta P_B}$$ Sign matters for classification:

• $XED>0$: Substitutes (e.g. Coke and Pepsi: if Coke prices rise, Pepsi demand rises)
• $XED<0$: Complements (e.g. coffee and creamer: if coffee prices rise, creamer demand falls)
• $XED=0$: Unrelated goods

Worked Example

A 10% rise in the price of cereal leads to a 15% fall in cereal quantity demanded, a 5% rise in demand for oatmeal, and a 20% fall in demand for milk. A 8% rise in consumer income leads to a 4% rise in cereal demand. Calculate all relevant elasticities:

• $|PED{|}_{cereal}=\frac{15\%}{10\%}=1.5$ (elastic)
• $XE{D}_{oatmeal,cereal}=\frac{5\%}{10\%}=0.5$ (substitutes)
• $XE{D}_{milk,cereal}=\frac{-20\%}{10\%}=-2$ (complements)
• $YE{D}_{cereal}=\frac{4\%}{8\%}=0.5$ (normal necessity good)

5. Government intervention — price ceilings, floors, taxes

Government policy interventions distort free market equilibrium, reduce total surplus, and create deadweight loss (lost value from transactions that no longer occur due to the policy).

Price Ceilings

A price ceiling is a legal maximum price sellers can charge for a good. It is only binding if set below the equilibrium price $P^{\ast }$. Binding price ceilings create shortages, non-price rationing (queues, black markets), and reduced quantity exchanged. Common real-world example: rent control.

Price Floors

A price floor is a legal minimum price buyers must pay for a good. It is only binding if set above the equilibrium price $P^{\ast }$. Binding price floors create surpluses, reduced quantity exchanged, and often require government to purchase excess supply. Common real-world examples: minimum wage, agricultural price supports.

Per-Unit Taxes

A per-unit tax is a fixed tax charged on each unit of a good sold. It creates a wedge between the price buyers pay ($P_b$) and the price sellers receive ($P_s$), where $P_b=P_s+t$ ($t$ = tax value). Tax incidence (the economic burden of the tax) is determined solely by the relative elasticity of supply and demand, regardless of who the tax is legally levied on: the more inelastic side of the market pays a larger share of the tax. The share of tax paid by consumers is calculated as: $$\text{Consumer Burden Share}=\frac{PES}{PES+|PED|}$$

Worked Example

The market for gasoline has $|PED|=0.3$ (inelastic demand) and $PES=1.2$ (elastic supply). If the government imposes a $\$1$ per gallon tax, what share of the tax do consumers pay? $$\text{Consumer Burden Share}=\frac{1.2}{1.2+0.3}=0.8=80\%$$ Consumers pay 80% of the $\$1$ tax ($\$0.80$ per gallon), while sellers pay the remaining 20% ($\$0.20$ per gallon).

6. Consumer and producer surplus

Total welfare in a market is measured as the sum of consumer and producer surplus, which is maximized at free market equilibrium.

Consumer Surplus (CS)

Consumer surplus is the difference between the maximum price a consumer is willing to pay for a good (their reservation price) and the actual market price they pay. Graphically, it is the triangular area below the demand curve, above the equilibrium price $P^{\ast }$, up to the equilibrium quantity $Q^{\ast }$. For a linear demand curve: $$CS=\frac{1}{2}\times (\text{Max willingness to pay}-P^{\ast })\times Q^{\ast }$$

Producer Surplus (PS)

Producer surplus is the difference between the minimum price a seller is willing to accept for a good (their marginal cost of production) and the actual market price they receive. Graphically, it is the triangular area above the supply curve, below the equilibrium price $P^{\ast }$, up to the equilibrium quantity $Q^{\ast }$. For a linear supply curve: $$PS=\frac{1}{2}\times (P^{\ast }-\text{Min willingness to accept})\times Q^{\ast }$$

Total Surplus

$$\text{Total Surplus}=CS+PS$$ Any government intervention (price control, tax, quota) reduces total surplus by the amount of the deadweight loss, the value of transactions that would have occurred in the free market but are blocked by the policy.

Worked Example

The market for notebooks has a maximum consumer willingness to pay of $\$10$, equilibrium price $P^{\ast }=\$3$, equilibrium quantity $Q^{\ast }=200$, and minimum producer willingness to accept of $\$1$. Calculate CS, PS, and total surplus:

• $CS=\frac{1}{2}\times (10-3)\times 200=\$700$
• $PS=\frac{1}{2}\times (3-1)\times 200=\$200$
• Total Surplus = $700+200=\$900$

7. Common Pitfalls (and how to avoid them)

• Pitfall 1: Confusing movement along the supply/demand curve with a shift of the curve when price changes. Why students do it: They mix up "quantity demanded/supplied" (movement along the curve) with "demand/supply" (the entire curve). Correct move: Only non-price determinants shift the entire curve; price changes only cause movement along the existing curve.
• Pitfall 2: Assuming all price ceilings and floors impact market outcomes. Why students do it: They forget the binding condition. Correct move: Price ceilings only bind if set below $P^{\ast }$; price floors only bind if set above $P^{\ast }$. Non-binding controls have no effect on market outcomes, so you will not earn marks for claiming they cause shortages or surpluses.
• Pitfall 3: Using the negative sign of PED to classify elasticity. Why students do it: They forget the law of demand makes PED inherently negative. Correct move: Always use the absolute value of PED for elastic/inelastic classification; only retain the sign for YED and XED to classify good types.
• Pitfall 4: Assuming tax incidence depends on who the government levies the tax on. Why students do it: They confuse legal incidence (who sends the tax payment to the government) with economic incidence (who bears the cost). Correct move: Tax burden is determined solely by relative elasticity of supply and demand, regardless of legal assignment.
• Pitfall 5: Calculating CS/PS for all theoretical units instead of only units that are actually exchanged. Why students do it: They forget only completed transactions generate surplus. Correct move: Always cap CS and PS calculations at the actual quantity traded in the market, especially after government interventions reduce traded quantity.

8. Practice Questions (AP Microeconomics Style)

Question 1

The market for reusable face masks has demand function $Q_d=500-10P$ and supply function $Q_s=200+5P$. (a) Calculate equilibrium price and quantity. (b) If the government imposes a price ceiling of $\$15$, calculate the resulting shortage or surplus, and the new quantity of masks exchanged.

Solution 1

(a) Set $Q_d=Q_s$: $500-10P=200+5P$ → $300=15P$ → $P^{\ast }=\$20$, $Q^{\ast }=500-10(20)=300$ units. (b) The price ceiling of $\$15$ is below $P^{\ast }=\$20$, so it is binding. At $P=\$15$: $Q_d=500-150=350$, $Q_s=200+75=275$. Shortage = $350-275=75$ units. Only 275 units are exchanged, as sellers will only supply 275 units at $\$15$.

Question 2

A 12% rise in consumer income leads to a 6% decrease in demand for generic frozen dinners, and an 18% increase in demand for gym memberships. (a) Calculate YED for both goods. (b) Classify each good as inferior, normal necessity, or normal luxury.

Solution 2

(a) $YE{D}_{genericfrozendinners}=\frac{-6\%}{12\%}=-0.5$. $YE{D}_{gymmemberships}=\frac{18\%}{12\%}=1.5$. (b) Generic frozen dinners have negative YED, so they are inferior goods. Gym memberships have positive YED greater than 1, so they are normal luxury goods.

Question 3

The government imposes a $\$2$ per unit tax on packs of chewing gum. Pre-tax equilibrium price is $\$3$, $|PED|=0.5$, $PES=2.0$. (a) What percentage of the tax burden falls on producers? (b) What price will consumers pay after the tax, and what price will producers receive?

Solution 3

(a) Producer burden share = $\frac{|PED|}{PES+|PED|}=\frac{0.5}{2.0+0.5}=0.2=20\%$. (b) Producers pay 20% of $\$2$ = $\$0.40$, so they receive $\$3-\$0.40=\$2.60$. Consumers pay the remaining 80% of $\$2$ = $\$1.60$, so they pay $\$3+\$1.60=\$4.60$. You can verify: $\$4.60-\$2.60=\$2$, which equals the tax value.

9. Quick Reference Cheatsheet

Core Formulas

Key Rules

1. Price changes cause movement along S/D curves; non-price determinants shift curves.
2. Binding price ceiling = < $P^{\ast }$ → shortage; Binding price floor = > $P^{\ast }$ → surplus.
3. Total surplus is maximized at free market equilibrium; all interventions create deadweight loss.

10. What's Next

This supply and demand model is the foundational building block for all remaining topics in the AP Microeconomics syllabus. You will apply these concepts to analyze market failure (externalities, public goods, common resources) in Unit 3, firm behavior and market structures (perfect competition, monopoly, oligopoly, monopolistic competition) in Units 4 and 5, factor markets for labor and capital in Unit 6, and the effects of government policies such as tariffs and quotas in international trade in Unit 7. Mastering the content in this guide is non-negotiable: 20-30% of your AP Micro exam multiple choice and 1-2 of the 3 free response questions will directly test supply and demand concepts, and nearly all other questions rely on a working understanding of this model.

If you struggle with any of the concepts, worked examples, or practice questions in this guide, you can get personalized, step-by-step help from Ollie at any time by visiting the homepage. Ollie can walk you through additional practice problems, explain tricky elasticity calculations, or test your knowledge of supply and demand shifters to make sure you are fully prepared for exam day.