# COURSE: AP MICROECONOMICS

# UNIT 2: SUPPLY AND DEMAND

# 1. INTRODUCTION

Imagine you are standing in line for the latest smartphone release. Why is the price set at $999 and not $10 or $10,000? Why does the price of gasoline fluctuate every week, while the price of a can of soda remains stable for years? These questions are at the heart of Supply and Demand, the most fundamental model in economics.

In this unit, we explore how markets function. A market is not necessarily a physical place; it is any arrangement that allows buyers and sellers to exchange goods and services. We will move beyond simple observations to understand the mathematical and behavioral mechanics that drive market prices and quantities. By the end of this chapter, you will be able to predict how global events—like a heatwave in Brazil affecting coffee prices or a new tax on sugar—impact the choices of millions of people. We will build from the simple "Law of Demand" to complex analysis of government interventions and international trade, providing you with the analytical tools used by policymakers and business leaders worldwide.

# 2. ALL KEY CONCEPTS, TERMS, AND PRINCIPLES

To master this unit, you must first become fluent in the "language of the market."

• Law of Demand: The inverse relationship between price and quantity demanded ($P \uparrow, Q_d \downarrow$), assuming all other factors are constant (ceteris paribus).
• Law of Supply: The direct relationship between price and quantity supplied ($P \uparrow, Q_s \uparrow$), assuming all other factors are constant.
• Ceteris Paribus: A Latin phrase meaning "all other things being equal." It allows economists to isolate the effect of one variable.
• Quantity Demanded ($Q_d$) vs. Demand ($D$): $Q_d$ is a specific point on the curve (changed only by price); $D$ is the entire curve (changed by outside factors).
• Elasticity: A measure of responsiveness. It tells us how much $Q$ changes when $P$, income, or other prices change.
• Equilibrium: The point where $Q_s = Q_d$. The "market-clearing" price where there is neither a shortage nor a surplus.
• Consumer Surplus (CS): The difference between what a consumer is willing to pay and what they actually pay.
• Producer Surplus (PS): The difference between the market price and the minimum price a seller is willing to accept.
• Deadweight Loss (DWL): The loss of total economic surplus caused by an inefficient allocation of resources (usually due to taxes or price controls).
• Normal vs. Inferior Goods: Demand for normal goods increases as income rises; demand for inferior goods decreases as income rises.
• Substitutes vs. Complements: Substitutes are goods used in place of one another; complements are goods used together.

# 3. IN-DEPTH EXPLANATION

# 2.1 – Demand

The Demand Curve represents the behavior of consumers. On a standard graph, Price ($P$) is on the vertical Y-axis and Quantity ($Q$) is on the horizontal X-axis. The curve slopes downward ($D$).

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  "title": "The Demand Curve",
  "subtitle": "Inverse relationship between Price and Quantity Demanded",
  "xLabel": "Quantity (Units)",
  "yLabel": "Price ($)",
  "labels": ["0", "10", "20", "30", "40", "50"],
  "datasets": [
    { "label": "Demand (D)", "data": [50, 40, 30, 20, 10, 0], "tension": 0 }
  ]
}

Why does it slope downward?

1. Substitution Effect: As the price of Good A rises, consumers switch to cheaper substitutes.
2. Income Effect: As the price of a good falls, the consumer's "real income" (purchasing power) increases, allowing them to buy more.
3. Law of Diminishing Marginal Utility: As you consume more of a good, the additional satisfaction (utility) from each extra unit decreases. Therefore, you are only willing to buy more units if the price is lower.

Shifters of Demand (The "TRIBE" Acronym):

• Tastes and Preferences.
• Related Goods (Prices of substitutes and complements).
• Income (Normal vs. Inferior).
• Buyers (Number of consumers in the market).
• Expectations (Future prices or income).

# 2.2 – Supply

The Supply Curve represents the behavior of producers. It slopes upward ($S$).

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  "title": "The Supply Curve",
  "subtitle": "Direct relationship between Price and Quantity Supplied",
  "xLabel": "Quantity (Units)",
  "yLabel": "Price ($)",
  "labels": ["0", "10", "20", "30", "40", "50"],
  "datasets": [
    { "label": "Supply (S)", "data": [0, 10, 20, 30, 40, 50], "tension": 0 }
  ]
}

Why does it slope upward?
As price increases, the potential for profit increases, encouraging firms to produce more. Furthermore, due to the Law of Increasing Opportunity Costs, producing additional units requires more resources, which costs more; thus, a higher price is needed to cover those costs.

Shifters of Supply (The "ROTTEN" Acronym):

• Resource/Input Costs (Labor, raw materials).
• Other goods' prices (Alternative items the firm could produce).
• Technology (Improvements increase supply).
• Taxes and Subsidies (Taxes $\downarrow S$; Subsidies $\uparrow S$).
• Expectations of producers.
• Number of Sellers.

# 2.3 – Price Elasticity of Demand (PED)

Elasticity measures how "sensitive" consumers are to a change in price.

The Formula:
$$PED = \left| \frac{% \Delta Q_d}{% \Delta P} \right|$$
Note: We use absolute value because PED is always negative due to the Law of Demand.

Ranges of Elasticity:

1. Elastic ($PED > 1$): Consumers are very responsive (e.g., luxury goods, goods with many substitutes).
2. Inelastic ($PED < 1$): Consumers are not very responsive (e.g., medicine, gasoline, addictive goods).
3. Unit Elastic ($PED = 1$): % change in $Q$ equals % change in $P$.
4. Perfectly Inelastic ($PED = 0$): Vertical demand curve.
5. Perfectly Elastic ($PED = \infty$): Horizontal demand curve.

Total Revenue Test:

• If demand is Inelastic: $P \uparrow \implies TR \uparrow$ (Price and Revenue move in the same direction).
• If demand is Elastic: $P \uparrow \implies TR \downarrow$ (Price and Revenue move in opposite directions).

# 2.4 – Price Elasticity of Supply (PES)

PES measures how responsive producers are to a change in price.
$$PES = \frac{% \Delta Q_s}{% \Delta P}$$
The primary determinant of PES is Time. In the short run, supply is often inelastic because firms cannot easily change factory size. In the long run, supply becomes more elastic.

# 2.5 – Other Elasticities

Income Elasticity ($E_i$):
$$E_i = \frac{% \Delta Q_d}{% \Delta Income}$$

• $E_i > 0$: Normal Good.
• $E_i < 0$: Inferior Good.

Cross-Price Elasticity ($E_{xy}$):
$$E_{xy} = \frac{% \Delta Q_{d, \text{Good A}}}{% \Delta P_{\text{Good B}}}$$

• $E_{xy} > 0$: Substitutes.
• $E_{xy} < 0$: Complements.

# 2.6 – Market Equilibrium and Surplus

Equilibrium occurs where the Supply and Demand curves intersect ($Q_s = Q_d$). At this point, the price is $P^$ and quantity is $Q^$.

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  "title": "Market Equilibrium",
  "subtitle": "Point of intersection where Qd = Qs",
  "xLabel": "Quantity (Q)",
  "yLabel": "Price (P)",
  "labels": ["0", "5", "10", "15", "20"],
  "datasets": [
    { "label": "Demand", "data": [20, 15, 10, 5, 0], "tension": 0 },
    { "label": "Supply", "data": [0, 5, 10, 15, 20], "tension": 0 }
  ]
}

• Consumer Surplus (CS): The area below the demand curve and above the price line.
  $$CS = \frac{1}{2} \times (\text{Base}) \times (\text{Max WTP} - \text{Market Price})$$
• Producer Surplus (PS): The area above the supply curve and below the price line.
  $$PS = \frac{1}{2} \times (\text{Base}) \times (\text{Market Price} - \text{Min Selling Price})$$

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    { "type": "label", "x": 420, "y": 60, "text": "S" },
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    { "type": "label", "x": 250, "y": 370, "text": "Qe" }
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• Total Surplus: $CS + PS$. Efficiency is achieved when total surplus is maximized.

# 2.7 – Disequilibrium

• Surplus: If $P > P^*$, then $Q_s > $Q_d$. Sellers must lower prices to clear inventory.
• Shortage: If $P < P^*$, then $Q_d > $Q_s$. Buyers will bid prices up.

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  "subtitle": "Price Floor (Surplus) and Price Ceiling (Shortage) effects",
  "xLabel": "Quantity",
  "yLabel": "Price",
  "labels": ["0", "5", "10", "15", "20"],
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    { "label": "Supply", "data": [0, 5, 10, 15, 20] },
    { "label": "Price = 15 (Surplus Area)", "data": [15, 15, 15, 15, 15], "dashed": true },
    { "label": "Price = 5 (Shortage Area)", "data": [5, 5, 5, 5, 5], "dashed": true }
  ]
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# 2.8 – Government Intervention

1. Price Ceiling: A legal maximum price (e.g., Rent Control). To be binding, it must be set below equilibrium. It causes a shortage.

2. Price Floor: A legal minimum price (e.g., Minimum Wage). To be binding, it must be set above equilibrium. It causes a surplus.

3. Excise Taxes: A tax on per-unit production. It shifts the Supply curve up/left by the amount of the tax.

   • Tax Incidence: The burden of the tax depends on elasticity. The less elastic side of the market pays more of the tax.

4. Deadweight Loss (DWL): Taxes and non-equilibrium pricing create a "triangle" of lost transactions that would have benefited both parties.

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    { "type": "polygon", "points": [[150, 125], [150, 275], [250, 200]], "fill": "#9ca3af", "label": "DWL" },
    { "type": "line", "x1": 150, "y1": 125, "x2": 150, "y2": 275, "color": "#000", "width": 4, "label": "Tax Wedge" },
    { "type": "axes", "cx": 50, "cy": 350, "length": 350, "xLabel": "Quantity", "yLabel": "Price" },
    { "type": "label", "x": 100, "y": 125, "text": "Price Paid by Buyers" },
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# 2.9 – International Trade

• If World Price ($P_w$) > Domestic Price ($P_d$): The country will export. Producers gain, consumers lose, but total surplus increases.
• If World Price ($P_w$) < Domestic Price ($P_d$): The country will import. Consumers gain, producers lose, but total surplus increases.

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  "title": "International Trade: Importing",
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  "xLabel": "Quantity",
  "yLabel": "Price",
  "labels": ["0", "5", "10", "15", "20"],
  "datasets": [
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    { "label": "Domestic Supply", "data": [0, 5, 10, 15, 20] },
    { "label": "World Price (Pw = 5)", "data": [5, 5, 5, 5, 5], "dashed": true }
  ]
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• Tariff: A tax on imports. It raises the price for domestic consumers, helps domestic producers, generates government revenue ($Tariff \times Quantity \text{ Imported}$), but creates DWL.

# 4. EXAMPLES

# Demand and Supply Basics

Example 1: The Substitution Effect
If the price of Pepsi increases from $1.50 to $2.50, and Coke remains at $1.50, consumers will switch to Coke. This illustrates a movement along the Pepsi demand curve (decrease in $Q_d$) and a rightward shift in the Coke demand curve (increase in $D$).

Example 2: Diminishing Marginal Utility
A student buys 1 slice of pizza for $3 because they are hungry (Utility = 10 utils). The 2nd slice gives 5 utils. The 3rd gives 1 util. The student will only buy the 3rd slice if the price drops to $0.50.

Example 3: Market Demand Summation
If Alice wants 2 apples at $1 and Bob wants 3 apples at $1, the Market Demand at $1 is $2 + 3 = 5$ apples.

Example 4: Change in Input Costs
The price of lithium (used in batteries) triples. This increases the cost of producing Electric Vehicles (EVs). The Supply curve for EVs shifts to the left ($S \downarrow$).

Example 5: Expectations (Demand)
Consumers expect the price of houses to rise 20% next month. Current Demand ($D$) shifts right as people try to buy before the price hike.

Example 6: Expectations (Supply)
Oil producers expect prices to double next week. They withhold current supply to sell later. Current Supply ($S$) shifts left.

# Elasticity Calculations

Example 7: Calculating PED (Midpoint Method)
Price rises from $10 to $12. $Q_d$ falls from 100 to 80.
$$% \Delta Q = \frac{80 - 100}{(80 + 100)/2} = -0.222$$
$$% \Delta P = \frac{12 - 10}{(12 + 10)/2} = 0.181$$
$$PED = |-0.222 / 0.181| = 1.22 \text{ (Elastic)}$$

Example 8: Total Revenue Test
A cinema charges $10 and sells 100 tickets ($TR = $1000$). It raises the price to $12 and sells 90 tickets ($TR = $1080$). Since $P \uparrow$ and $TR \uparrow$, demand is inelastic.

Example 9: Income Elasticity (Inferior Good)
Your income increases by 10%, and your demand for "Instant Noodles" falls by 5%.
$$E_i = -5% / 10% = -0.5$$
Because $E_i$ is negative, Instant Noodles are an inferior good.

Example 10: Cross-Price Elasticity (Complements)
The price of printers rises by 20%. The demand for ink cartridges falls by 30%.
$$E_{xy} = -30% / 20% = -1.5$$
Because $E_{xy}$ is negative, printers and ink are complements.

Example 11: Cross-Price Elasticity (Substitutes)
The price of butter rises 10%. Demand for margarine increases 15%.
$$E_{xy} = 15% / 10% = +1.5$$
Because $E_{xy}$ is positive, they are substitutes.

# Equilibrium and Shifts

Example 12: Single Shift (Demand)
A study proves coffee prevents aging. Demand shifts right ($D \uparrow$). $P^$ increases, $Q^$ increases.

Example 13: Single Shift (Supply)
A new robot makes shoe manufacturing 50% faster. Supply shifts right ($S \uparrow$). $P^$ decreases, $Q^$ increases.

Example 14: Double Shift (Opposite Directions)
Demand increases (due to popularity) and Supply decreases (due to a strike).

• $D \uparrow \implies P \uparrow, Q \uparrow$
• $S \downarrow \implies P \uparrow, Q \downarrow$
• Result: Price increases definitely; Quantity is ambiguous.

Example 15: Double Shift (Same Direction)
Demand increases and Supply increases.

• $D \uparrow \implies P \uparrow, Q \uparrow$
• $S \uparrow \implies P \downarrow, Q \uparrow$
• Result: Quantity increases definitely; Price is ambiguous.

Example 16: Surplus Calculation
Equilibrium is $10. The government sets a floor at $15. At $15, $Q_s = 50$ and $Q_d = 20$. The surplus is $50 - 20 = 30$ units.

Example 17: Shortage Calculation
Equilibrium is $10. A ceiling is set at $5. At $5, $Q_d = 100$ and $Q_s = 40$. The shortage is $100 - 40 = 60$ units.

# Surplus and Welfare

Example 18: Consumer Surplus on a Graph
Demand: $P = 20 - Q$. Supply: $P = Q$.
Equilibrium: $20 - Q = Q \implies 2Q = 20 \implies Q^* = 10, P^* = 10$.
$CS = 0.5 \times (10) \times (20 - 10) = 50$.
$PS = 0.5 \times (10) \times (10 - 0) = 50$.
Total Surplus = 100.

Example 19: Deadweight Loss from Price Ceiling
Using the previous example, set a price ceiling at $P = 8$.
At $P=8$, producers only supply $Q=8$ (from $P=Q$).
Consumers want $Q = 12$ (from $8 = 20-Q$).
Actual units traded = 8.
$DWL$ is the triangle between $Q=8$ and $Q=10$.
Value of 8th unit to consumer = $20-8=12$.
$DWL = 0.5 \times (10-8) \times (12-8) = 0.5 \times 2 \times 4 = 4$.

# Government Intervention and Taxes

Example 20: Tax Incidence (Inelastic Demand)
Insulin has perfectly inelastic demand. The government imposes a $1 tax per dose on sellers. Sellers pass the entire $1 to consumers. Consumers pay 100% of the tax.

Example 21: Tax Incidence (Elastic Demand)
A tax is placed on a specific brand of water with many substitutes. Sellers cannot raise prices without losing all customers. Sellers pay most of the tax.

Example 22: Calculating Tax Revenue
A $2 tax is placed on 1,000 units sold. Tax Revenue = $$2 \times 1,000 = $2,000$.

Example 23: The "Wedge"
A tax of $T creates a vertical wedge between the price consumers pay ($P_c$) and the price producers receive ($P_p$). $P_c - P_p = T$.

Example 24: Subsidy Effect
A $1 subsidy per bushel is given to corn farmers. Supply shifts right. $P$ for consumers falls, $Q$ increases. Total surplus increases, but the cost to the government creates a "deadweight loss of taxation" elsewhere.

# International Trade

Example 25: Domestic Market without Trade
Price of wheat in the US is $5. World price is $5. No trade occurs.

Example 26: Exporting
US price of wheat is $5. World price is $8. The US exports wheat.
Domestic price rises to $8.
US consumers lose $CS$. US producers gain $PS$. Net gain to US = total surplus increase.

Example 27: Importing
US price of shirts is $20. World price is $10. The US imports shirts.
Domestic price falls to $10.
US consumers gain $CS$. US producers lose $PS$. Net gain to US = total surplus increase.

Example 28: Tariff Effects
World price = $10. Tariff = $2. New domestic price = $12.
Imports decrease. Government gets revenue. Domestic production increases.

Example 29: Quota vs. Tariff
A quota limits imports to 100 units. A tariff is set to result in the same 100 imports.
Difference: The "Revenue" under a tariff goes to the government. Under a quota, the "Quota Rent" goes to whoever holds the import licenses.

# Advanced Mastery Level

Example 30: Why Slope $\neq$ Elasticity
On a linear demand curve $Q = 10 - P$, the slope is constant ($-1$).
At $P=9, Q=1$, $PED = |\frac{% \Delta Q}{% \Delta P}|$. Near this point, because the base quantity is so small, a tiny price change causes a massive percentage change in quantity, making demand highly elastic ($PED > 1$). At low prices and high quantities (e.g., $P=1, Q=9$), the same price change represents a large percentage change in price but a small percentage change in quantity, making demand inelastic ($PED < 1$).

Example 31: Binding vs. Non-binding Floor
Equilibrium wage is $15. The city passes a $10 minimum wage. This is non-binding; the market stays at $15. If they pass a $20 wage, it is binding and causes unemployment (surplus of labor).

Example 32: Welfare Analysis of a Tariff
A tariff reduces imports from 500 to 200.
The DWL consists of two triangles:

1. Production Inefficiency: Producing at home costs more than buying abroad.
2. Consumption Inefficiency: Consumers buy less than the socially optimal amount.

Example 33: Perfect Elasticity Scenario
A farmer in a perfectly competitive wheat market faces a horizontal demand curve. If they raise price by $0.01, $Q_d$ drops to zero. $PED = \infty$.