Demand Theory: Laws, Elasticity & Consumer Choice

TAKEAWAYS

• Demand curves slope downward because of the law of demand (inverse price-quantity relationship), but shifts occur due to non-price determinants (income, tastes, substitutes).
• Price elasticity of demand (PED) measures responsiveness to price changes: $E_d=\frac{\%\Delta Q_d}{\%\Delta P}$. If $|E_d|>1$, demand is elastic (e.g., luxury goods like smartphones); if $|E_d|<1$, it’s inelastic (e.g., salt).
• Consumer equilibrium occurs where the budget line is tangent to the highest indifference curve, balancing marginal utility per rupee spent.
• Total utility (TU) rises until marginal utility (MU) equals price ($MU=P$), then falls due to diminishing marginal utility.
• Real-world examples: eSewa’s dynamic pricing for electricity uses price elasticity, while Daraz’s discounts on bulk orders exploit income effect.
• Exam focus: Memorize determinants of demand (5+), elasticity formulas, and budget constraint math (e.g., $M=P_{x}X+P_{y}Y$).

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1. The Law of Demand and Demand Curve

Definition & Explanation

The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, and vice versa. This inverse relationship arises due to:

• Substitution effect: Consumers switch to cheaper substitutes (e.g., tea → coffee if tea prices rise).
• Income effect: Higher prices reduce purchasing power (e.g., if Daraz raises laptop prices, fewer buyers can afford them).
• Diminishing marginal utility: Each additional unit consumed yields less satisfaction (e.g., the 10th cup of tea gives less happiness than the first).

Demand Schedule vs. Demand Curve

A demand schedule is a table showing quantities demanded at different prices. The demand curve plots this data, always sloping downward from left to right.

Worked Example: Tea Demand in Nepal

Assume Nepal’s tea demand follows:

Plot this as a demand curve (see below). If the price rises from Rs. 200 to Rs. 300, quantity demanded falls from 100 kg to 80 kg due to substitution and income effects.

Non-Price Determinants of Demand (Shifters)

While price moves along the demand curve, these factors shift the entire curve:

1. Consumer income (normal vs. inferior goods):
   • Normal goods (e.g., smartphones): Demand ↑ if income ↑.
   • Inferior goods (e.g., public transport): Demand ↓ if income ↑.
2. Prices of related goods:
   • Substitutes (tea vs. coffee): If coffee price ↑, tea demand ↑.
   • Complements (cars vs. petrol): If petrol price ↑, car demand ↓.
3. Consumer preferences/tastes: Marketing (e.g., Daraz ads for winter jackets) shifts demand right.
4. Future expectations: If consumers expect prices to rise (e.g., NEPSE stock crash), current demand ↑.
5. Number of buyers: More households (e.g., post-earthquake reconstruction) → demand ↑.

Visualizing Shifts:

```figure
{"type":"curves","lines":[{"label":"Original Demand (D₁)","from":[1,9],"to":[9,1]},{"label":"Increased Demand (D₂)","from":[1,10],"to":[9,2]},{"label":"Decreased Demand (D₃)","from":[1,8],"to":[9,0]}],"xlabel":"Quantity of Tea (kg)","ylabel":"Price (NPR)","caption":"Demand shifts for tea in Nepal: Rightward (income rise), Leftward (income fall)"}

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2. Price Elasticity of Demand (PED)

Measures how responsive quantity demanded is to price changes. Formula: [ E_d = \frac{%\Delta Q_d}{%\Delta P} = \frac{\Delta Q / Q}{\Delta P / P} ]

Types of Elasticity

| Elasticity Type | ( |E_d| ) | Slope of Demand Curve | Example (Nepal) | |-----------------------|--------|--------------------------------|-------------------------| | Perfectly Elastic | ∞ | Horizontal line | Agricultural commodities (e.g., wheat) | | Elastic | >1 | Flatter slope | Luxury cars (Toyota Fortuner) | | Unit Elastic | =1 | 45° angle | Electricity (short-run) | | Inelastic | <1 | Steeper slope | Salt, medicine | | Perfectly Inelastic| 0 | Vertical line | Life-saving drugs |

Worked Example: eSewa Electricity Bill

Suppose eSewa charges Rs. 10 per unit. If price rises to Rs. 12, quantity demanded falls from 100 units to 90 units. Calculate ( E_d ): [ E_d = \frac{(90-100)/100}{(12-10)/10} = \frac{-10%}{20%} = -0.5 ] Since ( |E_d| = 0.5 < 1 ), demand is inelastic. eSewa can increase revenue by raising prices (price ↑ × quantity ↓ = revenue ↑).

Determinants of PED

1. Availability of substitutes: More substitutes → more elastic (e.g., smartphones vs. salt).
2. Necessity vs. luxury: Luxuries (e.g., holidays) are elastic; necessities (e.g., insulin) are inelastic.
3. Proportion of income spent: Big-ticket items (e.g., houses) have more elastic demand.
4. Time period: Longer time → more elastic (consumers find substitutes).

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3. Consumer Behavior: Utility and Choice

Key Concepts

• Total Utility (TU): Total satisfaction from consuming a good.
• Marginal Utility (MU): Additional satisfaction from one more unit (( MU = \Delta TU / \Delta Q )).
• Diminishing Marginal Utility: Each extra unit yields less extra satisfaction (e.g., the 5th burger gives less happiness than the 1st).

Utility Maximization Rule

Consumers allocate spending to maximize utility when: [ \frac{MU_x}{P_x} = \frac{MU_y}{P_y} ] Example: If ( MU_{tea} = 20 ) and ( P_{tea} = Rs. 10 ), and ( MU_{coffee} = 30 ) with ( P_{coffee} = Rs. 15 ), you should switch from tea to coffee because ( 20/10 > 30/15 ).

Budget Constraint and Indifference Curves

• Budget Line: Shows all combinations of two goods a consumer can buy with a fixed income. [ M = P_x X + P_y Y ] Example: If income ( M = Rs. 40,000 ), ( P_x = Rs. 800 ), ( P_y = Rs. 1,600 ), the budget line equation is: [ 800X + 1600Y = 40,000 \quad \text{or} \quad X + 2Y = 50 ]

• Indifference Curve: Shows combinations of goods yielding the same utility. Key properties:
  1. Higher curves = higher utility.
  2. Downward sloping (trade-off between goods).
  3. Convex to origin (diminishing marginal rate of substitution).

```figure
{"type":"curves","lines":[{"label":"Indifference Curve (IC₁)","from":[1,10],"to":[10,1],"fn":{"expr":"10 - x","label":""}},{"label":"Indifference Curve (IC₂)","from":[1,12],"to":[12,1],"fn":{"expr":"12 - x","label":""}}],"points":[{"x":4,"y":6,"label":"Consumer Equilibrium","xmark":"X*","ymark":"Y*"}],"xlabel":"Quantity of Good X (e.g., Rice)","ylabel":"Quantity of Good Y (e.g., Dal)","caption":"Budget constraint tangency with higher indifference curve (Nepalese staples example)"}

Consumer Equilibrium

Occurs where:

1. The budget line is tangent to the highest possible indifference curve.
2. $\frac{M{U}_x}{P_x}=\frac{M{U}_y}{P_y}$.

Worked Example: Daraz vs. Hamrobazaar Suppose a consumer spends Rs. 40,000 on laptops (X) and tablets (Y):

• $P_X=Rs.80,000$, $P_Y=Rs.16,000$.
• Budget line: $X+2.5Y=50$.
• At equilibrium, $M{U}_X/80,000=M{U}_Y/16,000$. If $M{U}_X=40$ and $M{U}_Y=8$, the consumer should buy more tablets to balance utility per rupee.

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4. Real-World Applications in Nepal

1. eSewa’s Dynamic Pricing for Electricity

• Idea Used: Price elasticity of demand.
• How: During peak hours (6–9 PM), eSewa increases electricity tariffs (e.g., Rs. 12/kWh → Rs. 15/kWh). Since demand for electricity is inelastic ($|E_d|<1$), revenue rises even as some consumers reduce usage.
• Data: Post-price hike, quantity demanded fell by 8% (from 100 units to 92 units), but total revenue increased by 7% (from Rs. 1,200 to Rs. 1,284).

2. Daraz’s Discount Strategies

• Idea Used: Income effect and substitution.
• How: Daraz offers bulk discounts (e.g., "Buy 2, Get 1 Free") to exploit:
  • Income effect: Consumers feel "richer" and buy more.
  • Substitution: Shoppers switch from Hamrobazaar to Daraz for better deals.
• Example: A Rs. 5,000 product on sale for Rs. 3,000 increases quantity demanded by 40% (elastic demand).

3. NTC’s Telephone Tariff Adjustments

• Idea Used: Cross-price elasticity.
• How: When Ncell reduced call rates, NTC lowered landline prices to retain customers. The complementary relationship between mobile and landline services means a price cut in one can increase demand for the other if they’re substitutes for some users.

4. Kathmandu Traffic Congestion (Negative Demand Shock)

• Idea Used: Shift in demand curve.
• How: During Dashain/Tihar, the number of vehicles on roads increases (shift right), causing congestion. The supply of road space is fixed, leading to higher "opportunity cost" (time lost in traffic). This is a real-world example of demand exceeding supply at a fixed price (zero toll).

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5. Exam Tip: How to Score Full Marks

Common Mistakes to Avoid

1. Confusing "change in demand" vs. "change in quantity demanded":
   • Quantity demanded = movement along the curve (price change).
   • Demand = shift of the curve (non-price factors).
2. Forgetting ceteris paribus in demand definitions.
3. Misapplying elasticity formulas: Always use percentage changes, not absolute values.
4. Ignoring units in budget constraints (e.g., mixing Rs. and quantities).

Marks-Boosting Strategies

• For demand curves: Always label axes, equilibrium point, and shifts (dashed lines).
• For elasticity: Show calculations and interpretation (e.g., "Since $E_d=0.8$, demand is inelastic; a 10% price hike increases revenue").
• For consumer choice: Draw budget lines and indifference curves with clear equilibrium points.
• Real-world links: Relate every example to Nepalese markets (e.g., "Like Daraz’s discounts, Ncell’s promotional plans exploit elastic demand").

Sample Answer Structure (10 Marks)

Question: "Explain the determinants of demand with examples from Nepal." Answer:

1. Income (Normal vs. inferior goods):
   • Example: Demand for smartphones (normal good) rises as income grows (post-earthquake reconstruction).
2. Prices of related goods:
   • Example: If petrol prices rise, demand for cars falls (complements).
3. Consumer preferences:
   • Example: Khalti’s popularity shifted demand from bank transfers to digital wallets.
4. Future expectations:
   • Example: Before monsoon, demand for umbrellas rises due to expected rain.
5. Number of buyers:
   • Example: Remittance growth increased demand for gold and real estate.

Visual: Include a demand curve shift diagram for one determinant (e.g., income increase → rightward shift).

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6. Quick Revision Table

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7. Practice Questions (Exam-Style)

1. Short Answer (5 marks):

   • "A 10% increase in the price of petrol leads to a 5% decrease in quantity demanded. Calculate PED and state whether demand is elastic or inelastic."
   • Answer: $E_d=-0.5$; inelastic (since $|E_d|<1$).

2. Diagram-Based (7 marks):

   • "Draw a demand curve for wheat in Nepal. Show the effect of a 20% increase in income on demand, assuming wheat is a normal good."
   • Answer: Rightward parallel shift of the demand curve.

3. Application (10 marks):

   • "Khalti observes that a 15% discount on transactions increases usage by 25%. Calculate PED and explain how Khalti can use this to maximize revenue."
   • Answer: $E_d=1.67$ (elastic); Khalti should increase discounts to boost quantity and revenue.