Demand and Supply

Demand and Supply

🟢 Lite — Quick Review (1h–1d)

Demand = quantity consumers will buy at each price (given income, tastes, related goods prices, expectations, number of buyers). Supply = quantity producers will sell at each price (given costs, technology, government policies, expectations, number of sellers).

Law of Demand: Inverse P–Qd relationship, ceteris paribus. Law of Supply: Direct P–Qs relationship, ceteris paribus.

Equilibrium price clears the market where Qd = Qs. Below it → shortage pushes price up; above it → surplus pushes price down.

Price Elasticity of Demand: Ed = (ΔQ/ΔP) × (P/Q). Midpoint formula: Ed = [(Q₂−Q₁)/(Q₂+Q₁)] ÷ [(P₂−P₁)/(P₂+P₁)]. Ed > 1 = elastic (price fall ↑ total revenue); Ed < 1 = inelastic (price rise ↑ total revenue).

RBI Grade B pointers: (1) Identify which curve shifts and new equilibrium — curves often drawn with parallel shifts; (2) Always check units and signs in elasticity calculations; (3) Normal goods have income elasticity > 0; inferior goods < 0.

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🟡 Standard — Regular Study (2d–2mo)

Law of Demand: Mechanism and Exceptions

The Law of Demand states that, holding all else constant (ceteris paribus), as price rises, quantity demanded falls. This inverse relationship operates through two effects: the substitution effect (the good becomes relatively more expensive vs. substitutes) and the income effect (higher price reduces real purchasing power, lowering effective buying capacity).

Two important exceptions exist in micro theory. Veblen goods (luxury goods like premium watches) show a direct price-demand relationship — higher prices signal status and boost demand. Giffen goods are inferior staples where a price increase forces consumers to buy more of that good because they cannot afford better alternatives.

Law of Supply: Mechanism

The Law of Supply establishes a direct, positive relationship: as price rises, quantity supplied increases. Producers respond to higher prices because it becomes profitable to expand output using existing capacity, while new producers enter the market. The supply curve is upward-sloping for this reason.

Equilibrium: The Market-Clearing Mechanism

Equilibrium occurs where the demand and supply curves intersect. At this price (P*), quantity demanded equals quantity supplied — no inherent pressure for price to change. A price below P* creates excess demand (shortage), bidding price upward; a price above P* creates excess supply (surplus), pushing price downward.

Price Elasticity of Demand (PED)

The midpoint/mid-arc formula provides a consistent elasticity measure regardless of direction:

Ed = [(Q₂ − Q₁) / (Q₂ + Q₁)] ÷ [(P₂ − P₁) / (P₂ + P₁)]

• |Ed| > 1: Elastic — percentage change in Qd exceeds percentage change in P
• |Ed| = 1: Unit elastic
• |Ed| < 1: Inelastic — percentage change in Qd is smaller than in P

This has direct implications for Total Revenue (TR = P × Q): for elastic demand, lowering price raises TR; for inelastic demand, raising price raises TR.

Factors Influencing PED

1. Necessity vs. luxury — necessities (medicine, electricity) are inelastic; luxuries are elastic
2. Availability of substitutes — more close substitutes → more elastic
3. Proportion of income spent — higher share → more elastic
4. Time period — longer run → more elastic as consumers adjust
5. Definition of the good — broader definitions (food) are more inelastic than narrow ones (mangoes)

Factors That Shift the Demand Curve (Not Price)

• Change in income (normal goods shift right, inferior goods shift left)
• Change in tastes and preferences
• Change in price of related goods (substitutes and complements)
• Change in expectations (future price or income)
• Change in number of buyers

Factors That Shift the Supply Curve (Not Price)

• Change in input costs (raw materials, wages)
• Change in technology
• Change in government policies (taxes, subsidies)
• Change in producer expectations
• Change in number of sellers

Exam Pattern for RBI Grade B

Economics carries roughly 3% weight in the Phase-II General paper, translating to approximately 4-5 questions from demand-supply analysis. Recent RBI Grade B papers have featured case-based questions requiring simultaneous shift analysis and numerical elasticity calculations. Expect one question requiring a demand/supply graph with both axes labeled and initial and new equilibrium marked.

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🔴 Extended — Deep Study (3mo+)

Shifts vs. Movements: The Critical Distinction

This is the most-tested conceptual trap in competitive exams. A movement along the demand curve is caused only by a change in the good’s own price — price rises, point slides leftward (quantity demanded falls); price falls, point slides rightward (quantity demanded rises). This is a change in quantity demanded.

A shift of the demand curve occurs when any non-price determinant changes — income rises, taste shifts in the product’s favour, a substitute’s price rises, or consumer numbers increase. The entire curve shifts rightward (increase in demand) or leftward (decrease in demand). At each and every price, a different quantity is now demanded. This is a change in demand.

The same distinction applies to supply: price changes cause movements along the supply curve; non-price factors cause the supply curve to shift.

Cross Elasticity of Demand

Cross elasticity measures how demand for good A responds to a price change in good B:

Eₓᵧ = (% change in Qd of good A) / (% change in price of good B)

• Positive cross elasticity → substitutes (price of Pepsi rises → demand for Coca-Cola rises)
• Negative cross elasticity → complements (price of printers rises → demand for ink cartridges falls)
• Zero → unrelated goods

RBI questions often test whether students can correctly pair goods — confusing substitutes and complements is a common error.

Income Elasticity of Demand

Income elasticity classifies goods:

• Normal goods: Eᵧ > 0 — demand rises as income rises (clothing, dining out)
• Inferior goods: Eᵧ < 0 — demand falls as income rises (cheap instant noodles, public transport in high-income brackets)
• Luxury goods: Eᵧ > 1 — demand grows faster than income (designer bags, premium travel)

This classification is relative and can change with income levels — a motorcycle may be a normal good for low-income consumers but an inferior good for high-income consumers who upgrade to cars.

Consumer and Producer Surplus: Welfare Analysis

Consumer surplus is the area between the demand curve and the equilibrium price, up to the quantity purchased. It equals the maximum amount a consumer would pay minus what they actually pay — the “bargain” benefit. For example, if a student would pay ₹500 for a textbook but pays ₹350, consumer surplus = ₹150 per book.

Producer surplus is the area between the supply curve and the equilibrium price, up to the quantity sold. It equals actual price minus the minimum price the producer would accept. This represents the benefit producers receive from participating in the market.

Combined, they measure total welfare or social surplus. Any market distortion — price floor, price ceiling, tax — reduces this surplus, creating deadweight loss. RBI descriptive questions frequently ask candidates to shade these areas on a graph and calculate total welfare.

Common Mistakes to Avoid

1. Blurring movement and shift — if the question mentions income, tastes, costs, or technology, the curve shifts; only price changes cause movement along the curve.
2. Forgetting ceteris paribus — any elasticity or demand-supply statement assumes other factors held constant; violating this assumption invalidates the analysis.
3. Sign errors in elasticity — income elasticity for inferior goods must be negative; cross elasticity for complements must be negative.
4. Using arithmetic mean instead of midpoint — the midpoint formula requires averaging both quantity and price pairs before computing the ratio.
5. Confusing normal vs. inferior classification — a rising income that reduces demand for a good makes it inferior; a rising income that increases demand makes it normal.

Practice Prompts

1. Shift analysis prompt: Suppose the government imposes a per-unit subsidy on electric vehicle manufacturers. Using demand-supply analysis, show what happens to the equilibrium price and quantity in the EV market. Explain which curve shifts, by how much, and why the new equilibrium is determined at the intersection of the new supply curve and the unchanged demand curve. Label all axes and curves clearly.

2. Elasticity and revenue prompt: A restaurant raises the price of its signature dish from ₹400 to ₹500, and quantity sold falls from 200 to 150 units per day. Calculate the price elasticity of demand using the midpoint formula. Is this elastic or inelastic? What happened to the restaurant’s daily total revenue, and what does this imply about the restaurant’s pricing strategy for maximizing revenue?

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