Demand and Supply

Demand and Supply

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Demand = quantity consumers are willing and able to buy at each price, ceteris paribus (other things unchanged). Supply = quantity producers are willing and able to sell at each price, ceteris paribus.

Law of Demand: Price ↑ → Quantity Demanded ↓ (inverse relationship). Law of Supply: Price ↑ → Quantity Supplied ↑ (direct relationship).

Equilibrium: Where Qd = Qs. Price where no tendency to change.

PED Formula (2 marks in CA Foundation): PED = (ΔQ/Q) ÷ (ΔP/P). Interpret values: E>1 = elastic, E<1 = inelastic, E=1 = unitary.

Key distinction for exams: Movement along the curve = change in quantity demanded/supplied (due to price change). Shift of the curve = change in demand/supply (due to other factors like income, technology).

CA Foundation exam pattern: Expect calculation of equilibrium price/quantity and PED from given data. Watch for shift-versus-movement questions in MCQs — a very common trap.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Law of Demand

The law states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, and vice versa. Two effects drive this:

1. Substitution effect: Good becomes relatively more expensive than substitutes, so consumers switch away.
2. Income effect: Higher price reduces real purchasing power, lowering quantity demanded.

Law of Supply

As price rises, suppliers offer more quantity (ceteris paribus). Higher price increases producer profitability per unit, incentivising greater output.

Movement vs. Shift — The Critical Distinction

Cause	Effect on Curve
Price of good changes	Movement along the curve (Qd or Qs changes)
Income, tastes, technology, input costs change	Shift of the entire curve (demand or supply changes)

Confusing these is the most common error in CA Foundation exams.

Price Elasticity of Demand (PED)

PED measures consumer sensitivity to price changes:

• Point elasticity: PED = (dQ/dP) × (P/Q) — use when data from a specific point
• Arc elasticity: PED = [(Q₂ − Q₁)/((Q₂+Q₁)/2)] ÷ [(P₂ − P₁)/((P₂+P₁)/2)] — use when comparing two points

Arc elasticity uses the midpoint formula, avoiding dependence on which point is the starting point.

Determinants of Demand and Supply

Demand shifts: Income (normal vs. inferior goods), tastes and preferences, price of related goods (substitutes/complements), expectations, number of buyers.

Supply shifts: Input costs, technology, expectations, number of sellers, government policies.

Equilibrium

At equilibrium: Qd = Qs. If P is above equilibrium → Excess Supply (pressure to lower price). If P is below equilibrium → Excess Demand (pressure to raise price).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Linear Demand and Supply Functions

CA Foundation numericals frequently use linear functions:

• Demand: Qd = a − bP (intercept a, slope −b)
• Supply: Qs = c + dP (intercept c, slope +d)

To find equilibrium: set a − bP = c + dP → solve for P, then substitute back to find Q.

Types of Elasticity

Type	Coefficient	Meaning
Perfectly elastic	E = ∞	Smallest price rise → quantity demanded collapses to zero
Relatively elastic	E > 1	Proportionate Q change > proportionate P change
Unitary	E = 1	Quantity and price change in equal proportion
Relatively inelastic	0 < E < 1	Quantity change < price change
Perfectly inelastic	E = 0	Quantity demanded unchanged regardless of price

Factors Affecting PED

1. Availability of substitutes — more substitutes → more elastic
2. Necessity vs. luxury — necessities are inelastic
3. Proportion of income — higher income share → more elastic
4. Time period — longer run → more elastic (consumers adjust)

Consumer and Producer Surplus

At equilibrium, consumer surplus = area between the demand curve and the equilibrium price (welfare gain above what consumers actually pay). Producer surplus = area between the supply curve and the equilibrium price (gain above the minimum sellers would accept). Both are maximised at equilibrium.

Common Mistakes to Avoid

• Forgetting the ceteris paribus assumption when explaining a shift
• Treating income effect and substitution effect as mutually exclusive — they work simultaneously
• Applying the wrong elasticity formula (point vs. arc) for the given data
• Assuming constant elasticity along a linear demand curve — it changes at every point
• Mixing up normal goods (income ↑ → demand ↑) and inferior goods (income ↑ → demand ↓)

CA Foundation Exam Strategy

Paper 3 tests demand-supply through calculation-based questions (2–4 marks each) requiring PED computation, equilibrium solving, and curve-shift identification. Paper 1 may test conceptual understanding in the business economics section. Solve at least 5 past-year questions on equilibrium determination and 5 on elasticity calculations before the exam.

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