Consumer Surplus & PED

— Economics · 8-mark "Analyse" question —

★ Exam Tip: This is an 8-mark question: K = 3m, A = 3m, E = 2m. Define first, then show the effect of elastic and inelastic demand each with a diagram + area, and finish with a conclusion.

1. The Question & How to Attack It

Q18 marks · analyse

Analyse, using Price Elasticity of Demand, how consumer surplus changes if demand is elastic and inelastic.

Mark split: AO1 → K → 3m · AO2 → A → 3m · AO3 → E → 2m (K = knowledge, A = application/analysis, E = evaluation)

Board slip: the page-1 plan gives A → 3m but the last page writes A → 2m. K 3 + A 2 + E 2 = 7, not 8 — so 3 + 3 + 2 = 8 is the split that adds up.

Answer plan (in this order)

Define PED + give its formula
Define Consumer Surplus + its formula
Define elastic / inelastic demand
Elastic demand → C.S. ↑ increases → diagram + area
Inelastic demand → C.S. ↓ decreases → diagram + area
Conclusion → willingness can change for an individual → advertisement, taste / patterns

the six-step skeleton of the answer

2. What is Consumer Surplus (C.S.)?

Definition: Consumer surplus is the difference between the price you are willing to pay and the price you actually pay.

FormulaC.S. = Willingness (W) − Actual Price (AP)

Worked example (with graph)

C.S. = triangle P A a — above price level, below the demand curve

C.S. = W − AP W = $200 AP = $170 C.S. = 200 − 170 C.S. = $30

C.S. = area PAa
Willingness (W) = area 0Aaq
Actual price paid (AP) = area 0Paq

Note: C.S. is the area below the demand curve and above the price level — it can never sit below the price line.

3. Price Elasticity of Demand (PED)

Definition: PED measures the responsiveness of the % change in quantity demanded caused by a % change in price.

FormulaPED = % Δ in Q_D% Δ in Price

	Elastic demand	Inelastic demand
PED	greater than 1 (PED > 1)	less than 1 (PED < 1)
Responsiveness	high	low
Substitutes	more available	fewer / less available
Switching if price changes	consumers have more chances to switch	consumers have fewer chances to switch

4. C.S. with Elastic Demand

Substitutes ↑

consumers less willing to pay higher prices

typically a luxury product

Price ↓ 10%, Q_D ↑ 50%, Total Revenue (TR) ↑

C.S. INCREASES

Diagram 1 — price falls P→P₁, C.S. grows from PAa to P₁Ab

Say it: elastic ⇒ price cut brings a big rise in quantity ⇒ TR ↑ and C.S. ↑.

Area: before the change C.S. = PAa; after it C.S.₁ = P₁Ab (bigger triangle, because the price line has dropped to P₁). The demand curve is flat — Q_D reacts strongly (50%) to a small price change (10%).

Why: with more substitutes consumers are less willing to pay a higher price than the selling price (compared with inelastic demand), so the surplus ends up with the consumer rather than being turned into producer revenue.

Careful — board wording clash: the board's written elastic paragraph says is "reducing" C.S., but the plan, the table and Diagram 1 all show C.S. INCREASE (P₁Ab bigger than PAa). Go with the diagram: elastic → C.S. ↑.

5. C.S. with Inelastic Demand

Substitutes ↓

consumers more willing to pay higher prices

typically a necessity product (habitual in nature)

Price ↑ 50%, Q_D ↓ 10%, Total Revenue (TR) ↑

C.S. DECREASES

Diagram 2 — price rises P→P₁, C.S. shrinks from PAa to P₁Ab

Say it: inelastic ⇒ price rise barely dents quantity ⇒ TR ↑, but C.S. ↓ (turned into producer revenue).

Area: before C.S. = PAa (large triangle); after C.S.₁ = P₁Ab (smaller, price line is higher). The demand curve is steep — Q_D reacts only a little (10%) to a big price change (50%).

Why: the producer can raise price by a larger percentage, which contracts quantity demanded by a smaller percentage ⇒ total revenue ↑ and consumer surplus ↓ from PAa to P₁Ab. With fewer substitutes, consumers are ready to pay more than the selling price, so the C.S. can be converted into producer revenue by charging higher prices.

Careful — board wording clash: the opening line of the board's inelastic paragraph says "increasing consumer surplus", but the next lines, the table and Diagram 2 all show C.S. DECREASE (P₁Ab smaller than PAa). Use inelastic → C.S. ↓.

6. Side-by-Side: Elastic vs Inelastic

	C.S. with elastic demand	C.S. with inelastic demand
Substitutes	↑ more	↓ fewer
Willingness to pay higher prices	less willing	more willing
Type of product	luxury	necessity
Price / Q_D / TR	Price ↓ (10%), Q_D ↑ (50%), TR ↑	Price ↑ (50%), Q_D ↓ (10%), TR ↑
C.S.	INCREASE: PAa → P₁Ab ↑	DECREASE: PAa → P₁Ab ↓
Demand curve	flat curve (F.C.)	steep curve (S.C.)

Quick relation: in both cases TR rises — elastic: big Q↑ outweighs the small price cut; inelastic: big price rise outweighs the small Q↓. The difference is who keeps the extra: the consumer (C.S. ↑) or the producer (C.S. ↓).

7. The Extreme Cases

★ Perfectly elastic demand

1000s of substitutes — same product, same price
Firm is a price taker
Price — (no change), Q_D ↑ (10%), TR ↑ (10%)
C.S. = ZERO → Willingness = Actual price (W = AP)

★ Perfectly inelastic demand

No substitute available — a monopoly, firm is a price maker
Price ↑ (10%), Q_D — (no change), TR ↑ (10%)
Consumers willing to pay higher prices as there is no substitute
C.S. = INFINITE → willingness cannot be determined in numbers

perfectly elastic: horizontal D, no area above the price

perfectly inelastic: vertical D, surplus is unbounded

Reason (perfectly elastic): perfect substitutes exist, so the amount a person is willing to pay and the amount actually paid are equal ⇒ C.S. = 0.

Reason (perfectly inelastic): no substitutes, so the amount the person is willing to pay cannot be determined in numbers ⇒ C.S. is infinite.

8. Additional: Marginal Consumer Surplus

Example — buying juice one glass after another at an unchanged price of $5:

Juice no.	1	2	3	4	5	6
W (willingness)	$10	$9	$8	$7	$6	$5
A (actual price)	$5	$5	$5	$5	$5	$5
C.S.	$5	$4	$3	$2	$1	$0

Chain: Quantity ↑ → willingness ↓ → satisfaction ↓ → marginal C.S. ↓

Each extra unit consumed gives less willingness and less satisfaction, so the surplus on that extra unit — the marginal consumer surplus — keeps falling.

At the 6th glass (circled on the board): Willingness = Actual price ⇒ C.S. = 0.

blue bar = willingness, green part above the $5 line = C.S. — shrinks to 0 at glass 6

Link to PED: when the surplus on each extra unit shrinks to zero (willingness = price) this is the perfectly elastic situation.

9. Model Answer — How to Write It (8 marks)

Knowledge (3m) — definitions

Price Elasticity of Demand measures the responsiveness of % change in quantity demanded due to a % change in price. Formula: PED = %ΔQ_D ÷ %ΔPrice.

Consumer surplus is the difference between the price you are willing to pay and what you actually pay. Formula: C.S. = Willingness − Actual Price.

Elastic demand is when PED > 1 — high responsiveness, more substitutes available, so consumers have more chances to switch if prices change. Inelastic demand is when PED < 1 — low responsiveness, fewer substitutes, so consumers have less chance to switch.

Application / analysis (3m) — both cases + diagrams

Elastic paragraph: more substitutes, product is a luxury ⇒ consumers are less willing to pay higher prices than the selling price ⇒ producer cuts price, Q_D rises by a larger %, TR ↑, C.S. rises from PAa to P₁Ab as shown in Diagram 1.

Inelastic paragraph: fewer substitutes, product is a necessity and habitual ⇒ consumers are more willing to pay higher prices ⇒ producer raises price by a larger %, Q_D contracts by a smaller %, TR ↑, C.S. falls from PAa to P₁Ab as shown in Diagram 2.

Contrast sentence: in elastic demand consumers are less willing than the selling price (surplus stays with consumers); in inelastic demand they are ready to pay more than the selling price, so C.S. is converted into producer revenue.

Extremes: perfectly elastic — perfect substitutes, willingness = actual price, so C.S. = 0. Perfectly inelastic — no substitutes, willingness cannot be measured in numbers, so C.S. is infinite.

Evaluation (2m) — conclusion

Conclusion: "To conclude, in my opinion consumer surplus can change due to a change in the willingness of an individual, as it can be influenced by advertisements and by changes in consumers' tastes and patterns."

Why it earns the E marks: it steps beyond PED alone — willingness to pay itself is not fixed, so the demand curve can move and C.S. changes even when price does not.

10. Quick Recap — Full Cheat-Sheet

✓ Formulas & Definitions

C.S. = Willingness − Actual Price (difference between what you'd pay and what you pay)
PED = %ΔQ_D ÷ %ΔPrice
Elastic: PED > 1 (more substitutes, high responsiveness) · Inelastic: PED < 1 (fewer substitutes, low responsiveness)
Areas: C.S. = PAa, W = 0Aaq, AP = 0Paq · C.S. is always below demand, above price
Worked: 200 − 170 = $30

Case	Substitutes	Price / Q_D / TR	C.S.	Curve
Elastic	more	P ↓10%, Q ↑50%, TR ↑	increases (PAa → P₁Ab ↑)	flat (F.C.)
Inelastic	fewer	P ↑50%, Q ↓10%, TR ↑	decreases (PAa → P₁Ab ↓)	steep (S.C.)
Perfectly elastic	1000s, same product & price	P —, Q ↑10%, TR ↑10% · price taker	zero (W = AP)	horizontal (H.C.)
Perfectly inelastic	none · monopoly	P ↑10%, Q —, TR ↑10% · price maker	infinite	vertical (V.C.)

✓ Remember for the exam

8 marks = K 3 + A 3 + E 2: define → elastic (diagram + area) → inelastic (diagram + area) → conclude
Marginal C.S. falls as you consume more (juice: $5, 4, 3, 2, 1, 0) — zero when W = actual price
Conclusion line: willingness can change → advertisement, taste / patterns
If your paragraph wording and your diagram disagree, trust the diagram: elastic ↑ / inelastic ↓