IB Economics: Market Failure and Government Intervention - Tutor Lesson Plan (external cost, tax, deadweight loss)

IB Economics: Market Failure and Government Intervention – Tutor Lesson Plan (external cost, tax, deadweight loss)

2026-11-05

Students can draw the two cost curves yet freeze when asked why the welfare triangle equals 18. One fixed numerical question ties external cost, tax and triangle together.

Objectives — the student can write MSC from MPC plus an external cost, solve for the free-market and social optimum quantities, and read the deadweight loss as a triangle that matches the unit tax.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Linear equations	Solve 20 + 0.5Q = 50 – 0.5Q	30
Triangle area	Area of a triangle with base 6 and height 6	18
Law of demand	What happens to quantity demanded when price rises?	It falls

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Two cost curves	The difference between MPC and MSC	Name where the external cost sits
8-22 min	Free-market equilibrium	20 + 0.5Q = 50 – 0.5Q	Get Q 30 and P 35
22-36 min	Social optimum	Add 6 to MSC and solve again	Get Q 24 and P 38
36-48 min	Welfare loss	Triangle with base 6 and height 6	Get 18
48-56 min	Policy tools	Unit tax, subsidy, regulation	Match three tools
56-60 min	Exit check	Three questions marked	Finish alone

03 Teaching script (read it out)

Scene 1: External cost shifts the whole MSC curve up

With MPC = 20 + 0.5Q and an external cost of 6 per unit, MSC = 26 + 0.5Q. The slope is unchanged and the whole line moves up by 6.
Make the student say which line moved: the cost line shifts, demand stays, so the equilibrium quantity falls rather than rises.

Student explains why the slope does not change (the external cost per unit is constant).

Scene 2: Two equilibria: solve first, then discuss policy

Free market: 20 + 0.5Q = 50 – 0.5Q gives Q = 30 and P = 35.
Social optimum: 26 + 0.5Q = 50 – 0.5Q gives Q = 24 and P = 38. The market over-produces by 6 units, and that gap is the size of the failure.

Note: both solutions use the same demand line and differ only in the cost line; swapping the wrong line is the classic error.

Scene 3: Welfare loss: base is the quantity gap, height is the external cost

Base 30 – 24 = 6, height 6, area 0.5 times 6 times 6 = 18.
Link it to policy: a unit tax of 6 lifts the producer cost onto MSC, so output falls back to 24 without any quota.

Student calculates the triangle if the external cost rises to 8 (0.5 times 5 times 8 = 20).

04 Question types and worked steps

Type	Example	Steps	Answer
Equilibrium quantity	20 + 0.5Q = 50 – 0.5Q	Set cost equal to demand and solve	Q = 30
Social optimum	Add 6 to MSC and solve	Write MSC before solving	Q = 24, P = 38
Welfare loss	base 6, height 6	Half base times height	18
Policy matching	tax, subsidy, regulation	Decide whether the externality is positive or negative	A negative externality takes a tax

Sanity check (teach this): The welfare triangle always sits between the free-market quantity and the social optimum. A triangle starting at zero means the whole consumer surplus was mistaken for the loss.

05 Independent practice (3 questions)

Question	Answer	What to watch
Free-market Q and P	Q 30, P 35	Used MPC, not MSC
Social optimum Q	24	Added the 6 to MSC
Welfare loss area	18	Base is the quantity gap, not the total quantity

06 Common mistakes and what to say

Mistake	Student writes	Say this
MSC drawn steeper	quantity computed as 20	A constant external cost per unit shifts the line up and leaves the slope alone.
Base of 30 used	area computed as 90	The base is the difference between the two quantities, which is 6.
Tax picked at random	tax set at 3	The tax must equal the marginal external cost of 6 for output to return to 24.
Only price mentioned	no fall in quantity	The effect of the tax is on quantity; the price change is a result.

07 Homework (paper, section and question numbers)

IB economics chapter index — Use the three questions on this page

08 Exit check (last 5 minutes)

Question	Answer	Decision
Free-market Q and P	Q 30, P 35	All three right: move to evaluating intervention
Social optimum Q	24	One wrong: re-teach the MSC shift
Welfare loss	18	Two wrong: repeat scene 3

09 Tutor FAQ

Must the diagram be drawn?

Almost always, and both quantities plus the triangle must be labelled.

Tax or regulation?

IB wants evaluation: a tax raises revenue but is hard to set, regulation is clear but rigid.

What goes in the evaluation?

Information gaps, the size of the elasticity, and the difficulty of measuring the external cost.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.