DSE Economics | Market Equilibrium Teacher Prep Guide | Equilibrium price, shortage and surplus in 3 steps

2026-11-04

Tutor lesson-prep guides · Secondary Economics

Students can draw two crossing lines but stop when asked how large the shortage is. This lesson pins three lines down, equilibrium, ceiling and floor, all checked with the same algebra.

Objectives — Students can: (1) compute equilibrium price and quantity from demand and supply functions; (2) compare a control price with the equilibrium and state whether a shortage or surplus appears, with its size; (3) explain in one sentence how the price mechanism restores equilibrium.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Law of demand	What happens to quantity demanded when price rises	It falls along the same curve
Law of supply	What happens to quantity supplied when price rises	It rises
Solving equations	Set two expressions equal and solve	Find P, then substitute back

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Plot the curves	Demand downward, supply upward, quantity on x and price on y	Label both axes
8-20 min	Solve equilibrium	120 – 2P = 20 + 2P gives P = 25	Solve it themselves
20-32 min	Shortage and surplus	Below equilibrium is a shortage, above is a surplus; verify with the same algebra	Compute one number
32-46 min	Price ceiling	Ceiling $20: Qd = 80, Qs = 60, shortage 20	Compute it themselves
46-56 min	Price floor	Floor $30: Qs = 80, Qd = 60, surplus 20	Contrast with the ceiling
56-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Separate a movement along from a shift

A price change moves you along the same demand curve. Income, tastes or the price of a related good shift the whole curve.
Students merge the two constantly, so a price rise is reported as a fall in demand when it is only a fall in quantity demanded.

Students explain: after a cash handout, does the demand curve shift or do we move along it, and is it demand or quantity demanded.

Scene 2: Equilibrium is an equation, not a lucky reading

Set Qd = Qs: 120 – 2P = 20 + 2P, so P = 25 and Q = 70.
Always substitute back into the other equation. That single check catches most arithmetic slips.

Students do: after solving for P, substitute into both equations to confirm the same Q.

Scene 3: Control prices: only binding ones matter

A ceiling below equilibrium ($20 against $25) binds and creates a 20 unit shortage; a floor above equilibrium ($30 against $25) binds and creates a 20 unit surplus.
A ceiling above equilibrium or a floor below it is non-binding, and the market stays at equilibrium.

Students do: fill a 2×2 table of Qd and Qs at both control prices, then decide which one binds.

04 Question types and worked steps

Type	Example	Steps	Answer
Equilibrium	Qd = 120 – 2P, Qs = 20 + 2P	Set equal, solve P, substitute	P = 25, Q = 70
Shortage	Ceiling at $20	Qd minus Qs	80 – 60 = 20 units
Surplus	Floor at $30	Qs minus Qd	80 – 60 = 20 units
Curve shift	Non-price factor changes	Decide whether the curve moves	Only a shift changes demand

Sanity check (teach this): Substitute back into the other equation: both Q values must match. A ceiling above equilibrium or a floor below it does not bind, so no shortage or surplus should be computed.

05 Independent practice (3 questions)

Question	Answer	What to watch
Find equilibrium	Set Qd = Qs, solve P, substitute	Check in both equations
Shortage and surplus	Subtract Qs from Qd, or the reverse	Shortage is Qd above Qs; surplus is Qs above Qd
Curve shifts	Demand against quantity demanded	Only non-price factors shift the curve
Policy effect	Is the control price binding	Compare with equilibrium first

06 Common mistakes and what to say

Mistake	Student writes	Say this
Swapping shortage and surplus	Calls the $20 ceiling a surplus of 20	“A ceiling holds price down, so buyers outnumber sellers. That is a shortage. Which of Qd and Qs is bigger?”
Merging demand with quantity demanded	Writes “demand falls” when price rises	“Did the curve move? If not, only quantity demanded fell, and saying demand costs marks.”
Computing with a non-binding price	Finds a shortage at a $40 ceiling	“That ceiling sits above equilibrium, so the market never reaches it. It does nothing.”
Not substituting back	Wrong P goes unnoticed	“Put it back into the other equation. If the two Q values differ, you already know.”

07 Homework (paper, section and question numbers)

Secondary Economics chapter index — All chapters and practice
DSE Economics practice paper — Demand, supply and intervention questions
Redo this chapter — Recompute equilibrium and the shortage under a $20 ceiling

08 Exit check (last 5 minutes)

Question	Answer	Decision
How to find equilibrium	Set Qd = Qs and solve for P	Saying “read the graph” means act one again
Effect of a ceiling below equilibrium	A shortage of 20 units	Saying surplus means act three again
Price rises: demand or quantity demanded	Quantity demanded, a movement along	Saying demand means act one again

09 Tutor FAQ

Must I use algebra?

With functions, yes: it is faster and safer than eyeballing a graph. Pure diagram questions need areas instead.

Which control price causes queues?

The ceiling, because the suppressed price leaves quantity demanded above quantity supplied.

Is the black market price above or below the ceiling?

Above, because buyers bid up when supply is short.

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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.