S1 Science | Energy Forms and Conversion: Efficiency, Power and Electricity Bills

2026-11-12

Every student has heard of saving energy, but few can say how much they saved. This lesson turns energy into countable units, from the efficiency of a lamp to the number on an electricity bill.

Objectives — By the end of 45 minutes students can list at least six energy forms, write a full conversion chain, calculate potential and kinetic energy, work out the efficiency of a device, and turn power and time into units of electricity and cost.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Unit sense	How many metres are in a kilometre?	They say 100 but then mix up 1000
Percentages	What percentage of 500 is 100?	They answer 5 per cent, forgetting to multiply by 100
Decimals	What is 2 times 1.5?	Usually right, but units get dropped

02 Lesson flow (45)

Time	Stage	Tutor does	Student does
0-5 min	Pull back a toy car	Pull back, release and ask where the energy came from	Say that pulling stored it
5-14 min	Seven forms	List seven forms with one example each and stress that energy changes form but is never lost	Write a three-step chain
14-24 min	Potential and kinetic	2 kg raised 5 m gives 100 J; at 4 m/s the same mass has 16 J; compare the two	Do one calculation alone
24-35 min	Efficiency	A lamp takes 500 J and gives 100 J of light, so 20 per cent, with 400 J lost as heat	Calculate and name where the loss goes
35-41 min	Power and cost	A 12 W lamp for 25 h is 0.3 units and 0.36 dollars; a 2 kW kettle for 15 min is 0.5 units and 0.60 dollars	Find the units and the cost
41-45 min	Exit check	Three questions: is energy lost, the efficiency formula, what one unit means	Answer orally

03 Teaching script (read it out)

Scene 1: Minutes 5-14 | Energy forms

Ask students to call out every kind of energy they have heard of and fill half the board, then group the answers into seven: kinetic, potential, thermal, light, sound, chemical and electrical. Each needs a concrete example plus the change involved: a torch runs chemical to electrical to light and heat, a cyclist downhill turns potential into kinetic. State the rule plainly: energy is never destroyed, only converted, which is conservation. Ask where the energy goes after friction and let them discover that it becomes heat in the surroundings, which is why no machine is perfect.

Students write a three-step conversion chain and state conservation

Scene 2: Minutes 14-24 | Potential and kinetic

Write PE = mgh and KE = half m v squared. With m = 2 kg, h = 5 m and g = 10, PE is 100 J. Then ask for the kinetic energy at 4 m/s: half times 2 times 16 is 16 J. Challenge them: if the object fell from 5 m, the kinetic energy at the bottom should be about 100 J, so why does 4 m/s give only 16 J? Let them find that the speed must be larger, and confirm it is about 10 m/s giving roughly 100 J. This is the hinge of the lesson, where formulas meet conservation.

Students compute both quantities and reconcile the numbers

Scene 3: Minutes 24-35 | Efficiency

Write efficiency as useful output over total input times 100 per cent. The lamp takes 500 J of electricity and gives 100 J of light, so 20 per cent, and the other 400 J becomes heat, which is why the shade is warm. Compare an LED that gives the same 100 J of light from 125 J, at 80 per cent. Ask why old bulbs waste so much and make them answer with the word efficiency. Insist that answers give a percentage and say where the lost energy went, because that is a common lost mark.

Students calculate efficiency and state that the loss becomes heat

Scene 4: Minutes 35-45 | Power and cost

Power is energy per second, in watts. One unit of electricity is 1 kWh, meaning 1000 W for one hour. A 12 W lamp for 25 hours is 12 times 25 divided by 1000, which is 0.3 units, costing 0.36 dollars at 1.2 dollars per unit. A 2000 W kettle for 15 minutes is 0.5 units and 0.60 dollars. Ask which costs more and draw out that 15 minutes of a kettle beats 25 hours of a lamp. Close with: saving electricity is power multiplied by time, not just switching off lights.

Students compute units and cost and compare two appliances correctly

04 Question types and worked steps

Type	Example	Steps	Answer
Potential energy	A 3 kg mass is raised 4 m. Find its potential energy with g = 10.	1. PE = mgh. 2. 3 x 10 x 4. 3. Answer in joules.	120 J
Kinetic energy	A 2 kg bicycle moves at 3 m/s. Find its kinetic energy.	1. KE = half m v squared. 2. 0.5 x 2 x 9. 3. Answer in joules.	9 J
Efficiency	A fan takes 60 J and delivers 45 J of motion. Find the efficiency.	1. 45 over 60. 2. Multiply by 100. 3. Give a percentage.	75 per cent
Cost	A 1500 W heater runs for 2 hours at 1.2 dollars per unit. Find the cost.	1. 1500 x 2 divided by 1000 is 3 units. 2. 3 x 1.2.	3.60 dollars

Sanity check (teach this): Three sanity checks: efficiency must be below 100 per cent, and anything above means the fraction is upside down; useful energy plus losses must equal the input; convert watts to kilowatts before costing, or the bill comes out a thousand times too high.

05 Independent practice (3 questions)

Question	Answer	What to watch
A 5 kg mass is raised 2 m. Find the potential energy.	100 J	Did they mix g = 9.8 with g = 10
A motor takes 400 J and outputs 300 J. Find the efficiency and the loss.	75 per cent, with 100 J lost as heat and sound	Did they give only a percentage
A 100 W fan runs 8 hours a day for 30 days at 1.2 dollars per unit. Find the cost.	24 units and 28.80 dollars	Did they forget the days or the division by 1000

06 Common mistakes and what to say

Mistake	Student writes	Say this
Believing energy runs out	Once the electricity is used, it is gone	Energy is not destroyed; it becomes heat and sound and spreads out. That is why we say wasted, not gone.
Efficiency above 100 per cent	Input 300 J, output 400 J, written as 133 per cent	Output must be smaller than input. 133 per cent means the fraction is upside down.
Confusing power and energy	A 2000 W appliance must use more electricity than a 12 W one	Multiply by time: 2000 W for 15 minutes is 0.5 units, while 12 W for 25 hours is only 0.3 units.
Forgetting the division by 1000	A 12 W lamp for 25 hours written as 300 units	Convert watts to kilowatts first; that number would run a whole flat for a year.

07 Homework (paper, section and question numbers)

Secondary Science hub — Other chapters in the same subject; finish the three practice items first
Same subject: S1 heat and temperature — Every loss in this chapter ends up as heat
Same subject: S3 forces and simple machines — Machine efficiency follows the same logic

08 Exit check (last 5 minutes)

Question	Answer	Decision
Can energy be destroyed?	No, it changes form and the total is conserved	If they say it runs out, repeat friction turning to heat
How is efficiency calculated?	Useful output over total input times 100 per cent	If they forget the 100, redo the lamp
What is one unit of electricity?	1 kWh, or 1000 W for one hour	If unclear, recalculate the kettle

09 Tutor FAQ

Should g be 9.8 or 10?

Use 10 in junior science unless the question gives 9.8. Warn students never to switch mid-question, or the numbers stop matching.

Is a proof of conservation needed?

No. At S1 an accounting approach is enough, and the electricity bill example convinces faster than theory.

Which tariff should be used?

Any stated rate works; the method is power times time divided by 1000, then times the rate. Real tariffs change and need not be memorised.

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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.