The shape of the lesson
By the end of the lesson, learners can
- Same job, different speed: Set the energy to transfer and the time taken, then run it.
- Rate, not amount: The defining equation and its rearrangement for energy:
- Power: the rate of doing work or transferring energy.
- Unit: watt (W), where 1 W = 1 J/s.
- Rearrange: energy = power × time; time = energy ÷ power.
One picture every idea returns to
Power is the rate of doing work, or the rate of transferring energy: P = W ÷ t = ΔE ÷ t. It is measured in watts (W), where one watt is one joule per second.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Energy, but how fast.. Two motors can lift the same crate to the same shelf, transferring the same energy, yet one finishes in a second and the other takes a minute. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Same job, different speed | Set the energy to transfer and the time taken, then run it. Project tab 5 of the lab (Power) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Rate, not amount | The defining equation and its rearrangement for energy: Model the worked example Worked example on the board, one line at a time. | Rally Coach |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Rate, not amount, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Power is not the same as energy. | Energy, in joules, is the amount transferred; power, in watts, is how quickly it is transferred. Two machines can transfer exactly the same energy while having very different powers, simply because one takes longer than the other. Always divide the energy by the time. |
Power (P = W/t): worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Define power, write an equation for it, and state its unit.
A motor does 900 J of work in 15 s. Calculate its power.
A 60 W lamp is left on for 120 s. Calculate the energy it transfers.
A crane transfers 6000 J of energy to a load in 12 s. Calculate the power of the crane.
Two motors do the same amount of work. Motor A finishes in 5.0 s and motor B in 10 s. State which motor is more powerful and explain why.
A 200 W device transfers 5000 J of energy. Calculate the time this takes.
Answer key and marking notes
- Power is the rate of doing work, or rate of transferring energy. ✓
- P = W ÷ t (= ΔE ÷ t); unit watt (W), 1 W = 1 J/s. ✓
- P = W ÷ t = 900 ÷ 15
60 W
- E = P × t = 60 × 120
7200 J
- P = ΔE ÷ t = 6000 ÷ 12
500 W
- Motor A is more powerful. ✓
- It does the same work in less time, and P = W ÷ t, so a smaller time gives a greater power. ✓
- t = E ÷ P = 5000 ÷ 200
t = 25 s
1 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A motor transfers 1200 J of energy in 8.0 s. What is its power?
- A0.0067 W
- B150 W
- C1200 W
- D9600 W
Quiz key
P = E ÷ t = 1200 ÷ 8.0 = 150 W. Option C ignores the time; D multiplies energy by time.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.