The shape of the lesson
By the end of the lesson, learners can
- Measure the push: Connect a voltmeter across the source and across components to compare the energy per charge.
- Given out, then used up: The source gives energy to each charge as e.m.f.; components take energy from each charge as p.d.
- Voltage: energy per coulomb, in volts (one volt is one joule per coulomb).
- e.m.f. and p.d.: e.m.f. is energy given to each coulomb; p.d. is energy given up in a component.
- Series rule: the e.m.f. equals the sum of the p.d.s. The voltmeter goes in parallel.
One picture every idea returns to
The electromotive force of a source is the electrical energy given to each unit of charge by the source. The potential difference across a component is the energy transferred from each unit of charge in it. Both are measured in volts by a voltmeter connected in parallel.
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 per charge.. A battery does not store current, it stores a push: a fixed amount of energy handed to every unit of charge that passes through. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Measure the push | Connect a voltmeter across the source and across components to compare the energy per charge. Project tab 5 of the lab (Ammeter and voltmeter) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Given out, then used up | The source gives energy to each charge as e.m.f.; components take energy from each charge as p.d. | Jigsaw |
| 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, 14 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 Given out, then used up, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| A voltmeter is connected in parallel, across the component, not in series. | The volt is a joule per coulomb, the energy carried by each unit of charge. |
EMF and potential difference: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by the potential difference across a component, and name the unit in which it is measured.
A student wants to measure the p.d. across a lamp.
(a) Name the instrument used.
(b) State how it must be connected.
A cell of e.m.f. 9.0 V is connected to two lamps in series. The p.d. across the first lamp is 4.0 V. Calculate the p.d. across the second lamp.
Explain the difference between the e.m.f. of a cell and the p.d. across a component, in terms of energy and charge.
Three components are connected in series with a battery. The p.d.s across them are 2.0 V, 3.5 V and 1.5 V. Calculate the e.m.f. of the battery, and state the principle you have used.
A student connects a voltmeter in series with a lamp, expecting it to read the p.d. across the lamp. Explain why this is wrong, and describe the correct connection. State, in energy terms, what the reading represents when done correctly.
Answer key and marking notes
- The energy transferred from each unit of charge (each coulomb) as it passes through the component. ✓
- It is measured in volts (V). ✓
(a) A voltmeter. ✓
(b) In parallel, connected across the lamp. ✓
- e.m.f. = V1 + V2, so V2 = 9.0 − 4.0
V2 = 5.0 V
- The e.m.f. is the energy the cell gives to each coulomb of charge. ✓
- The p.d. is the energy each coulomb gives up as it passes through that component. ✓
- e.m.f. = 2.0 + 3.5 + 1.5
e.m.f. = 7.0 V
conservation of energy: the e.m.f. equals the sum of the p.d.s
- A voltmeter must be connected in parallel, across the lamp, not in series. ✓
- Connected in series it would not measure the p.d. across the lamp (and would block the current). ✓
- Read correctly, it shows the energy transferred from each coulomb passing through the lamp. ✓
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 voltmeter is used to measure the potential difference across a resistor. How should it be connected?
- Ain parallel with the resistor
- Bin series with the resistor
- Cin series with the cell only
- Dacross the open switch
Quiz key
A voltmeter is connected in parallel, across the resistor, to measure the potential difference between its ends. Connecting it in series (like an ammeter) would not give the p.d. across the resistor.
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.