The shape of the lesson
By the end of the lesson, learners can
- Watch the wire jump: Send current through the wire in the field and see which way the force pushes it.
- Size and direction: The size depends on current and field strength; the direction follows the left-hand rule.
- The effect: a current-carrying wire in a field feels a force.
- Reversal: reverse the current OR the field and the force reverses.
- Size: bigger with more current or a stronger field; largest at right angles.
One picture every idea returns to
A wire carrying a current in a magnetic field experiences a force, the motor effect. The force is larger with a bigger current or a stronger field, and it reverses if the current or the field is reversed. The left-hand rule relates the directions of force, field and current.
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, A force from a current.. Lay a current-carrying wire between two magnets and it jumps. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Watch the wire jump | Send current through the wire in the field and see which way the force pushes it. Project tab 3 of the lab (Motor effect) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Size and direction | The size depends on current and field strength; the direction follows the left-hand rule. | Think-Pair-Share |
| 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, 13 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 Size and direction, 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.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
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 |
|---|---|
| The force is greatest when the current is at right angles to the field, and zero when the current runs along the field. | Reversing either the current or the field reverses the force, but reversing both leaves it unchanged. |
The motor effect: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Describe a simple experiment to show that a force acts on a wire carrying a current in a magnetic field.
State two changes that would increase the size of the force on the wire.
A wire is pushed to the left when a current flows. State two separate changes, each of which on its own would make the wire push to the right instead.
A wire carrying a current is placed so that it lies along the direction of the magnetic field. State the size of the force on the wire and explain your answer.
Explain how the motor effect is used in a loudspeaker to produce sound.
Extended: In a motor-effect demonstration the magnetic field points from left to right and the current in the wire flows out of the page towards you. Use the left-hand rule to state the direction of the force on the wire.
Answer key and marking notes
- Place a wire between the poles of a magnet, at right angles to the field. ✓
- Pass a current through the wire. ✓
- The wire is pushed (it moves), showing a force acts on it. ✓
- Increase the current. ✓
- Use a stronger magnet (a stronger field). ✓
- Reverse the direction of the current. ✓
- Reverse the magnetic field by swapping the magnet poles. ✓
- The force is zero. ✓
- The force is largest when the wire is at right angles to the field, and zero when it lies along the field. ✓
- A changing current from the music passes through a coil in a magnetic field. ✓
- The force on the coil pushes it back and forth, moving the cone and making sound waves. ✓
- First finger right (field), second finger out of the page (current). ✓
- The thumb points upward, so the force on the wire is upward. ✓
2 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 wire carrying a current is placed between the poles of a magnet, at right angles to the field. What happens to the wire?
- AIt experiences a force, which can push it out of the field.
- BIt heats up but does not move.
- CIt becomes permanently magnetised.
- DNothing happens to it.
A current-carrying wire sits at right angles to a magnetic field and feels a force. Which change would reverse the direction of this force?
- Areversing the direction of the current
- Bincreasing the size of the current
- Cusing a longer length of wire in the field
- Dusing a stronger magnet without changing the directions
Quiz key
A current-carrying conductor in a magnetic field experiences a force (the motor effect), whose direction is given by Fleming's left-hand rule. Reversing the current or the field reverses the force.
Reversing the current (or reversing the field) reverses the force direction, as given by Fleming's left-hand rule. Increasing the current or field strength changes only the size of the force, not its direction.
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.