The shape of the lesson
By the end of the lesson, learners can
- Map a field with compasses: Switch the source between a bar magnet, a straight wire and a solenoid.
- Three sources, one set of rules: However the field is made, the same rules for field lines apply.
- On a conductor: F = BIL sinθ; the tesla is defined from B = F/(IL) (N A⁻¹ m⁻¹).
- On a charge: F = BQv sinθ, always perpendicular to v, so the path is a circle with r = mv/BQ.
- Induction: Φ = BA, flux linkage = NΦ, induced e.m.f. = rate of change of flux linkage (Faraday); direction opposes the change (Lenz).
One picture every idea returns to
A magnetic field line points the way the north pole of a small compass points. Lines run out of a north pole and into a south pole, never cross, and crowd together where the field is stronger. Around a straight current-carrying wire the lines are concentric circles, with their direction set by the right-hand grip rule.
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, Drawing the invisible.. A magnetic field is a region where a magnet, a moving charge or a current feels a force. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Map a field with compasses | Switch the source between a bar magnet, a straight wire and a solenoid. Project tab 1 of the lab (Fields and field lines) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Three sources, one set of rules | However the field is made, the same rules for field lines apply. | Frayer Model |
| 30 to 43 min | Check and plenary | The quick-check questions from the student lesson, answered on mini whiteboards one at a time; re-teach the one most get wrong, then a random call on the last. | Individual, then whole class |
| 43 to 45 min | Exit and homework | Set the worksheet below (1 questions, 12 marks) from the topic practice set. 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 Three sources, one set of rules, 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| Field lines never cross, because the field has a single direction at each point. | Closer lines mean a stronger field (greater flux density), so do not draw them evenly spaced near a magnet. The lines are continuous closed loops: outside a magnet they go north to south, inside they go south to north. Reversing the current or flipping the magnet reverses every arrow but keeps the same shape. |
Magnetic fields and field lines: topic worksheet
1 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced. These questions cover the whole topic, so sibling lessons share them.
A straight wire passes at right angles through the uniform field between the poles of a magnet, as shown in Fig. 1.1. The flux density is 0.25 T and the length of wire in the field is 4.0 cm.
(a) State two rules that magnetic field lines always obey. [2]
(b) Define magnetic flux density. [2]
(c) The wire carries a current of 3.0 A. Calculate the force on the wire. [2]
(d) State the direction of this force, and the rule you used. [2]
(e) The wire is now turned so that the current makes an angle of 30° with the field. Calculate the new force. [2]
(f) State the angle between the current and the field for which the force is a maximum. [1]
(g) State the value of the force when the current is parallel to the field. [1]
Answer key and marking notes
- (a)Any two: they run from north to south outside a magnet; they never cross; they are closer where the field is stronger ✓✓
- (b)The force per unit current per unit length on a conductor at right angles to the field ✓✓
- (c)F = BIL = 0.25 × 3.0 × 0.040 ✓ = 0.030 N ✓
- (d)The force is vertical (up or down depending on the current sense) ✓; from Fleming's left-hand rule ✓
- (e)F = BIL sinθ = 0.25 × 3.0 × 0.040 × sin30° ✓ = 0.015 N ✓
- (f)90° (current perpendicular to the field) ✓
- (g)Zero ✓
The lesson's own checks as a quiz
The quick-check and exam-style questions from the student lesson, printable with the key.
A magnetic field line shows the direction in which:
- Athe north pole of a small compass placed there points
- Belectrons drift along a wire
- Cthe field is weakest
- Dgravity acts on the magnet
The magnetic field pattern around a long straight wire carrying a steady current is:
- Aconcentric circles centred on the wire
- Bstraight lines parallel to the wire
- Cradial lines pointing away from the wire
- Da single loop passing through the wire
Two magnetic field lines can never cross because at any point the field has:
- Aa single, definite direction
- Bzero strength
- Ctwo equal sources
- Da fixed magnitude
Where magnetic field lines are drawn closer together, the field is:
- Astronger
- Bweaker
- Czero
- Dreversed in direction
Quiz key
A field line gives the direction of the force on a free north pole, which is the way a plotting compass needle's north end points.
The field forms concentric circles around the wire. The right-hand grip rule gives the direction, and the circles spread out (weaker field) farther away.
A field line gives the unique direction of the field at a point. A crossing would mean the field pointed two ways at once, which is impossible.
The spacing of field lines represents the field strength (flux density): closer lines mean a stronger field, as near the poles of a magnet.
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.