The shape of the lesson
By the end of the lesson, learners can
- Two wires, one force: Choose the direction and size of each current and the separation.
- Three patterns and a core: Know the field shape of each arrangement, and what iron does.
- 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 straight wire makes a field of concentric circles; a flat coil makes a field like a short bar magnet; a solenoid makes a strong, nearly uniform field inside. A ferrous core greatly increases the flux density. Because each wire sits in the other's field, parallel currents in the same direction attract and in opposite directions repel.
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, Fields that push back.. Currents make magnetic fields, and those fields then push on other currents. 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: Two wires, one force | Choose the direction and size of each current and the separation. Project tab 2 of the lab (Fields due to currents) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Three patterns and a core | Know the field shape of each arrangement, and what iron does. | Cooperative Concept Mapping |
| 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 (2 questions, 23 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 patterns and a core, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| "Same direction attracts" is the opposite of two bar magnets, so do not guess from poles. | If you reverse both currents, the force is unchanged (still attraction or still repulsion), because their relative direction is the same. A soft iron core increases the field; it does not change its shape. The field inside a long solenoid is uniform, drawn as parallel, equally spaced lines. |
Magnetic fields due to currents: topic worksheet
2 questions, 23 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.
Two long straight parallel wires P and Q are a few centimetres apart and carry currents in the same direction.
(a) Describe the shape of the magnetic field around a single long straight wire, and how its direction is found. [2]
(b) State whether P and Q attract or repel, and explain why in terms of their fields. [3]
(c) State what happens to the force if the current in Q alone is reversed. [1]
(d) State what happens to the force if the currents in both P and Q are reversed. [1]
(e) Describe the magnetic field inside a long solenoid. [2]
(f) State the effect on this field of inserting a soft iron core. [1]
This question links three lessons of the topic. A long solenoid carries a current and a coil of 500 turns is wound around its middle.
(a) Describe the magnetic field inside the solenoid, and state the effect of placing a soft iron core inside it. [2]
(b) The current is switched off, and the flux linkage of the 500-turn coil falls from 0.080 Wb to zero in 0.040 s. Calculate the average e.m.f. induced. [3]
(c) State the direction of the induced current relative to the change, and the law used. [2]
(d) An electron now enters the solenoid's field of flux density 0.015 T at 2.0 × 10⁶ m s⁻¹, at right angles to the field. Calculate the radius of its circular path. [3]
(e) State and explain the effect on this radius of doubling the flux density. [2]
(f) State why the magnetic force on the electron does not change its speed. [1]
Answer key and marking notes
- (a)Concentric circles centred on the wire ✓; direction from the right-hand grip rule (thumb along the current, fingers curl the field) ✓
- (b)They attract ✓; between the wires the two circular fields point in opposite senses and partly cancel ✓, so each wire is pushed toward the weaker field, that is toward the other wire ✓
- (c)The force becomes a repulsion ✓
- (d)Unchanged: they still attract ✓
- (e)Strong and almost uniform along the inside ✓, shown by parallel, equally spaced lines; like a bar magnet outside ✓
- (f)It greatly increases the flux density ✓
- (a)Strong and almost uniform inside (parallel, equally spaced lines) ✓; a soft iron core greatly increases the flux density ✓
- (b)ε = Δ(NΦ)/Δt. Here the quoted flux linkage falls from 0.080 to 0 Wb ✓; ε = 0.080 / 0.040 ✓ = 2.0 V ✓
- (c)The induced current acts to oppose the fall in flux (it tries to maintain it) ✓; this is Lenz's law ✓
- (d)r = mv/BQ = (9.11 × 10⁻³¹ × 2.0 × 10⁶)/(0.015 × 1.6 × 10⁻¹⁹) ✓✓ = 7.6 × 10⁻⁴ m (0.76 mm) ✓
- (e)The radius halves ✓, since r = mv/BQ is inversely proportional to B ✓
- (f)The force is always perpendicular to the velocity, so it does no work ✓
The lesson's own checks as a quiz
The quick-check and exam-style questions from the student lesson, printable with the key.
Two long straight parallel wires carry current in the same direction. The force between them is:
- Aan attraction
- Ba repulsion
- Czero
- Dalong the wires
Placing a soft iron core inside a current-carrying solenoid:
- Agreatly increases the magnetic flux density
- Breverses the field direction
- Chas no effect on the field
- Dmakes the field non-uniform inside
The magnetic field well inside a long current-carrying solenoid is best described as:
- Astrong and almost uniform
- Bzero everywhere
- Ccircular, centred on the axis
- Dstrongest at the centre and zero at the ends
Two parallel wires attract each other. If the current in both wires is reversed, the wires now:
- Astill attract
- Brepel
- Cfeel no force
- Dtwist about their axes
Quiz key
Parallel currents in the same direction attract: each wire is pushed toward the other by the field of its neighbour.
A ferrous core becomes strongly magnetised and greatly increases the flux density, without changing the field's shape or direction.
Inside a long solenoid the field is strong and nearly uniform, shown by parallel, equally spaced field lines along the axis.
Reversing both currents keeps them in the same direction relative to each other, so the force is unchanged: they still attract.
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.