Lesson focus: magnetic fieldThe area around a magnet where magnetic materials feel a force. model the direction and strength of forces around magnets, currents and the Earth.
Map magnetic fields, use field-line conventions and distinguish permanent magnetA magnet that stays magnetic all the time, like a fridge magnet. from induced magnetismWhen a magnetic material, such as an iron nail, becomes a magnet because it is near another magnet..
Build on earlier learning
Forces at a distanceYear 7 classified magnetism as a non-contact force. This lesson develops field direction, shape and evidence.
Practical
Mapping a magnetic field
- Place a bar magnet under paper.
- Move a plotting compass around it and mark the direction of the needle each time.
- Join the marks to draw field lines from north to south.
Record: A field-line drawing with arrows.
Plotting a magnetic field with a compass
Put a plotting compass near the north pole and mark where the needle points. Move the compass so its tail is on the mark and repeat. Join the marks to get a field line from N to S.
Controls
Field lines go from the north pole to the south pole and never cross. They are closest together at the poles, where the field is strongest.
You will be successful when you can…
- Identify poles and magnetic materials.
- Use field-line direction.
- Map a field with a compass.
- Explain attraction by induced magnetism.
Words that carry the science
- magnetic field
- A region where a magnet, magnetic material or moving charge experiences magnetic force.
- magnetic pole
- A region of a magnet where the field is concentrated, labelled north-seeking or south-seeking.
- field line
- A model line whose tangent gives field direction and whose spacing represents relative field strength.
- permanent magnet
- A material that retains magnetisation after an external field is removed.
- induced magnetism
- Temporary or permanent magnetisation caused by an external magnetic field.
- compass
- A small magnet that aligns with the local magnetic field.
Field lines encode direction and relative strength
01Field lines encode direction and relative strength
Outside a bar magnet, field direction is conventionally drawn from the north-seeking pole to the south-seeking pole. Lines form continuous loops through the magnet and never cross, because the field at one point has one direction.
Closer field-line spacing represents a stronger field in the model. A plotting compassA small magnet on a pivot that points along a magnetic field, such as towards the north. points tangentially along the field; iron filings show shape but not direction.
02Pole interactions follow superposed fields
Unlike poles attract and like poles repel. Between unlike poles, field lines connect through the gap. Between like poles, the patterns bend apart and a weak or zero-field point may occur between equal magnets.
The force generally becomes weaker with distance, but field-line sketches are qualitative unless calibrated measurements are provided.
03Magnetic materials can be induced
Iron, steel, nickel and cobalt are common magnetic materials. An unmagnetised iron object can be attracted to either pole because the external field induces a nearer opposite pole.
Soft iron magnetises and demagnetises readily; steel tends to retain magnetisation. Earth has a magnetic fieldThe area around a magnet where magnetic materials feel a force., so a compass north-seeking pole points approximately toward geographic north, which is magnetically south-like in pole convention.
Use a compass to map direction
A plotting compass is placed at several points around a bar magnet.
- Mark the direction indicated by the compass north-seeking end.
- Move the compass so its tail begins at the previous arrow head.
- Repeat to trace a smooth field line.
- Add arrows from the magnet’s north pole toward its south pole outside the magnet.
How to make the reasoning visible: Several starting positions reveal the field pattern; one compass position is not a complete map.
Magnetic-field mapping
Use the observations to infer field shape and material behaviour.
| Test | Observation | Inference | Limitation |
|---|---|---|---|
| compass near north pole | north end points away | field leaves north pole | single point only |
| compass near south pole | north end points inward | field enters south pole | Earth field also present |
| iron nail near either pole | attracted | induced magnetism | does not prove permanent magnet |
| two north poles | repel | like-pole interaction | distance not calibrated |
| iron filings | curved dense pattern near poles | shape and relative strength | no direction shown |
Why can a nail be attracted to both poles?
Why should the magnet be kept away from steel furniture during mapping?
Check your understanding
Answer from memory, then check against the suggested answer.
Field direction outside a magnet?
What shows direction during mapping?
Do field lines cross?
A metal object is attracted to a magnet. Why is this not enough to call it a magnet?
Why does a compass north end point roughly toward geographic north if unlike poles attract?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student maps the magnetic field around a bar magnet.
Describe how to map the field.
(2)Explain how the map shows field direction and strength.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., describe the measurement before interpreting the drawing conventions.