20.1Magnetic Fields
Magnets and Magnetic Fields — American System Physics Revision Notes & Topical Questions
10 min read 7 parts Lesson 3 of 5 1 quiz
Lesson summary
Describe the forces between magnetic poles and between magnets and magnetic materials.Like poles repel; unlike poles attract.Field lines point from N to S outside a magnet.Field lines never cross; closer lines mean a stronger field.Magnetic flux density B is measured in tesla (T)…
- Law of magnetic poles
- Magnetic field
- Non-magnetic material
- Induced magnetism
- Soft magnetic material
- Hard magnetic material
Section 1 of 7Magnets and Magnetic Fields
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2
Every magnet has two poles. Cutting a magnet in half produces two smaller magnets, each with N and S poles; single poles do not exist.
3
A freely suspended magnet lines up north–south. The end pointing north is called the north-seeking (N) pole.
4
A magnetic field exists in the space around a magnet. Its direction at any point is the way a compass needle (a tiny N pole) points.
5
Iron filings sprinkled around a magnet show the shape of the field; plotting compasses show both shape and direction.
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Between two unlike poles facing each other the field lines go straight across; between two like poles the lines push apart and there is a neutral point where the field is zero.
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A magnet attracts an unmagnetised piece of iron by induction: the iron becomes a magnet with the opposite pole nearest, so they attract.
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In a magnetic material the domains are randomly arranged when unmagnetised; in a strong field they line up, and the material becomes a magnet.
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Soft iron's domains realign easily and randomise again when the field is removed, so it is ideal for electromagnets that must be switched off.
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Steel's domains stay aligned, so it is used for permanent magnets.
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Earth behaves as if it has a huge bar magnet inside it; a compass points to magnetic north, which is actually a magnetic south pole.
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The strength of a field is shown by the spacing of field lines: the field is strongest at the poles where lines are closest.
Section 2 of 7Process & sequence
1
Place a bar magnet on a sheet of paper and draw round it.
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Put a small plotting compass near the N pole and mark a dot at the tip of the needle.
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Move the compass so its tail is on the dot and mark the new tip position.
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Repeat until you reach the S pole or the paper edge.
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Join the dots with a smooth line and add an arrow from N to S.
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Repeat from different starting points to build a field pattern; lines closer together show a stronger field.
How a magnet attracts an iron nailwrite these steps in this order; each step is a marking point.
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The N pole of a magnet is brought near an unmagnetised iron nail.
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The magnetic field lines up domains in the nail.
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The end of the nail nearest the magnet becomes an S pole (induced magnetism).
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Unlike poles attract, so the nail is pulled towards the magnet.
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When the magnet is removed, the soft iron loses most of its magnetism.
Section 3 of 7Compare
| Property | Soft iron (temporary) | Steel (permanent) |
|---|---|---|
| Ease of magnetising | Easy | Harder |
| Keeps magnetism? | No — loses it when field removed | Yes |
| Uses | Electromagnet cores, relays, transformer cores | Fridge magnets, compass needles, loudspeakers, motors |
| Process | Method |
|---|---|
| Magnetise | Place in a solenoid with d.c. current; or stroke repeatedly in one direction with a magnet |
| Demagnetise | Place in a solenoid carrying a.c. and slowly withdraw it; or heat strongly; or hammer repeatedly |
Only repulsion proves an object is a magnet; attraction could just be a magnetic material.
Section 4 of 7Worked examples
Not a magnet
A student finds that end X of a metal rod is attracted to both the N and S poles of a magnet. What can be concluded?
- The rod is made of a magnetic material but is not a magnet (it is magnetised by induction each time).
- A magnet would repel one of the poles.
2 cm from the pole
On a field diagram, the lines 2 cm from a pole are twice as close together as the lines 6 cm away. Where is the field stronger and roughly by what factor?
- Closer lines mean stronger field, so the field is stronger 2 cm from the pole, roughly twice as strong (line density ∝ B).
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Magnet · Magnetic pole · Law of magnetic poles · Magnetic field · Magnetic field line · Direction of a magnetic field · Magnetic field strength (flux density) · Magnetic material · Non-magnetic material · Induced magnetism · Soft magnetic material · Hard magnetic material · Magnetic domain
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Words to avoid
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a magnet attracts all metalsa magnet attracts magnetic materials such as iron, steel, nickel and cobalt.
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field lines go from S to Noutside a magnet, field lines point from N to S.
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attraction proves it is a magnetonly repulsion proves an object is a magnet.
4
soft iron is a weak magnetsoft iron is easily magnetised and easily demagnetised (temporary magnet).
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the field is where the magnet isa magnetic field is the region in which a magnetic pole experiences a force.
Instant feedbackCheck yourself
Q1. Which material would be best for the core of an electromagnet?
- A. Steel
- B. Copper
- C. Soft iron
- D. Aluminium
Show answer
C. Soft iron is easily magnetised and demagnetised, so the magnet can be switched off.
Q2. What happens when two N poles are brought together?
- A. They attract
- B. They repel
- C. Nothing
- D. They attract then repel
Show answer
B. Like poles repel.
Q3. The direction of a magnetic field line is defined as
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A. the direction of force on a north pole
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B. the direction of force on a south pole
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C. from S to N outside the magnet
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D. towards the stronger pole
Show answer
A. By definition, field direction is the direction of force on a N pole.
Q4. Which of these is NOT a magnetic material?
- A. Nickel
- B. Cobalt
- C. Iron
- D. Copper
Show answer
D. Copper is non-magnetic.
Q5. How can a steel bar be demagnetised?
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A. Place it in a solenoid carrying d.c.
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B. Cool it in ice
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C. Stroke it with a magnet
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D. Slowly withdraw it from a solenoid carrying a.c.
Show answer
D. The alternating field randomises the domains as the bar is withdrawn.
Q6. What shows that a magnetic field is strongest at a point?
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A. Field lines are farthest apart
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B. Field lines are closest together
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C. Field lines cross
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D. There are no field lines
Show answer
B. Line density represents field strength.
Marked practiceQuizzes for this lesson
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