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Magnetic field of magnets. Warm-up. 1 Three ring magnets are inserted into a vertical rod on a wooden stand. Which of the following results is/are reasonable?. Red & blue sides denote the 2 poles of a magnet. C. A. B. Warm-up.

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Magnetic field of magnets

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warm up

1 Three ring magnets are inserted into a vertical rod on a wooden stand. Which of the following results is/are reasonable?

Red & blue sides denote the 2 poles of a magnet.




warm up3

2 4 objects in bar shape are wrapped with paper so that they look the same.

2 of them are bar magnets, 1 is an iron bar & 1 is a copper bar. Without using any other tools, how can you identify all of them?

0 introduction
0 Introduction

Lodestone (磁石)

  • a naturally-occurring ore
  • attracts iron objects
  • points north-south when freely hung
  • has been used to make compass for centuries
1 poles of a magnet
1 Poles of a magnet

Paper clips are attracted to bar magnets.

The magnetic force seems to come from 2 ends.

poles (極)

How are they called?

1 poles of a magnet6
1 Poles of a magnet
  • If a magnet is hung by a piece of thread,

it swings until it lies pointing north-south.

North Pole

north pole (N-pole)

south pole (S-pole)

poles of a magnet
Poles of a magnet

If 2 bar magnets are brought together…



unlike poles attract

like poles repel

Like poles repel each other;

unlike poles attract each other.

poles of a magnet8
Poles of a magnet
  • A magnet attracts some metallic objects but not all. These objects are called magnetic objects.
  • A magnet exerts a force on nearby magnetic objects even if they are not in contact. Magnetic force, like electrostatic force, is an action-at-a-distance force.
  • We say that a magnet sets up a magnetic field(磁場), or B-field for short.
experiment 16 a
Experiment 16a

Magnetic fields of permanent magnets

Investigate the magnetic fields produced by permanent magnets.

iron filings

perspex board

2 slab-shaped magnets on steel yoke

bar magnet on underside of perspex board

perspex board

iron filings

magnetic fields
Magnetic fields

Iron filings are attracted by magnet, settle into a special pattern.

These patterns show the magnetic fieldsof the magnets.

magnetic field patterns
Magnetic field patterns

Field lines run from the N-pole round to theS-pole.



  • When field lines are closely-spaced, field is strong.
  • When field lines are widely-spaced, field is weak.
magnetic field patterns12
Magnetic field patterns





neutral point

field lines between unlike poles

field lines between like poles

Neutral point: the field from one magnet cancels the field from another (No magnetic field exists)

magnetic field patterns13
Magnetic field patterns

What is its magnetic

field pattern?



uniform magnetic field

  • The slab-shaped magnets produce a strong & uniform magnetic field.
the earth s magnetic field
The Earth's magnetic field

geographic North Pole

magnetic South Pole


The earth has a weak magnetic field.

  • It is like the field around a huge bar magnet.

Note: N-pole of compass needle points north

 S-pole of the 'earth magnet' is actually in the north!

example 1
Example 1

Bar magnets

2 bar magnets placed side by side with like poles facing each other.

(a) Mark in the diagram the positions of all neutral points.



example 116
Example 1

Bar magnets

(b) Explain briefly why there are neutral points in these positions.

At P, the magnetic field due to the 2 N-poles & the 2 S-poles cancel each other out.

The same explanation applies to the neutral point at Q.



example 117
Example 1

Bar magnets

(c) What happens when a plotting compass is put in these positions?



The magnetic field at P and Q is zero. The needle of the compass will lie in whatever direction it settles down to.

q1 in which direction does a
Q1 In which direction does a...

In which direction does a magnet point if it is hung freely at the North Pole?

A The N-pole of the magnet points downwards.

B The S-pole of the magnet points downwards.

C The S-pole of the magnet points horizontally.

D Any direction.

q2 true or false every magnet
Q2 True or false: Every magnet...

True or false: Every magnet has a north pole and a south pole, just as every coin has 2 sides, a 'head' & a 'tail'. If you break a magnet into 2, you get 2 smaller magnets, each again having both a north pole & a south pole.


q3 using iron filings we may
Q3 Using iron filings, we may...



Using iron filings, we may be able to figure out the __________ and the ___________, but not the __________ of a magnetic field in a region. (strength/direction/pattern)


q4 in a region where a
Q4 In a region where a...



In a region where a magnetic field is uniform, the field lines must be _________ (convergent/parallel/divergent) and __________ (evenly/unevenly) spaced.

Draw a uniform magnetic field in the box.

q5 a strong magnet attracts
Q5 A strong magnet attracts...

A strong magnet attracts a paper clip with a certain force.

(a) Does the paper clip exert a force on the magnet? (Yes/No)

(b) If no, why not? If yes, does it exert as much force on the magnet as the magnet exerts on it? Why?

Yes, because they are an action-reaction pair.