Magnetic field of magnets

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# Magnetic field of magnets - PowerPoint PPT Presentation

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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### 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

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

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

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 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

If 2 bar magnets are brought together…

attraction

repulsion

unlike poles attract

like poles repel

Like poles repel each other;

unlike poles attract each other.

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 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

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

These patterns show the magnetic fieldsof the magnets.

Magnetic field patterns

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

S

N

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

N

N

N

S

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 patterns

What is its magnetic

field pattern?

S

N

uniform magnetic field

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

geographic North Pole

magnetic South Pole

11.5

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

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.

P

Q

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.

P

Q

Example 1

Bar magnets

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

P

Q

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...

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...

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.

(T/F)

Q3 Using iron filings, we may...

pattern

strength

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)

direction

Q4 In a region where a...

parallel

evenly

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...

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.