SECTION 6: MAGNETISM AND

ELECTROMAGNETISM

Chapter 21Magnetism and

Electromagnetism

Magnets attract magnetic materials but not non-magnetic materials.

Magnetism is a non-contact force (acts at a distance).

Magnetic materials:• iron• steel

• nickel

• cobalt

The region around a magnet where a magnetic force can be felt is called the magnetic field.

The magnetic field is strongest at the poles of a magnet.

north-seeking pole or

north pole

south-seeking pole or

south pole

Like poles

repel

Unlike poles

attract

REPULSION

ATTRACTION

REPULSION

ATTRACTION

S

SS

S

S

S S

S

N N

NN

NN

N N

A magnetic field around a bar magnet has a shape and direction.

The magnetic field is represented using magnetic field lines (lines of force, flux lines) that show the shape, direction and strength of the field.

Investigate the shape of the magnetic field between two bar magnets.

A

B

C D

N

N

N

NN

N

N

N S

S

S

S S

S

S

N S

UNIFORM MAGNETIC FIELD.

Magnetic field lines are parallel and equally spaced apart.

PERMANENT MAGNETS TEMPORARY MAGNETS

Made from magnetically hard materials, such as steel.

Retain their magnetism once they have been magnetised.

Made from magnetically soft materials, such as iron.

They are easy to magnetise but lose their magnetism easily.

Magnetism can be induced in some materials when they are placed in a magnetic field.

ElectromagnetismA wire carrying a current has a magnetic field around it.

The magnetic field around a current carrying wire has a circular shape.

The right hand grip rule

The magnetic field around a flat coil

Solenoids

The magnetic field around a solenoid has the same shape as the field around a bar magnet.

The field inside the solenoid is very strong and uniform.

It can be used to magnetise objects.

Polarity of a coil

The strength of the magnetic field around a solenoid can be increased by:

• Increasing the current.• Increasing the number of turns on the

solenoid.• Using a magnetically soft core such as

iron.• Bringing the poles together.

Using electromagnets

The electric bell

Circuit breaker

An electromagnetic door lock

There is an iron bar keeping the door locked.

If the security officer agrees to let you in, he presses a switch which allows a current to flow through the solenoid.

What happens to the iron bar?

The electromagnetic relay