Magnetism & Electromagnetism Notes

Edexcel IGCSE Physics: Revision notes

Key facts

  • Like poles repel, unlike poles attract; magnetic materials are always attracted because of induced magnetism.
  • Magnetically soft materials (iron) magnetise and demagnetise easily; hard ones (steel) keep their magnetism.
  • A current in a conductor produces a magnetic field; a solenoid has a bar-magnet field.
  • A current-carrying wire in a field feels a force: First finger = Field, seCond finger = Current, thuMb = Motion (left hand).
  • The motor effect drives d.c. motors and loudspeakers.

Magnets and fields

Like poles repel and unlike poles attract. Magnetic materials are always attracted, because the magnet induces magnetism in them.

A magnetic field line shows the direction a free north pole would move. Outside a magnet the lines run from north to south, and are closer together where the field is stronger.

Induced magnetism: an unmagnetised magnetic material (iron, steel, cobalt, nickel) in a field becomes temporarily magnetised, so it is attracted, never repelled.

Magnetically hard (steel)

Behaviour:
Difficult to magnetise
Retention:
Keeps its magnetism
Use:
Permanent magnets

Magnetically soft (iron)

Behaviour:
Easy to magnetise
Retention:
Loses magnetism quickly
Use:
Electromagnet cores

Which material is best for the core of an electromagnet?

Electromagnets

A current in a conductor produces a magnetic field around it. A coil with an iron core makes an electromagnet that can be switched off.

The field round a straight wire is concentric circles. A solenoid has a field like a bar magnet, with north and south poles at the ends and a roughly uniform field inside.

An electromagnet is a coil around a soft iron core. Increase the current or number of turns to strengthen it.

Wire
Field around a straight wire: concentric circles.

Straight wire

  • Concentric circles

Flat coil

  • Stronger field through the centre

Solenoid

  • Like a bar magnet
  • Uniform field inside

How can the strength of an electromagnet be increased?

Force on a current

A current-carrying wire in a magnetic field feels a force, unless the current is parallel to the field. The left-hand rule gives its direction.

A moving charge in a magnetic field feels a force if its motion is not parallel to the field. In a wire, the force acts on the wire as a whole.

The force is bigger with a stronger field or larger current, and it reverses if either the current or the field is reversed.

  1. 1

    First finger

    points along the Field

  2. 2

    seCond finger

    points along the Current

  3. 3

    thuMb

    gives the direction of Motion (force)

Fleming's left-hand rule

The current in a wire in a field is reversed. The force

Motors and loudspeakers

The motor effect turns the coil in a d.c. motor and makes the coil in a loudspeaker vibrate.

In a d.c. motor, the current flows in opposite directions on opposite sides of the coil, so the forces are opposite and the coil turns.

In a loudspeaker, a varying current in a coil gives a varying force. The coil and cone vibrate, making sound.

d.c. electric motor

Set-up:
Coil carrying current in a field
Force:
Opposite forces on opposite sides
Result:
Coil rotates continuously

Loudspeaker

Set-up:
Coil carrying varying current in a field
Force:
Varying force
Result:
Coil and cone vibrate, making sound

What makes the cone of a loudspeaker vibrate?

Try an exam question

Explain how a loudspeaker produces sound.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.