Simple Phenomena of Magnetism Notes

Cambridge IGCSE Physics: Revision notes

Key facts

  • Like poles repel; unlike poles attract.
  • A magnetic field is a region where a magnetic pole feels a force; its direction is that of the force on a north pole.
  • Field lines run from north to south outside a magnet; closer lines mean a stronger field.
  • Soft iron is magnetised easily and loses magnetism quickly; steel keeps it.
  • Magnetic forces are caused by the interaction of magnetic fields.

Magnetic poles

Like poles repel and unlike poles attract. The force is stronger when the poles are closer.

Every magnet has a north pole and a south pole. The forces occur because poles create magnetic fields. Magnetised materials have distinct poles; unmagnetised materials have none.

  1. 1

    Bring two poles close

    each pole has a magnetic field

  2. 2

    Unlike poles (N and S)

    attract

  3. 3

    Like poles (N and N, or S and S)

    repel

  4. 4

    Closer poles

    the force is stronger

What happens when two magnets are brought together.
  • Like polesrepel
  • Unlike polesattract

Two north poles are brought together. What happens?

Induced magnetism

A magnetic material placed in a magnetic field becomes a temporary magnet, and is always attracted to the magnet that induced it.

Induced magnetism disappears when the material leaves the field. Soft iron magnetises easily and loses its magnetism quickly, so it suits electromagnets. Steel is harder to magnetise but keeps its magnetism, so it makes permanent magnets.

  1. 1

    Paperclip near a magnet

    it is in the magnet's field

  2. 2

    Temporarily magnetised

    it develops its own north and south poles

  3. 3

    Attracted

    the induced magnet is attracted to the original magnet

  4. 4

    Magnet removed

    the paperclip loses its magnetism and falls off

Induced magnetism: a paperclip near a magnet

Soft iron

  • Easy to magnetise
  • Loses magnetism quickly
  • Electromagnets, relays

Steel

  • Harder to magnetise
  • Keeps magnetism
  • Compass needles, bar magnets

A paperclip is attracted to a magnet. Why?

Magnetic fields

A magnetic field is a region where a magnetic pole feels a force. Field lines show its direction: north to south outside the magnet.

The direction at a point is the direction of the force on a north pole placed there. Plot it with a plotting compass (the needle lines up with the field) or with iron filings (they form the pattern of the lines). Lines never cross.

−3−2−1123−3−2−1123xyNS
Field lines of a bar magnet (schematic); outside the magnet they run from N to S

Outside a magnet, field lines run...

Field strength

Closer field lines mean a stronger field; wider spacing means a weaker field.

The field is strongest at the poles, where the lines are closest. It weakens as the lines spread out away from the magnet. Between attracting poles the lines are dense; between repelling poles the field is weak in the middle.

123456246810xyfield strength
Illustrative: the field is strongest at the pole (distance 0) and weakens with distance as the field lines spread out.
  • Closer field linesstronger field

Strongest

  • At the poles
  • Lines closest together

Weakest

  • Far from the magnet
  • Lines furthest apart

Where is the field of a bar magnet strongest?

Permanent and electromagnets

Use a permanent magnet for a constant field without power. Use an electromagnet when the field must be switched or varied.

An electromagnet has a soft iron core, so it can be switched off. A scrapyard electromagnet picks up cars when the current is on and drops them when it is off.

  1. 1

    Current switched on

    the coil makes a magnetic field

  2. 2

    Soft iron core

    becomes strongly magnetised and lifts iron

  3. 3

    Current switched off

    the core loses its magnetism and the load drops

An electromagnet: a field you can switch on and off.

Permanent (steel)

  • Compass needles
  • Bar magnets
  • No power needed

Electromagnet

  • Motors, relays, doorbells
  • Loudspeakers, magnetic locks
  • Lifting iron, then releasing it

Why is an electromagnet used to lift scrap cars?

Why magnets exert forces

Magnetic forces come from the interaction of magnetic fields, so magnets do not need to touch.

Each magnet has a field. When two come near, their fields interact. Forces act through non-magnetic materials such as wood, plastic and paper. The field weakens with distance, and the force depends on the strength of each field.

  1. 1

    Each magnet

    has a magnetic field around it

  2. 2

    Fields interact

    when two magnets come near

  3. 3

    Force on each magnet

    acts through wood, plastic and paper; weaker as the distance grows

How magnets exert forces without touching.
  • Magnetic forceinteraction of magnetic fields

A magnet can pull a paperclip through paper because...

Try an exam question

A steel bar and a soft iron bar are each placed in a strong magnetic field and then removed. Describe what happens to each, and state one use of the soft iron.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.