PHYSICS TOPIC 6; magnetism & electromagnetism

Magnetic field

the volume of space around a magnet where magnetism can be detected

Magnetic field lines show. . ?

• SHAPE of a magnetic field

• STRENGTH - closer together lines = stronger & vice versa.

• DIRECTION - North to South

Where is a magnetic field strongest in a bar magnet

At the ends where the magnetic field lines are the closest together

Properties of a magnetically hard material

• Keeps its magnetism once it’s been magnetised

• Always magnetic, even when it’s not near other magnets

• E.g steel

Properties of a magnetically soft material

• Can be magnetized, but loses it’s magnetism as soon as it’s removed from the field

• Unsuitable for a permanent magnet

• E.g iron

Why do magnetic field lines never intersect?

Because the magnetic field only goes in 1 direction. If they were to intersect the magnetic field would go in 2 different directions, which is impossible.

How to investigate a magnetical field?

METHOD 1 - Iron filings

• place a sheet or paper on top of a bar magnet

• sprinkle iron filings on the paper

• gently tap/shake the paper - filings will move to show the magnetic field pattern

Shows shape and size & where it’s strongest according to how close the lines are. Doesn’t show direction.

METHOD 2 - plotting compases

• place a bar magnet on a piece of paper

• place a large amount of plotting compasses around the magnet

• Look at the pattern shown by the needles & draw it onto the paper with a pencil

Shows direction (North to South) but not the shape or size.

What is a uniform magnetic field & how do you create one?

When magnetic field lines are the same distance apart from each other - strength & direction is the same everywhere. You can do this by putting 2 opposing poles together

Permanent vs induced magnets

• Permanent magnets produce their own magnetic field & can attract and repel other magnets.

• Induced magnets become a magnet when placed inside a magnetic field & lose their magnetism when removed. Only cause attraction.

What does an electric current in a conductor do?

Produces a magnetic field around itself.

How to increase the strength of the magnetic field around a current carrying wire?

• increase current in the wire

• wrapping the wire so it becomes a coil / solenoid (long coil)

How to increase strength of the field around a solenoid?

• increase current in the solenoid

• increase number of turns in the solenoid

• wrap the solenoid around a magnetically soft core, e.g iron

What is an electromagnet?

A magnetically soft core (e.g iron) wrapped in a coil of wire.

Properties of a solenoid

• More current / more turns in the coil = stronger field

• Magnetic field is the same shape as a bar magnet’s, but can be reversed by reversing the direction of the current.