2.3 Magnetism

What are the two poles of a magnet?

North and South poles

What happens when two like poles are brought together?

They repel

What happens when two opposite poles are brought together?

They attract

What is a permanent magnet?

A magnet that is always magnetic and has fixed poles

What is an induced magnet?

A material that becomes magnetic only when placed in a magnetic field

Do induced magnets have permanent poles?

No

How can a temporary magnet be made?

By stroking with a permanent magnet

What happens to domains when a material is magnetised?

They align in the same direction

Name three magnetic materials

Iron Nickel Cobalt

What direction do magnetic field lines go?

From North to South

What happens to magnetic field strength with distance?

It decreases

What direction does the field point near a magnet?

Away from North and towards South

What is a plotting compass?

A small compass used to show magnetic field direction at a point

Why does Earth have a magnetic field?

Because of its magnetic core

What happens to a freely suspended compass?

It aligns with Earth's magnetic field

Does a compass point to the geographic North Pole?

No

Where is Earth's magnetic north actually located?

Near North Canada

Why is Earth's magnetic north actually a south pole?

Because opposite poles attract

What happens when current flows through a wire?

A magnetic field is produced

What rule gives the direction of the field around a wire?

Right hand grip rule

What shape is the magnetic field around a wire?

Circular

How does current affect magnetic field strength?

Greater current gives stronger field

How does distance affect magnetic field strength?

Greater distance gives weaker field

What is a solenoid?

A coil of wire carrying current

What shape is the field of a solenoid?

Like a bar magnet

Why does coiling wire increase magnetic strength?

Fields combine into one strong field

What does an iron core do in a solenoid?

Increases magnetic strength

List factors affecting solenoid strength

Current Length Cross sectional area Number of turns Iron core

What is the motor effect?

Force produced when magnetic fields interact

Why does a wire experience a force in a magnetic field?

Because its field interacts with another field

What direction is the force on a wire?

At right angles to current and magnetic field

What is the shape of the magnetic field between magnets?

Straight lines

What is the shape of the field around a wire?

Circular

What is Fleming's Left Hand Rule used for?

To find direction of force current or field

How are the directions arranged in Fleming’s Left Hand Rule?

All at 90 degrees

What direction is conventional current?

Opposite to electron flow

What is the equation for force on a wire?

F = B I L

What does B stand for?

Magnetic flux density

What unit is magnetic flux density measured in?

Tesla

How does an electric motor work?

A current carrying coil in a magnetic field experiences forces that cause rotation

Why do opposite sides of the coil move differently?

Current flows in opposite directions

What rule can be used to predict motion in a motor?

Fleming’s Left Hand Rule

What is electromagnetic induction?

When a changing magnetic field induces a potential difference

When is a current induced?

When the circuit is complete

What does the induced current do?

Opposes the change causing it

How does a generator work?

A rotating coil in a magnetic field induces a potential difference

What type of current does an alternator produce?

Alternating current AC

What produces direct current in a generator?

Split ring commutator

What does a commutator do?

Reverses current each half turn

What do transformers do?

Change voltage of an AC supply

Why must transformers use AC?

Because changing magnetic fields are needed

What happens in a step up transformer?

Voltage increases

What happens in a step down transformer?

Voltage decreases

What determines voltage change in a transformer?

Number of turns in coils

How does a dynamic microphone work?

Movement of coil in magnetic field induces current from sound waves

What causes the coil to move in a microphone?

Sound pressure variations

What happens to the induced current in a microphone?

It is sent to a speaker

How does a loudspeaker work?

Current in a coil creates a magnetic field that moves the coil

What causes sound in a speaker?

Movement of the cone creates pressure waves

How are microphones and speakers related?

They work in reverse of each other