Physics- Electromagnetism and Motor effect

What happens when an electric current flows in a wire?

It produces a magnetic field around the wire.

What affects the strength of the magnetic field around a wire?

The size of the current (bigger current = stronger field)

What shape is the magnetic field around a straight wire?

Concentric circles

What is an electromagnet

An electromagnet has a current carrying coil of wire (solenoid) wound around a soft iron core.

How can you make an electromagnet stronger?

• -Increase current

• -Increase number of turns

• -Add an iron core

What happens in an electromagnet when current is switched off?

The electromagnet loses its magnetism

What does the magnetic field look like around a straight wire? Description

Circular field lines around the wire.

What does the magnetic field look like around a straight wire? Picture

What does the magnetic field look like around a flat coil? Description

Like a bar magnet- field lines loop from one side to the other.

What does the magnetic field look like around a flat coil? Picture

What does the magnetic field look like around a solenoid? Description

Inside: Strong, uniform field

Outside: like a bar magnet

What does the magnetic field look like around a solenoid?

What is a uniform field?

A magnetic field where the strength and direction are the same at every point in space.

When does a charged particle feel a force in a magnetic field?

When it moves across the field (not parallel)

What rule gives the direction of force?

Flemmings left-hand rule

What happens if the charge is negative?

Force is in the opposite direction

What do the fingers represent in Flemmings left hand rule? Description

First finger = magnetic field (N —> S)

Second finger = current

Thumb = force (motion)

What do the fingers represent in Flemmings left hand rule? Diagram

What is the motor effect?

A force on a current carrying wire in a magnetic field

Why does the motor effect happen?

Because moving charges experience a force in a magnetic field.

How do you increase the force on a wire?

• -Stronger magnetic field

• -Larger current

• -More turns in coil

How can you change the direction of the force?

Reverse current or magnetic field

How does a DC motor work?

A current flows through the coil.
This creates a magnetic field around the wires of the coil.
This field interacts with the magnetic field of the permanent magnets. And producing a force on the wire.
The force is upwards on one side of the coil and downwards on the other side.
Which side is up and which is down can be predicted using Flemmings left hand rule.
The split ring communicator reserves the direction of the current in the coil, every half turn, this keeps the force in the same direction so the coil keeps spinning in the same way.
The brushes rest against the communicator, but are not stuck to it, which allows the coil to keep rotating whilst still allowing current to flow.

Why does the coil keep spinning in a DC motor?

The communicator reserves current every half turn.

What do brushes do?

Provide electrical contact with the rotating coil.

What is inside a loudspeaker?

A coil attached to a cone in a magnetic field.

How does a loudspeaker work?

An alternating current, which changes direction continuously, is passed through the coil.
This current in the coil creates an alternating magnetic field.
Which produces an alternating force on the cone.
This causes the cone to vibrate.
This causes the air to vibrate forming a longitudinal wave.
Which is sound

What happens if motion is parallel to the field?

No force.

Bigger current = ?

Bigger magnetic field AND bigger force

More coil turns = ?

Stronger electromagnet AND greater force