Magnetism

Question 1

What happens when a switch is closed?

Answer 1

Electric current flows through the circuit.

Question 2

What happens when current flows through a conducting wire?

Answer 2

A magnetic field is produced around the wire.

Question 3

How do you prove that there is a magnetic field around a wire?

Answer 3

Place a compass close to the wire.
When current is turned off, the compass needle lines up with the Earth’s magnetic field. However, if we turn the current on, the needle deflects away from the wire.

Question 4

What affects the strength of the magnetic field.

Answer 4

Size of the current.

Strongest closest the wire.

Question 5

What happens if we change the direction of the current?

Answer 5

We change the direction of the magnetic field.

A compass placed near this wire would deflect in the opposite direction.

Question 6

How to work out the direction of the magnetic field produced by the wire?

Answer 6

Right hand grip rule.
Thumb pointing in the direction of the conventional current.
Now fingers will be pointing in the direction of the magnetic field.

Question 7

How to increase the strength of a magnetic field.

Answer 7

Coiling the wire or adding more coils (solenoid).

Question 8

What happens when we turn on the current in a solenoid?

Answer 8

There will be a strong and uniform magnetic field inside the solenoid.
The magnetic field around the solenoid is a similar shape to the field around. bar magnet.

Question 9

What are the ways of increasing the strength of the magnetic field produced by a solenoid?

Answer 9

Increasing the size of the current.
Increasing the number of turns of the coil.
Place a piece of iron inside the solenoid (iron core)

Question 10

What is an electromagnet and why are they useful?

Answer 10

A solenoid containing an iron core.
We can change the strength of the magnetic field by changing the size of current.
We can also turn an electromagnet on and off.

Question 11

Give two examples of electromagnets:

Answer 11

Doorbells

Relays

Question 12

Why are relays used?

Answer 12

High voltage circuits are dangerous to turn on and off by just using a switch.
Risk of electrocution.
Sparking.
So we use a relay to turn these circuits on and off.

Question 13

What is a relay?

Answer 13

Contains two separate circuits.
There is a low voltage circuit containing an electromagnet. This circuit is safe to be turned on and off.
On the high voltage circuit we have replaced the switch with two metal contacts. One of the contacts is connected to a spring which keeps the contacts apart. There is also an iron block next to the spring.
The low voltage circuit is turned off. So no current is flowing through the electromagnet and there is no magnetic field.
The high voltage circuit is turned off because the contacts are not touching.
Now switch on the low volt circuit.
Current now flows around the circuit, and there is now a magnetic field around the electromagnet. The magnetic field now attracts the iron block in the high voltage circuit. This causes the contacts to close and switches on the high voltage.
If we switch off the low volt circuit, there is no magnetic field. The contacts now spring apart and the high voltage circuit is turned off.

Question 14

How does a doorbell work?

Answer 14

The switch is closed when the buzzer is pressed. This causes a current to flow through the circuit. A magnetic field is produced by the electromagnet. The iron contact is now attracted to the magnetic field. When the contact moves towards the magnetic field, the clapper hits the bell.
This breaks the circuit. Now there is no current flowing through the circuit and there is no magnetic field. The iron contact now springs back into its original position. Now the circuit is complete again, and the process is repeated.

Question 15

What is the Motor Effect?

Answer 15

When you place a wire with current flowing through (and its own magnetic field) into another magnetic field (e.g. between magnets).
The magnetic field around the wire now interacts with the magnetic field between the magnets.
So the wire experiences a force. And the wire moves in the direction of this force.

Question 16

How do you calculate the size of the force produced by the motor effect?

Answer 16

force = magnetic flux density (T) x current x length

This formula applies to the wire which is at right angles to the magnetic field.

Question 17

What is magnetic flux density?

Answer 17

A measure of the strength of the magnetic field.

Question 18

How do you use Fleming’s Left Hand Rule to determine the direction of the force acting on the wire (motor effect).

Answer 18

Place your thumb, first and second finger so they are at right angles.
Point your first finger in the direction of the magnetic field from North to South.
Point your second finger in the direction of conventional current (+ to -)
The direction that your thumb is pointing shows the direction of the motion (the force).

Question 19

What if the wire or conductor is parallel to the magnetic field?

Answer 19

It will not experience a force.

Question 20

Give an example of a useful application of the motor effect.

Answer 20

Electric motors

Question 21

How is the motor effect used in loudspeakers and headphones?

Answer 21

Moving-coil loudspeaker

Question 22

How does a moving-coil loudspeaker work?

Answer 22

There is cone with a coil of wire wrapped around one end.
The coil of wire is connected to an AC electrical supply.
There is a permanent magnetic which goes inside the coil of wire.
As the current passes through the coil, it generates a magnetic field.
The magnetic field in the coil now interacts with the magnetic field from the permanent magnet- so they attract or repel each other.
This produces a resultant force, which causes the cone to move.
When the current switches direction, the direction of the force on the cone reverses.
This causes the cone to move in and out, generating sound waves.

Question 23

What affects the sound generated by a moving-coil loudspeaker?

Answer 23

By changing the frequency of the AC supply, we change the frequency that the cone vibrates.
Higher frequency will produce a higher pitched sound and vice versa.
If we increase the size of the current, then we increase the amplitude of the vibration - increases the volume of the sound.

Question 24

What is the generator effect?

Answer 24

Start with a wire below the magnetic field.
If we move the wire through the magnetic field, a potential difference is induced across the ends of the wire.
When the wire stops moving, the potential difference is lost.
If we move the wire back down the magnetic field, a potential difference is induced again, but this time the PD has reversed direction. (induced potential)

If we have a complete circuit, we induce a current -this is the generator effect.
The direction of current switches when the direction of movement switches.
If the movement stops, then current stops.

This whole idea also works if we move the magnetic field and hold the wire still instead.

Question 25

When can we see the generator effect?

Answer 25

Only if the wire passes through the magnetic field.

If the wire moves along the magnetic field, then there is no induced potential difference or current.

Question 26

What factors affect the size of the induced potential difference or current in the generator effect?

Answer 26

They are larger if:

• we use a stronger magnetic field
• we move the wire more rapidly
• we shape the wire into a coil (the greater the number of turns the greater the induced PD or current.

Question 27

What happens if we move a magnetic in and out of a coil of wire?

Answer 27

• produces and induced current
• the direction of the current changes when the direction of movement changes
• we can also switch the direction of the induced current if we switch the poles of the magnet
• the induced current creates it own magnetic field
• this field opposes the movement of the magnet
• so when we insert the north pole into the coil, that end of the coil also becomes a north pole
• this repels the magnet, making it harder to push the magnet in
• and when we pull the north pole out, that end of the magnet becomes a south pole
• this attracts the magnet, making it harder to pull it out

Because the induced current makes it harder to move the magnet, this means we are doing work
So we are transferring energy from the movement of the magnet to the movement of the current.