P15: Electromagnetism

Question 1

Where on the magnet are the magnetic forces the strongest?

Answer 1

At the poles of the magnet.

Question 2

What happens when you bring 2 magnets close together?

Answer 2

The exert a force on each other.
2 like poles repel.
2 unlike poles attract.
Attraction and repulsion are examples of non-contact forces

Question 3

What is a permanent magnet?

Answer 3

A magnet which produces its own magnetic field e.g. bar magnet - permanent magnets either attract or repel each other depending on the direction.

Question 4

What is an induced magnet?

Answer 4

An object which becomes a magnet when it is placed in a magnetic field.
Induced magnetism always causes a force of attraction - if permanent magnet is taken away, induced magnets lose most or all of their magnetism quickly.

Question 5

What are the magnetic materials?

Answer 5

Iron, Steel, Nickel, Cobalt

Question 6

What is a magnetic field?

Answer 6

A region around a magnet where a force acts on another magnet or magnetic material.

Question 7

What does the strength of the magnetic field depend on?

Answer 7

The distance of the object from the magnet.
The closer it is, the stronger the strength of the magnetic field
The closer the field lines, the stronger the force.

Question 8

How can the direction of a magnetic field be found?

Answer 8

Using a compass.
Compass contains a small bar magnet - if a compass is placed near a bar magnet, the magnetic field can be potted.

Question 9

How can you use a compass to map the magnetic field of a bar magnet?

Answer 9

Place the compass next to the north pole to the bar magnet. Then draw a cross at the north pole of the magnet, then move the compass so the south pole is on the cross. Then again draw another cross at the north pole of the compass. Continue doing this until we have plotted a complete magnetic field line. Connect all dots and show direction of field line using an arrow. Repeat at different points on the bar magnet.

Question 10

Where does the direction of a magnetic field run?

Answer 10

North to South

Question 11

How do we know that the Earth has its own magnetic field?

Answer 11

If a compass is held away from the bar magnet, it will point in one direction. This tells us that the Earth has its own magnetic field due to the magnetic elements in its core.

Question 12

What happens when a current flows through a CONDUCTING wire?

Answer 12

A magnetic field is formed around the wire

Question 13

How can we prove that there is a magnetic field around the wire?

Answer 13

Use a compass near the wire - it should point (deflect) in the direction of the magnetic field

Question 14

What does the strength of the magnetic field depend on?

Answer 14

The size of the current - larger current produces a stronger magnetic field.
It is strongest closer to the wire, as you move further away the strength decreases

Question 15

What happens if you change the direction of the current?

Answer 15

The direction of the magnetic field decreases.
A compass placed near this wire will deflect in the opposite direction to before.

Question 16

How would you work out the direction of a magnetic field on a wire?

Answer 16

Use the right hand grip rule - place right hand so thumb is in the direction of the conventional current, and the fingers will determine the direction of the magnetic field.

Question 17

How else might you increase the strength of the magnetic field?

Answer 17

Coil the wire - called a solenoid

Question 18

What happens when you turn on the current on a solenoid?

Answer 18

You get a strong and uniform magnetic field inside the solenoid

Question 19

What is the magnetic field of a solenoid like?

Answer 19

It has a similar shape to the field around a bar magnet.
To work out its direction, use the right hand grip rule again.

Question 20

How can you increase the strength of the magnetic field produced by a solenoid?

Answer 20

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

Question 21

What is an electromagnet?

Answer 21

A solenoid containing an iron core.

Question 22

What are the uses of electromagnets?

Answer 22

We can change the strength of the magnetic field by changing the size of the current.
We can turn it on or off

Question 23

Why is it dangerous to use a switch on high voltage circuits?

Answer 23

We can get sparking, which poses a risk of electrocution.

Question 24

What is a relay?

Answer 24

A device used to turn high voltage circuits on or off.
Contains 2 separate circuits - a low voltage circuit containing an electromagnet (safe to be switched on and off).

Question 25

How can you turn off a high voltage circuit?

Answer 25

Attach a spring (keeps contacts apart) and 2 metal contacts to the high p.d. circuit. There is also an iron block next to the spring.
Turning the low voltage circuit on produces a magnetic field around the electromagnet - this attracts the iron block on the high voltage circuit and causes the contacts to close, switching on the high voltage circuit.

Question 26

What appliances use electromagnets?

Answer 26

High voltage circuits
Doorbells

Question 27

Describe what happens inside a doorbell

Answer 27

When the buzzer is pressed, the switch closes - this creates a current and a magnetic field around the electromagnet. The iron contact attached to the spring is now attracted to the magnetic field. When the contact moves towards the magnetic field, the clapper now hits the bell and makes a sound and breaks the circuit, and the clapper springs back into its original position. The circuit is complete again, and the process is repeated.

Question 28

What is the motor effect?

Answer 28

When a single wire is placed among other wires which carry current in a different direction, the single wire experiences a resultant force.

Question 29

How can the size of the force be calculated?

Answer 29

F (N) = B(T)* I(A) * l(m)
Force = magnetic flux density * current * length
Applies to a wire which is at right angles to the magnetic field.
Magnetic flux density is a measure of the strength of the magnetic field.

Question 30

How can you determine the direction of the force through a wire?

Answer 30

Use Fleming’s left hand rule - place 3 fingers at right angles.
First finger corresponds to the direction of the field (remember north to south!)
Second finger corresponds to the direction of the current (conventional current, as in + to -)
The thumb shows the direction of the force.

Question 31

What happens if the conductor is parallel to the magnetic field?

Answer 31

It will not experience a force.

Question 32

Where is the motor effect used?

Answer 32

In electric motors.

Question 33

Describe what happens in an electric motor

Answer 33

A wire is arranged in a square…thing.
The direction of the current in opposite sides of the wire runs in the opposite directions.
When placed in a magnetic field, the left side experiences an upward force, and the right experiences a downward force, this causes a moment and the loop rotates. When it turns 90 degrees, it stops rotating. This can be solved by changing the current when the loop passes 90 degrees - using a split ring commutator (a split metal ring connected to conducting brushes - they allow the electric current to pass onto the ring).

Question 33

Describe the setup of a loudspeaker (found in the speakers on a stereo)

Answer 33

*Headphones contain a similar setup, only smaller
A cone which has a coil of wire wrapped around one end - the coil is connected to an ac electrical supply, and a permanent magnet which goes inside the coil of wire. As the current passes through the coil, it generates a magnetic field - the field from the coil interacts with the field from the magnet - these fields either repel or attract each other, producing a resultant force which causes the cone to move.
When the current switches direction, the direction of the force on the cone reverses, causing the cone to move in and out, generating sound waves.

Question 34

What happens if you change the frequency of the ac supply in a loudspeaker?

Answer 34

By changing the frequency of the ac, we can change the frequency that the cone vibrates - high frequency produces a higher pitch.
By increasing the size of the current, we can increase amplitude - bigger amplitude = louder sound

Question 35

What is induced potential?

Answer 35

By moving a wire up and down through a magnetic field, the potential difference is induced across the ends of the wire. When the wire stops moving, the potential difference is lost. If you move it up and down again, p.d. returns, but will reverse direction.
If we had a complete circuit, we introduce a current (generator effect)
Only happens when the wire passes through the magnetic field, not along it.

Question 36

How can you increase the size of the induced potential difference?

Answer 36

Use a stronger magnetic field
Move the wire faster
Coil the wire - greater number of turns, greater the induced potential difference and current

Question 37

How else can the direction of the induced current be changed?

Answer 37

Switch the poles of the magnet

Question 38

What happens when you introduce a magnet to a wire with a current through it?

Answer 38

It produces its own magnetic field.
When you insert the north pole of the magnet into a coil, the wire will produce a magnetic field (also north pole) which opposes the movement of the magnet and repel the magnet. If it is removed, the wire will change direction and attract the magnet

Question 39

What does it mean if the induced current makes it harder to move the magnet?

Answer 39

Work is done.
Transferring energy from the movement of the magnet, into the movement of the current.

Question 40

What is an alternator and how does it work?

Answer 40

A coil of wire rotating in a magnetic field. Coil is connected to two metal rings (commutators), which allow the current to pass out of the coil.
Potential difference is induced when the wire passes through the magnetic field.
Because two sides of the coil are attached to different rings, an alternator produces an alternating potential difference and an alternating current.

Question 41

How can you increase the size of the alternating current?

Answer 41

Increase the strength of the magnetic field.
Increase number of turns on coil.
Increase area of the coil.
Increase rotation speed of coil (increases both size and frequency)

Question 42

What is a dynamo and how does it work?

Answer 42

Produces a direct current.
Has a split ring commutator - has two sides separated by a gap, A and B. Side of the coil that is moving down is connected to part a and the part that is moving up is connected to part B. Because the coil is cutting through the magnetic field lines, a potential difference and current are induced. When the coil is vertical, it is moving parallel to the magnetic field, so p.d. is zero.
Potential difference and current do not reverse when coil rotates because each part is still connected to its respective on the split ring commutator, so we get a dc.

Question 43

What does the graph of a direct current look like?

Answer 43

2 peaks for each rotation (each side of the coil passes through magnetic field twice during each cycle of rotation).

Question 44

Describe the moving coil microphone

Answer 44

Similar to a moving coil loudspeaker.
Coil of wire is attached to a thin sheet of plastic (diaphragm), the end of the coil sits over a permanent magnet. When sound waves hit the diaphragm, they cause it to vibrate, now the coil of wire moves in and out through the magnetic field, inducing a potential difference across the end of the wire.