electromagnetism

Question 1

what is an induced magnet

Answer 1

An induced magnet is when a soft magnetic material becomes magnetic when placed in a magnetic field. The magnetism is lost when the material is removed from the magnetic field.

Question 2

What are permanent magnets

Answer 2

A magnetic material that creates its own magnetic field that cannot be turned on or off. Often used to make induced magnets. Eg iron.

Question 3

Soft magnetic materials

Answer 3

If material loses its magnetism quickly ( e.g iron)

Question 4

hard magnetic materials

Answer 4

keeps induced magnetism permanently ( e.g steel)

Question 5

why is iron used in transformer core

Answer 5

It is a soft magnetic material, which means that it can demagnetise and magnetise quickly, which it needs to do in a tranmsformer

Question 6

How to find magnetic field lines

Answer 6

1. draw three points at each pole of magnet
2. place plotting compass near bar magnet on one of the 6 dots.
3. draw a small dot where north pole of compass is pointing to and one where south is.
4. Move plotting compass to the dot to the north and draw a small dot where the north of compass is.
5. Repeat last step until compass is pointing off the page or is next to the magnet again.
6. draw a smooth line through the dots the compass has made and draw an arrow from north to south.
7. repeat steps 1-6 starting at another of the 6 original dots

Question 7

what is the motor effect

Answer 7

A wire carrying a current creates a magnetic field.

This can interact with another magnetic field, causing a force that pushes the wire at right angles. This is known as the motor effect.

Question 8

When is the size of the force in the motor effect increased?

Answer 8

• when the current in the wire increases;
  -when the strength of the magnetic field increases;
  -when the length of wire inside the magnetic field is increased.

Question 9

when is the force of the motor effect greatest?

Answer 9

The force is always greatest when the direction of the current is 90° to the direction of the magnetic field.

Question 10

When is there no motor effect?

Answer 10

There is no motor effect force if the current and magnetic field are parallel to each other.

Question 11

What do the fingers show in the left hand rule?

Answer 11

First finger - direction of field
Second finger - direction of current
thumb - Motion ( direction of force)

Question 12

How do A.C Generators work

Answer 12

1. Generators rotate a coil in a magnetic field ( or a magnet in a coil).
2. As the coil spins, a current is induced in the coil.
3. Instead of a split ring commutator, a.c generators have slip rings and brushes so the contacts don’t swap every half turn.
4. This means they produce a.c voltage. Faster revolutions produce more peaks and higher overall voltage.
   Power stations use a.c generators to produce electricity

Question 13

How do loudspeakers work

Answer 13

1. A.C electrical signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone.
2. The coil is surrounded by a permanent magnet, so the A.C signals cause a force on the coil and make it move back and forth.
3. These movements make the cone vibrate and this creates a sound.

Question 14

what is a split ring commutator

Answer 14

It swaps the contacts every half turn to keep the motor rotating in the same direction

Question 15

How to reverse the direction of rotation in a simple D.C motor

Answer 15

• swapping the negative and positive of the D.C supply
• swapping the magnetic poles over

Question 16

How to increase voltage in an electromagnet?

Answer 16

increase:
- strength of magnetic field
- number of turns on the coil
- speed of the movement
- increase the current

Question 17

equation linking primary and secondary current and voltage

Answer 17

VpIp = VsIs

Question 18

What do transformers do?

Answer 18

They can change the size of the voltage of an alternating current.
Transformers transfer electtrical energy from one circuit to another either increasing or decreasing the voltage.

Question 19

what happens in a transformer?

Answer 19

When an alternating voltage is applied across the primary coil, the magnetically soft iron core magnetises and demagnetises quickly. This induces an alternating voltage in the secondary coil.

Question 20

Step up transformers

Answer 20

increase the voltage.

Question 21

on which coil do step up transformers have more turns?

Answer 21

secondary coil

Question 22

step down transformer

Answer 22

decrease the voltage

Question 23

on which coil do step down transformers have more turns?

Answer 23

primary coil

Question 24

Equation for number of turns on a transformer

Answer 24

Vp/Vs = Np/Ns
where N is the number of turns on the coil and p = primary and s = secondary

Question 25

How do transformers transfer electricity

Answer 25

An alternating current passing through the primary coil generates an alternating magnetic field in the iron core.
The lines of the alternating magnetic field in the secondary field induces an alternating voltage between the terminals of the secondary coil
The core confines the field lines within the secondary coil.

Question 26

equation linking power, current and resistance

Answer 26

P = I^2R
or P = IV

Question 27

uses of an induced magnet

Answer 27

Relay switch
Circuit breaker
bell

Question 28

What is a circuit breaker?

Answer 28

They are resettable fuses that protect a circuit from overloading or short circuiting by detecting faults and then stopping the flow of electricity.

Question 29

How does a circuit breaker work?

Answer 29

When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. The electricity shuts off.

Question 30

What happens when there is a current bigger than normal (in terms of circuit breaker)?

Answer 30

1. Electromagnet becomes stronger because more current through solenoid wrapped around core
2. Iron bolt moves LEFT
3. Plunger ( which was being held by iron bolt) is now released and moves up
4. The push switch ( held down by the plunger) now goes off and the circuit is broken unless the plunger moves back down to close the switch.

Question 31

What is the correlation between distance and strength of magnet?

Answer 31

The further away you are from the magnet, the weaker the force of attraction is

Question 32

Define ‘Non contact force’

Answer 32

A force which acts on an object without coming into physical contact with it.

Question 33

Define ‘Magnetic field lines’

Answer 33

The area around a magnet where there is a force.

Question 34

What happens to the domains of a soft magnetic material when placed near a permanent magnet?

Answer 34

The domains are jumbled up when there is no magnet near. However, as soon as a magnet is near to the soft magnetic material, the domains will line up, and all point in the same direction.
In steel, which is hard, the domains will stay lined up even after the magnet is removed from the magnetic field lines of the other magnet

Question 35

Proportionality of force and magnetic field strength

Answer 35

F § B x I x L
Where B is the magnetic field strength,L is length of wire and I is current
§ = directly proportional

Question 36

What is magnetic field strength measured in

Answer 36

Tesla

Question 37

Which three elements are naturally magnetic

Answer 37

Iron, Nickel and cobalt