Unit 7 - Magnetic Fields

Question 1

Magnetic rule of attraction

Answer 1

Opposite poles attract (north attracts south)

Similar poles repeal (north repels north, south repels south)

Question 2

Describe the properties of a magnetically hard material

Answer 2

A magnetically hard material retains its magnetic properties for a long period of time/ permanently
They’re difficult to demagnetise.

Question 3

Describe the properties of magnetically soft materials

Answer 3

A magnetically soft material looses its magnetic properties almost as soon as it leaves a magnetic field.

Question 4

What are magnetic field lines

Answer 4

They represent the shape and direction of a magnetic field

They always go from north to south

Question 5

How to induce magnetism in a magnetic material

Answer 5

Stroke it repeatedly with a magnet to align the domains

Place it in a magnetic field to align the domains

Question 6

What types of materials are attracted to magnets and can be magnetised

Answer 6

Iron / steel
Nickel
Colbalt
Copper

Question 7

Magnetic field lines always go from

Answer 7

North to south

Question 8

describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets

Answer 8

To track field lines you can use iron filings, which are magnetic materials, which will line up along the magnetic field. and you can use compasses which will show that the lines go from north to south.

Question 9

How to produce a magnetic field (in a conductor)

Answer 9

By passing an electric current through a conductor it will produce a magnetic field around it

Question 10

Describe the constriction of electromagnets

Answer 10

A piece of wire is wrapped around a soft magnetic material. When there is a current in the wire, a magnetic field is induced in the metal. Electromagnets can be turned on and off by turning on/off the current

Question 11

How to increase the strength of an electromagnet

Answer 11

Increase the current

Increase the number of coils of wire around the soft iron core

Question 12

Magnetic field lines around a straight coil

Answer 12

circles round the wire (right hand thumb rule for direction)

Question 13

Magnetic field lines around a solenoid

Answer 13

Similar to a bar magnet

Question 14

Describe how to use two permanent magnets to produce a uniform magnetic field

Answer 14

If you put two bar magnets together with their north and south touching, then they will form the same magnetic field as if there were one bar magnet.

‘hold two opposite poles close to each other so that they attract as this makes a uniform magnetic field’

Question 15

Relationship of a transformer

Answer 15

Vp/Vs = Np/Ns

Input voltage / output voltage = primary turns / secondary turns

Question 16

Relationship for 100% efficiency

Answer 16

Input power = output power
VpIp = VsIs
Primary voltage X primary current = secondary voltage X secondary current

Question 17

When will there be a force on a charged particle (magnetic field)

Answer 17

The is a force on a charged particle when it moves into a magnetic field as long as it’s motion is not parallel to the field

Question 18

What is the motor effect

Answer 18

A wire carrying current in a field experiences force

Question 19

Why does the motor effect happen

Answer 19

The field from the current “squashes” the external field from the magnets. This “catapults” the wire out of the field

Question 20

How does an electric motor work

Answer 20

A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.
The direction of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or commutator. A coil of wire is used with lots of turns to increase the effect of the magnetic field.

Question 21

How does a loudspeaker work

Answer 21

When a fluctuating electric current flows through the coil, it becomes a temporary electromagnet, attracted and repelled by the permanent magnet. As the coil moves, it moves the cone back and forth, pumping vibrations (sound waves) into the air.

Question 22

Flemings left hand rule

Answer 22

Left hand
Pointer finger goes in the direction of the field (north to south)
Middle finger goes in the direction of the current (positive to negative)
Thumb is the direction of the force (where the wire moves to)

Question 23

How to increase the force of a current carrying conductor

Answer 23

Increase the size of the current

Increase the strength of the field around it

Question 24

How to induce voltage in a conductor or coil

Answer 24

Voltage is induced in a conductor or coil when it moves through a magnetic field or when a magnetic field changes through it. A voltage will only be generated if the wire moved perpendicular to the magnetic field (must cut across lines)

Question 25

How to increase the size of the voltage induced (generator)

Answer 25

Moving the wire more quickly
Using a stronger magnetic field
Increasing the length of the wire
Wrapping the wire around a soft iron core

Question 26

Describe the generation of electricity

Answer 26

Electricity is produced by the rotation of a magnet within a coil of wire or when a magnetic field changes through it, this indices a voltage between the ends of the coil. Voltage output is alternating (AC).

Question 27

Stricture of a transformer

Answer 27

It has two coils, primary (input) and secondary (output)

A soft iron core concentrates the field lines so transformers are almost 100% efficient (no movement)

Question 28

What is the basic purpose of a transformer

Answer 28

To increase (step up) or decrease (step down) the voltage by having a different number of turns on each side

Question 29

How does a transformer work

Answer 29

The primary coil is an electromagnet. The alternating current (50Hz) creates a changing magnetic field in the secondary coil so a voltage is induced. A step up transformer has more turns on the secondary coil (induced voltage is bigger)

Question 30

Why are transformers so important for the national grid

Answer 30

If electricity was transmitted at a low voltage, the current would be high and the energy lost as heat would be high (therefore expensive). A step up transformer makes voltage high and cheaper to transfer and a step down transformer makes is safe for use