Magnetism

Question 1

What is the law of magnets

Answer 1

Opposite poles attract, like poles repel

Question 2

Describe hard magnetic materials, eg

Answer 2

Retains magnetism well
Permanent magnet, Often alloys of iron, nickel, cobalt

Question 3

Describe soft magnetic materials

Answer 3

Loses magnetism easily
Temporary magnet

Question 4

What are 2 experiments to find the field patterns of bar magnets, describe them

Answer 4

Using iron fillings: Place bar magnet under a piece of paper. Sprinkle iron fillings on the paper. Tap the paper gently so that the iron fillings align along the magnetic field.
Using a compass: draw the shape of the magnetic field by placing a compass and drawing the direction of the needle

Question 5

What is a magnetic field, its features, what does magnetic field lines show

Answer 5

Volume of space where a magnetic force is extended
Magnetic fields never cross, always go from north to south and are always concentrated at each pole
Magnetic field lines show the shape of the field, direction of the field and strength of the field

Question 6

What is Uniform magnetic field and its features

Answer 6

Exerts a constant force over a region
Field consists of parallel equally spaced magnetic field lines
Found between a north & a south pole facing each other

Question 7

What is induced magnetism

Answer 7

Magnetism can be induced if a magnetic material is placed inside a magnetic field
Iron bar becomes a magnet if placed inside the magnetic field and this is temporary

Question 8

What is the right hand grip rule

Answer 8

Thumb is placed in the direction of flowing current
fingers show the direction of circular magnetic field

Question 9

What is a solenoid, and how the strength of its field increase

Answer 9

Coil of wire carrying electric current
It increases with the number of turns in the coil and increase of electric current

Question 10

What is a electromagnet

Answer 10

Coil of wire wrapped around an iron core connected to a power supply
Current carrying wire has a magnetic field around it

Question 11

What are 4 uses of electromagnets

Answer 11

Scrap yard crane, electric bell, relay switch, circuit breaker

Question 12

What happens when a charged particle enters the field

Answer 12

It is deflected

Question 13

What is the motor effect, decribe its process

Answer 13

When a current carrying wire placed in a magnetic field, it will experience a force
Current carrying wire has a magnetic field around it
field interacts with a permanent magnetic field causing force on rod and the rod moves
Direction of motion is determined using fleming’s left hand rule

Question 14

What is fleming’s left hand rule

Answer 14

index finger shows direction of magnetic field
Middle finger shows direction of current
Thumb shows direction of force

Question 15

How does a electric motor work

Answer 15

Current creates a magnetic field around the coil
Magnetic field around the coil interacts with the field produced by the magnets
Force is exerted on the coil
Current will flow in opposite directions on each side of the coil, the force produced by the coil will push one side of the coil up and the other side of the coil down
This will cause the coil to rotate

Question 16

What does the size of the force in a electric motor depend on?

Answer 16

Amount of current
Length of wire
Strength of magnetic field

Question 17

Direction of force in a electric motor depends on

Answer 17

Direction of current
Direction of magnetic field

Question 18

Name the uses of the electric motor

Answer 18

fans, blenders, washing machines, drills, hair dryers

Question 19

Name the process of the loud speaker

Answer 19

Current in the coil creates a magnetic field
Magnetic field interacts with the permanent magnet generating a force which pushes the cone outwards
Current is made to flow in the opposite direction (Alternating current)
Direction of magnetic field reverses
Force on the cone pulls it back in
Repeatedly alternating the current direction makes the cone in and out
Cone vibrations cause pressure variations in the air as sound waves

Question 20

What is electromagnetic induction?

Answer 20

Production of an induced voltage in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it
This is done by the conductor or coil cutting through the magnetic field lines of the magnetic field

Question 21

When do we use the term “change in magnetic flux linkage”

Answer 21

When magnetic field lines changes in a coil

Question 22

What is Faraday’s law

Answer 22

Voltage induced is proportional to the rate of change of magnetic flux linkage or rate of flux cutting

Question 23

Larger current is produced when.. (involving flux cutting)

Answer 23

If the magnet is moved at a faster speed in and out of the coil
A stronger magnet can be used
The turns in the coil is increased
An iron core can be inserted in the coil

Question 24

No voltage is produced when… (involving flux cutting)

Answer 24

If the magnetic field is stationary outside or inside the coil
If there no relative movement between the magnet and the coil

Question 25

Uses of electromagnetic induction

Answer 25

Transformers, Electrical power generators, Dynamo, Microphones, Forever flashlight

Question 26

What does a transformer do, what is a step up transformer, step down transformer

Answer 26

Transformers use electromagnetic induction to change the voltage of alternating currents
Step-up transformer increases the voltage
Step-down transformer reduces the voltage

Question 27

What is the structure of a transformer

Answer 27

It consists of at least 2 coils of wire wrapped around a laminated iron core

Question 28

How does a transformer work

Answer 28

When an alternating voltage, V p is applied to the primary coil of np turns, it causes an alternating current to flow in the primary coil.
This alternating current causes a changing magnetic field in primary coil.
The iron core increases the strength of the magnetic field which cuts the secondary coil of ns turns.
The changing magnetic field induces a changing voltage, Vs in the secondary coil
The induced voltage produces an alternating current in the external circuit

Question 29

Why can’t a transformer change the level of the voltage output of a battery?

Answer 29

Battery produces a steady (DC) voltage that would cause direct current in the primary coil of the transformer
This current would produce an unchanging magnetic field in the iron core
This unchanging magnetic field would NOT cause electromagnetic induction in the secondary coil

Question 30

What is the transformer equation

Answer 30

primary voltage/secondary voltage = primary turns/secondary turns

Question 31

What is the transformer power transfer equation

Answer 31

Vp * Ip = Vs * Is

Question 32

What does the Uk national grid do?

Answer 32

In the largest modern power stations electricity is generated at 25,000V (25 kilovolts = 25 kV) and stepped up in a transformer to 275 or 400 kV to be sent over long distances. Later the voltage is reduced by substation transformers for distribution to local users.

Question 33

What are the benefits of stepping up the voltage in power stations?

Answer 33

Less energy will be transferred to heat if the power is transmitted at the lower current and higher voltage

Question 34

Describe the process of a generator

Answer 34

The generator coil is initially in the horizontal position.
As the coil rotates in the clockwise direction a side of the coil moves up cutting the magnetic lines of force near the N- pole of the magnet and the other side moves down, cutting the lines of force near the S-pole of the magnet.
Due to this, induced current is produced and the current will move in the same direction
The current will move in the opposite direction. The generator produces and alternating current. We call this type of generator an alternator

Question 35

How much current is there when the coil is vertical?
How much current is there when the coil is horizontal?
(magnetic field vertical)

Answer 35

Vertical–0 as coil is parallel to magnetic field
Horizontal- maximum as coil is perpendicular to magnetic field and maximum rate of magnetic field line cutting occurs