P15 - Electromagnetism (PAPER 2)

Question 1

What is the relation between different poles

Answer 1

Opposite poles attract, like pols repel

Question 2

How do magnetic field lines of bar magnets exist

Answer 2

hey curve around from the north pole to south pole of the magnet

Question 3

what is induced magnetism

Answer 3

Magnetism created in an unmagnetised magnetic material when the material is placed in the magnetic field

Question 4

Why is steel used for permanent magnets instead of iron

Answer 4

Steel doesn’t lose magnetism easily, whilst iron does

Question 5

How do magnetic field lines around a wire exist

Answer 5

as circles centred on the wire in a plane perpendicular to the wire

Question 6

How do magnetic field lines in a solenoid exist

Answer 6

parallel to its axis and all in the same axis

Question 7

What is a uniform magnetic field

Answer 7

one where the magnetic field lines are parallel

Question 8

How does increasing the current affect the magnetic field

Answer 8

it makes it stronger

Question 9

How does reversing the direction of the current affect the magnetic field

Answer 9

reverses the magnetic field lines

Question 10

What is an electromagnet

Answer 10

a solenoid that has an iron core - insulated wire wrapped around an iron bar

Question 11

What is a solenoid

Answer 11

a long coil of insulated wire

Question 12

Where are some places electromagnets are commonly used

Answer 12

scrapyard cranes, circuit breakers, electric bells, relays

Question 13

How do electromagnets work in objects such as circuit breakers / relays / electric bells

Answer 13

By attracting an iron armature which opens a switch

Question 14

How can a force be increased in the motor effect (2)

Answer 14

if the current or the strengh of the magnetic field or length of conductor is increased

Question 15

How can a force be reversed in the motor effect

Answer 15

if the direction of current or magnetic field is reversed

Question 16

How do coils in an electric motor turn

Answer 16

When a current is passed through

Question 17

What is magnetic flux density

Answer 17

a measure of the strength of a magnetic field

Question 18

What is the equation for calculating force on a current carrying conductor at right angled to the lines of a magnetic field

Answer 18

F = B I L
force(N, Newtons) = Magnetic Flux Density (T, Tesla) x Current (A, Amperes) x length (m, metres)

Question 19

What is the generator effect

Answer 19

the effect of inducing a potential difference using a magnetic field

Question 20

What happens when a conductor crosses through the lines of a magnetic field

Answer 20

a potential difference is induced across the ends of the conductor

Question 21

What is the relation between the speed a conductor crossing through the lines of a magnetic field and the induced potential difference

Answer 21

the faster the speed, the larger the potential induced difference

Question 22

What is the relation between the direction of an induced current and the original change that caused it

Answer 22

They are opposite

Question 23

what is a simple a.c. generator made from

Answer 23

a coil that spins in a uniform magnetic field

Question 24

When is the waveform of the a.c. generator’s induced potential difference at its peak value

Answer 24

when the sides of the coil cross directly through the magnetic field lines

Question 25

When is the waveform of the a.c. generator’s induced potential difference at its zero value

Answer 25

when the sides of the coil move parallel tot he field lines

Question 26

What is a visual difference between a dynamo and an alternater

Answer 26

the dynamo has a split ring commutator whilst an alternator has two slip rings

Question 27

What is a dynamo and an alternater

Answer 27

dynamo - a d.c. generator
alternator - an a.c. generator

Question 28

What are transformers used for

Answer 28

increasing or decreasing the size of an alternating potential difference

Question 29

How are the sizes of alternating potential difference increased and decreased

Answer 29

increased - step up transformer
decreased - step down transformer

Question 30

Why do transformers only work with a.c.

Answer 30

because a changing magnetic field is necessary to induce a.c. in the secondary coil

Question 31

What are transformers made from

Answer 31

a primary coil, a secondary coil and an iron core

Question 32

what is the transformer equation

Answer 32

primary potential difference (Vp) / secondary potential difference (Vs) = Np / Ns

• where Np = number of primary turns
• where Ns = number of secondary turns

Question 33

What is the relation between the numbers of primary turns and secondary turns in step-up and step-down transformers

Answer 33

step-up - Ns > Np
step-down - Ns < Np

Question 34

What is the equation for a 100% efficient transformer

Answer 34

primary potential difference x primary current = secondary potential difference x secondary current

Question 35

What does a high grid potential difference do and why is it good

Answer 35

reduces the current needed, reducing power loss and making the system more efficient