magnetism

Question 1

which poles attract, and which repel?

Answer 1

LIKE
repel

UNLIKE
attract

Question 2

what is a permanent magnet?

Answer 2

a magnet that produces its own magnetic field

Question 3

what is an induced magnet?

Answer 3

a material that becomes a magnet when placed in a magnetic field

Question 4

define magnetic field

Answer 4

the region around a magnet where a force acts on a magnet/ another magnetic material

Question 5

the force between a magnet and a magnetic material is…

Answer 5

… always one of attraction

Question 6

give evidence, regarding a compass, that the earth’s core is magnetic

Answer 6

• a magnetic compass contains a small bar magnet
• the earth has a magnetic field
• the compass needle points in the direction of the magnetic field

Question 7

what happens when a current flows through a conducting wire?

Answer 7

a magnetic field is produced around the wire

Question 8

what determines the strength of a current-carrying wire?

Answer 8

• the magnitude of the current flowing through the wire
• distance from the wire

Question 9

what is an electromagnet?

Answer 9

a solenoid with an iron core

Question 10

explain what the motor effect is

Answer 10

when a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other

Question 11

state the direction each finger points in, during fleming’s left hand rule

Answer 11

THUMB- force
INDEX- magnetic field
MIDDLE- current

Question 12

recall factors that effect the size of the force on the conductor

Answer 12

force on the conductor increases as…
- current increase
- magnetic field strengths
- a longer conductor is used

Question 13

give four examples of magnetic materials

Answer 13

• iron
• nickel
• cobalt
• steel

Question 14

define the term solenoid

Answer 14

a coil of wire which, when current passes through, creates a strong magnetic field

Question 15

describe the magnetic field found inside a solenoid

Answer 15

strong and uniform

Question 16

what is the unit used for magnetic flux density?

Answer 16

tesla, T

Question 17

how does an electric motor work?

Answer 17

• a coil of wire, carrying a current, is placed in a magnetic field
• the forces on the two sides perpendicular to the field experience forces in opposite directions
• this causes a rotational effect

Question 18

what happens when an electrical conductor moves relative to a magnetic field?

Answer 18

a potential difference is induced across the ends of the conductor

Question 19

what is the requirement for an induced potential difference to cause a current flow?

Answer 19

conductor must from a closed loop

or…

be part of a competere circuit

Question 20

how is the generator effect used to generate different types of currents?

Answer 20

IN AN ALTERNATOR
to produce alternating current

IN A DYNAMO
to produce direct current

Question 21

describe the makeup of a basic transformer

Answer 21

a primary coil and a secondary coil of wire wrapped around an iron core

Question 22

why is iron used as the core for a transformer?

Answer 22

iron is easily magnetised

Question 23

explain how transformers work

Answer 23

• an alternating current flows through the primary coil
• this induces a changing magnetic field in the core
• this changing magnetic induces a current to flow in the secondary coil

Question 24

what happens to an electrical conductor when there is a change to the magnetic field that it is placed in?

Answer 24

a potential difference is induced across the ends of the conductor

Question 25

describe the direction of the magnetic field produced by an induced current?

Answer 25

- the direction of the field opposes the change that induced the current - the change is either the movement of the conductor or a change to the field

Question 26

what electromagnetic effect does a microphone take advantage of and how?

Answer 26

- the generator effect - converts pressure variations in sound waves into an alternating current in a circuit

Question 27

how do loudspeakers make use of the motor effect?

Answer 27

the motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound

Question 28

explain how loudspeakers work

Answer 28

- current passes through coil - generating a magnetic field - magnetic field of the coil interacts with the magnetic field of the permanent magnet - this produces a resultant force - causing the cone to move - when the current switches direction, the direction of the force on the wire reverses - generation sound waves

Question 29

how is the pitch from a loudspeaker changed?

Answer 29

- the frequency of the a.c current is altered - this creates a different frequency of vibration in the cone

Question 30

why must current flowing through a transformer be alternating?

Answer 30

- for current to be induced in the secondary coil, the magnetic field in the core must be continuously changing - for the magnetic field to be changing, the current in the primary coil must be alternating

Question 31

what can be said about a 100% efficient transformer?

Answer 31

the electrical power input equals the power output

Question 32

explain how a moving-coil loudspeaker produces a sound wave

Answer 32

- current creates a magnetic field around the coil… … which interacts with the magnetic field…. … producing a resultant force, causing the coil to move - when the direction of the current reveres, the direction of the resultant force reverses (producing a sound wave)

Question 33

explain why an alternating current is induced in a coil, when rotated

Answer 33

- the coil moves through the magnetic field - a potential difference is induced across the coil - there is a complete circuit, so a current is induced - every half turn, the potential difference reveres direction - so every half turn, the current changes direction

Question 34

suggest the purpose of slip rings

Answer 34

- provide a continuous connection between the transformer and the coils - prevents wires from twisting together

Question 35

what is the function of a microphone?

Answer 35

to convert the pressure variations in sound waves into variations in current

Question 36

explain how a moving-coil microphone works

Answer 36

- sound waves cause the diaphragm to vibrate - the diaphragm causes the coil/ wire to vibrate - the coil/ wire move through the magnetic field - a potential difference is induced across the ends of the coil / wire

Question 37

why does an alternator produce alternating current?

Answer 37

the two sides of the coil are attatched to different rings

Question 38

why does the movement of the magnet induce a potential difference across the coil?

Answer 38

the magnetic field across the coil changes

Question 39

describe the potential difference in a step-down transformer

Answer 39

the potential difference is GREATER in the PRIMARY COIL

Question 40

describe the potential difference in a step-up transformer

Answer 40

the potential difference is GREATER in the SECONDARY COIL