magnetism and electromagnetism

Question 1

4 ways to increase strength of electromagnet:

Answer 1

-plus no. of coils (keeping length of coil same
-less length of coil (keeping no. of coils same)
(stronger when coils densely packed)
-increase current that flows through coil
-put iron core inside coil (it is a soft magnetic material, becomes an induced magnet in presence of field

Question 2

3 ways to + size of induced p.d.

Answer 2

-plus no. of coils
-move wire or magnet faster
-use stronger magnets

Question 3

Explain how an electromagnetic induction (generator effect) is used in dynamos

Answer 3

-coil of wire is turned inside mag field

-induces a (mag field and) current in the coil of wire

-commutator switches connections every half-turn

-causes current to be in one direction (d.c.)

Question 4

how do loudspeakers work:

Answer 4

take elec signals, convert into vibrations, which then generate sound waves

Question 4

how do loudspeakers work:

Answer 4

take elec signals, convert into vibrations, which then generate sound waves

Question 5

how do microphones work:

Answer 5

detect vibrations caused by sound waves and use generator effect to turn them into elec signals

Question 6

explain how a loudspeaker converts electrical signals into sound waves:

Answer 6

-coil of wire carrying an alternating current is wrapped around a pole of a permanent magnet which is surrounded by the other pole, attached to a paper cone
-when current flows through wire, causes force which moves cone in one direction
-when current reverses, force reversed, cone moves in opp direction
-makes come vibrate, vibrates the air around it to create sound wave of same freq as the a.c

Question 7

explain how a dynamo generates current:

Answer 7

-coil made to rotate in mag f
-induces a p.d. across coil, causes current to flow through it
-split ring comm used to swap the connection each half turn to keep current flow in same direction

Question 8

state the generator effect:

Answer 8

-the induction of a p.d in a wire which is moving relative to a mag f or experiencing change in mag f

Question 9

what do step up transformers do?

Answer 9

-increase the p/d
bcs less turns on primary coil than sec coil

Question 10

what do step down transformers do?

Answer 10

-decrease p/d
bcs less turns on sec coil than primary coil

Question 11

how does a moving coil loudspeaker prod. a sound wave

Answer 11

-current prod. an m. field around the coil
-which interacts with the permanent magnet m. field
-resultant force causes the coil to move
-alternating current means alternating forces (direction reverses) - prods a sound wave

Question 12

what does changing the frequency of AC supply in a motor do

Answer 12

changes the frequency that the cone vibrates

Question 13

a high frequency will prod a …. pitch sound, a lower frequency will prod a …. pitch sound

Answer 13

-higher
-lower

Question 14

what does increasing size of current do in a loudspeaker

Answer 14

-increases the amplitude of the vibration
-this increases vol.

Question 15

what type of current do dynamos prod

Answer 15

bcs of split ring commutator, it prods. a direct current

Question 16

what type of current do alternators prod

Answer 16

bcs of the slip ring and brushes, they prod an alternating current

Question 17

why are iron cores used in elecmags

Answer 17

-Having an iron core in the center increases its strength as it is easier for magnetic field lines to pass through than air

Question 18

Factors that affect the strength

Answer 18

o Size of current
o Length
o Cross sectional area
o Number of turns (coils)
o Using a soft iron core

Question 19

what does a commutator do

Answer 19

allow the current to pass out of the coil

Question 20

what is generator effect

Answer 20

-induction of a p.d. (and current if circuit is complete) in a wire which is moving in a magnetic field, or experiencing a change in magnetic field

Question 21

Explain how Fleming’s left hand rule can be used to predict the direction in which the
copper rod will move

Answer 21

-  hold thumb first finger and second finger (of left hand) at right angles to each other
- second finger represents the current
- first finger represents the field
- thumb points in the direction of the force / thrust / acceleration

Question 22

what is induced current

Answer 22

when a coil/conductor is moved perpendicular to a magnetic field, there will be an induced voltage in the conductor/coil
-if conductor is part of a closed circuit then induced voltage will cause induced current in wire

Question 23

Explain how electrical signals in the transmitter produce a signal in the receiver.
[3 marks]

Answer 23

-electrical current in the transmitter producing radio waves
-which are absorbed by the receiver inducing electrical current in the receiver
-at the same frequency