physics magnetism and electromagnetism

Question 1

explain flemmings left hand rule

Answer 1

  hold thumb first finger and second finger (of left hand) at right angles to each other
second finger represents the current pointing out of the paper
first finger represents the field pointing downwards
thumb points in the direction of the force / thrust / acceleration
1
(therefore) the rod moves left to right

Question 2

how does a loudspeaker work

Answer 2

use the MOTOR EFFECT
current creates a magnetic field (around the coil)
1
(which) interacts with the permanent magnet field
1
producing a (resultant) force causing the coil/cone to move
1
(when the) direction of the current reverses, the direction of the (resultant) force
reverses (producing a sound wave)

Question 3

Explain why the results cannot be used to make a valid conclusion.

Answer 3

the student changed two variables at the same time
allow only one variable should be changed at a time
1
(so) it is not possible to know the effect of each variable

Question 4

why does the door shut

Answer 4

there is a current in the solenoid

creating a magnetic field
allow the solenoid / coil is magnetised

attracting the bolt

Question 5

The transformer can be adjusted to have different numbers of turns on the secondary coil.- why

Answer 5

to vary the (output) potential difference
so that you don’t need a different generator for each type of device

Question 6

The handle of the alternator is turned, causing the coil to rotate.
Explain why an alternating current is induced in the coil.

Answer 6

  the coil moves through the magnetic field
or
the coil cuts magnetic field lines
1
a potential difference is induced (across the coil)
1
there is a complete circuit, so a current is induced (in the coil)
1
every half turn the potential difference reverses direction
1
so (every half turn) the current changes direction

Question 7

why is the handle easier to turn after disconnection

Answer 7

(after disconnection) there is no induced current
1
so no magnetic field (produced around / by the coil)
1
to oppose the movement of the coil

Question 8

Explain how a moving-coil microphone works

Answer 8

  sound (waves) cause the diaphragm to vibrate
diaphragm moves is insufficient
1
the diaphragm causes the coil / wire to vibrate
do not accept moves the coil / wire up and down
if m.p.1 and m.p.2 do not score, allow sound (waves)
cause the coil / wire to vibrate for 1 mark
1
the coil / wire moves through the magnetic field
or
the coil / wire cuts magnetic field lines
1
a potential difference is induced (across the ends of the coil / wire)
allow induced current for induced p.d.

Question 9

why do the rods move apart

Answer 9

the rods become (induced) magnets
allow the rods are (temporarily) magnetised
ignore rods repel
do not accept rods become charged
1
with the same polarity (at each end)

Answer 10

the sides of the coil (parallel to the magnet) experience a force (in opposite directions) the forces cause moments that act in the same direction causing the coil to rotate (each half-revolution) the two halves of the (rotating) commutator swap from one
(carbon) brush to the other
(each half-revolution) the commutator reverses the current (in the coil)
or
keeping the forces in the same direction (keeping the coil rotating)