Magnetism & Electromagnetism

Question 1

what direction do magnetic field lines show & what does this indicate?

Answer 1

north to south
direction a force will act on a north pole placed at that point

Question 2

what is the difference between a permanent & induced magnet?

Answer 2

a permanent magnet produces its own magnetic field
an induced magnet turns into a magnet when in a magnetic field - temporary magnetic field

Question 3

what is a uniform magnetic field?

Answer 3

same strength & direction at every point

Question 4

why does adding an iron core increase magnetic field strength?

Answer 4

the iron core becomes an induced magnet

Question 5

what is the motor effect?

Answer 5

when a current is placed in a magnetic field (at right angles) it will experience a force because the magnetic fields interact

Question 6

describe a dc motor

Answer 6

1. the magnetic field goes from north to south
2. on one side of the wire, the current goes into the page so the force acts upwards
3. on the other side of the wire, the current goes out of the page so the force acts downwards
4. wire rotates & say direction
5. the split-ring commutator swaps connections every half turn to keep motor rotating in the same direction & the current flows in one direction

Question 7

what is another name for the generator effect?

Answer 7

electromagnetic induction

Question 8

what does induced current always do?

Answer 8

oppose the change that made it
the magnetic field produced by the induced current acts against the change in the opposite direction

Question 9

how does an alternator work?

Answer 9

1. a coil rotates in a magnetic field
2. the coil cuts field lines
3. this induces potential difference
4. the circuit is complete so current flows
5. the p.d. switches direction every half turn
6. so the current switches direction every half turn
7. it has slip rings & brushes so the contacts do not swap every half turn
8. so produces alternating current

Question 10

how does a dynamo work?

Answer 10

1. a coil rotates in a magnetic field
2. the coil cuts field lines
3. this induces potential difference
4. the circuit is complete so current flows
5. the p.d. switches direction every half turn
6. so the current switches direction every half turn
7. it has a split-ring commutator so the connections to the circuit swap every half turn
8. so current flows in one direction - direct current

Question 11

what happens if the frequency of revolutions is increased?

Answer 11

p.d. is increased & there are more peaks

Question 12

how do loud speakers work?

Answer 12

1. alternating current passes through coil of wire in magnetic field
2. coil surrounds one pole of a permanent magnet & is surrounded by the other pole
3. the alternating current causes a force on the cone
4. when current reverses, the force acts in the opposite direction
5. the cone vibrates, which makes the surrounding air vibrate causing variations in pressure that cause sound waves
6. the frequency of the sound wave is the same as the frequency of the ac

Question 13

how do microphones work?

Answer 13

1. sound waves hit the diaphragm attached to the coil of wire
2. this causes the coil to vibrate backwards & forwards in the magnetic field
3. this induces an alternating potential difference
4. the circuit is complete so alternating current is induced

Question 14

what do transformers do?

Answer 14

change the size of the p.d. of alternating current

Question 15

why is iron used?

Answer 15

easily magnetised & demagnetised

Question 16

how do transformers work?

Answer 16

1. an alternating p.d. & alternating current are applied across the primary coil
2. this magnetises & demagnetises the iron core
3. this causes a changing magnetic field
4. field lines are cut
5. so an alternating p.d. is induced in the secondary coil
6. circuit is complete so ac is induced