notes

Question 1

what rings does a d.c. motor use?

Answer 1

split rings

Question 2

what rings does an a.c. generator use?

Answer 2

slip rings

Question 3

what induces an electromotive force?

Answer 3

a conductor moving across a magnetic field

Question 4

electromotive force definition

Answer 4

the work done or energy per unit charge around the whole circuit by an energy source, such as a battery

Question 5

Faraday’s law

Answer 5

a wire close to a changing magnetic field will experience an induced electromotive force

Question 6

inducing an e.m.f: (2 points)

Answer 6

1. moving a magnet so that its field lines are cut by a wire
2. moving a wire across a magnetic field

Question 7

current diagram (dot/circle)

Answer 7

current out

Question 8

current diagram (cross)

Answer 8

current in

Question 9

Lenz’s law

Answer 9

the direction of the induced e.m.f. opposes the change that creates it

Question 10

current definition

Answer 10

rate of flow of charges (electrons) around a circuit

Question 11

alternating current

Answer 11

electrons continuously change directions

Question 12

direct current

Answer 12

electrons flow in one direction only

Question 13

A.C. GENERATOR - magnets

Answer 13

provide a constant magnetic field across the coil

Question 14

A.C. GENERATOR - coil

Answer 14

usually made from many turns of wire, it is rectangular so that its sides are perpendicular to the magnetic field

Question 15

A.C. GENERATOR - slip rings

Answer 15

cylindrical conductors that make constant contact with the coil during rotation. They allow the direction of the induced electromotive force (e.m.f.) to alternate and therefore cause an alternating current.

Question 16

A.C. GENERATOR - carbon brushes

Answer 16

make an electrical connection between the rotating coil and a circuit, avoiding the wires becoming twisted

Question 17

At a certain point, the output of an alternating current generator is +V. What will the output be 180 degrees later? (+ explanation)

Answer 17

-V. After half a rotation, the output electromotive force will be equal in size but will have an opposite direction

Question 18

When is a magnetic field created?

Answer 18

when an electric current (charge) flows through a wire

Question 19

to increase the strength of the magnetic field you can: (2 points)

Answer 19

1. increase the current through the solenoid
2. increase the number of turns of wire in the solenoid

Question 20

magnetic effects of a current in relays

Answer 20

when the electromagnet is supplied with a current, a magnetic field is generated. The field attracts the iron bar, which is pivoted and allows the conducting contacts to touch.

Question 21

magnetic effects of a current in loudspeakers

Answer 21

when an a.c. is supplied to the coil, the speaker cone oscillates left and right. The magnetic field due to the a.c. in the coil either attracts or repels a permanent magnet around, resulting in the vibrations necessary for sound.

Question 22

to increase the size of the force: (4 points)

Answer 22

1. increase the number of individual wires
2. increase the current in the wire
3. increase the strength in the magnetic field
4. increase the length of the wire in the magnetic field

Question 23

D.C. MOTOR - to make the motor spin more quickly (3 points)

Answer 23

1. Increase the strength of the magnets and thus the magnetic field
2. increase the number of turns of wire in the coil
3. increase the current to the coil from the power supply

Question 24

D.C. MOTOR - coil

Answer 24

rectangular and often made up of lots of turns of current-carrying wire

Question 25

D.C. MOTOR - magnets

Answer 25

bar magnet usually used, producing a field perpendicular to the coil, from N to S

Question 26

D.C. MOTOR - brushes

Answer 26

allow constant electrical contact with the inside of the split ring while the coil rotates - the wires would get twisted otherwise

Question 27

D.C. MOTOR - split ring commutator

Answer 27

causes the direction of current to stay the same so that the force also acts on the same direction.

Question 28

what is the name of the effect which produces a small current in the coil of wire when the magnet is moving?

Answer 28

electromagnetic induction

Question 29

the cross-sectional area of a current-carrying wire with a magnetic field is doubled. What factor does the strength of the magnetic field change by: a factor of 2 or a factor of 0.5? (+ explanation)

Answer 29

2. With double the cross-sectional area, resistance halved which means current is doubled. Magnetic field strength and current are directly proportional, and so the magnetic field strength also doubles.

Question 30

what is the main energy loss during transmission of electricity in a power distribution system due to?

Answer 30

heating of the wires

Question 31

formula for calculating energy loss from transmission

Answer 31

P= Iˆ2 x R

Question 32

use the right hand rule when...

Answer 32

e.m.f. is being induced

Question 33

why is voltage stepped up during electricity transmission?

Answer 33

to reduce energy lost as heat

Question 34

a rotating coil with slip rings within a magnetic field will induce what?

Answer 34

an alternating current

Question 35

when is a maximum e.m.f induced?

Answer 35

when the coil moves perpendicular to field lines

Question 36

How can the magnitude of an electromagnetic field be varied?

Answer 36

by increasing or decreasing the current or number of turns