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physics electromagnetic effects > notes > Flashcards

notes Flashcards

1
Q

what rings does a d.c. motor use?

A

split rings

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2
Q

what rings does an a.c. generator use?

A

slip rings

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3
Q

what induces an electromotive force?

A

a conductor moving across a magnetic field

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4
Q

electromotive force definition

A

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

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5
Q

Faraday’s law

A

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

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6
Q

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

A
  1. moving a magnet so that its field lines are cut by a wire
  2. moving a wire across a magnetic field
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7
Q

current diagram (dot/circle)

A

current out

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8
Q

current diagram (cross)

A

current in

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9
Q

Lenz’s law

A

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

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10
Q

current definition

A

rate of flow of charges (electrons) around a circuit

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11
Q

alternating current

A

electrons continuously change directions

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12
Q

direct current

A

electrons flow in one direction only

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13
Q

A.C. GENERATOR - magnets

A

provide a constant magnetic field across the coil

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14
Q

A.C. GENERATOR - coil

A

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

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15
Q

A.C. GENERATOR - slip rings

A

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.

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16
Q

A.C. GENERATOR - carbon brushes

A

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

17
Q

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

A

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

18
Q

When is a magnetic field created?

A

when an electric current (charge) flows through a wire

19
Q

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

A
  1. increase the current through the solenoid
  2. increase the number of turns of wire in the solenoid
20
Q

magnetic effects of a current in relays

A

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.

21
Q

magnetic effects of a current in loudspeakers

A

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.

22
Q

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

A
  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
23
Q

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

A
  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
24
Q

D.C. MOTOR - coil

A

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

25
Q

D.C. MOTOR - magnets

A

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

26
Q

D.C. MOTOR - brushes

A

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

27
Q

D.C. MOTOR - split ring commutator

A

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

28
Q

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

A

electromagnetic induction

29
Q

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)

A
  1. 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.
30
Q

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

A

heating of the wires

31
Q

formula for calculating energy loss from transmission

A

P= Iˆ2 x R

32
Q

use the right hand rule when…

A

e.m.f. is being induced

33
Q

why is voltage stepped up during electricity transmission?

A

to reduce energy lost as heat

34
Q

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

A

an alternating current

35
Q

when is a maximum e.m.f induced?

A

when the coil moves perpendicular to field lines

36
Q

How can the magnitude of an electromagnetic field be varied?

A

by increasing or decreasing the current or number of turns

physics electromagnetic effects (1 decks)

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