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Year 2 Electrical > Year 2 D1 > Flashcards

Year 2 D1 Flashcards

1
Q

Faraday’s law of electromagnetic induction

A

the magnitude of voltage induced in a turn of wire is proportional to the rate of change of flux passing through (or linked with) that turn.

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2
Q

Faraday’s formula

A

e =Blv
e = voltage generated
B = flux density of the field in tesla
l = active length of the conductor in the field in m
v = velocity of the conductor at right angles to the magnetic field in meters per second

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3
Q

Flux density

A

of the magnetic field is normally controlled by the magnitude of the current flowing through the field coils. Normally controlled by a series field rheostat

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4
Q

Lenz’s Law

A

the direction of the induced emf must be such that any current resulting from it will develop a flux that will oppose any change in the original flux

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5
Q

Induced voltage

A

anytime the conductor cuts flux lines at the rate of 1 weber per second, an emf of 1 volt will be induced in the conductor

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6
Q

The frequency of the generated emf

A

is directly proportional to the number of field poles in the alternator

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7
Q

Formula for frequency is

A 
f = P X N / 120
f = frequency of the electrical waveform in cycles per second or Hz
P = number of poles in the alternator
N = speed of the alternator in rpm
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8
Q

transformers have efficiencies in

A

95% to 99% range

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9
Q

AC

A

periodically changes in both magnitude and direction

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10
Q

Cycle

A

the distance from one point on a waveform to where it starts to repeat itself is a cycle

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11
Q

Alternation

A

one cycle of a sine wave contains two alternations

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12
Q

Period

A

is the time it takes to complete one cyle

the period of a waveform is equal to the number of seconds per cycle

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13
Q

Frequency

A

of a waveform is the number of complete cycles that occur in one second.
it is the reciprocal of the period

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14
Q

Amplitude

A

of the waveform is the max height of the waveform above or below the zero axis

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15
Q

peak value

A

max positive or negative value of the waveform

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16
Q

Peak to Peak

A

is twice the peak value

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17
Q

Instantaneous value

A

of the waveform is constantly changing

e = Em sin(theta) i = Im sin(theta)

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18
Q

Average value

A

of a waveform is .637 time peak

average value for one complete sine wave is zero

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19
Q

Effective value

A

of an AC waveform is equal to the DC source that would give the same heating effect in a resistive load.
.707 times peak value

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20
Q

in an AC cct effective resistance is made up of

A 
pure ohmic resistance
skin effect losses
hysteresis losses
eddy current losses
dielectric losses
radiation losses
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21
Q

opposition to AC current flow is called

A

impedance

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22
Q

The peak power in an AC lighting circuit is 1200 W. what is the effective power in the cct.

A

Peff = .5 x Pm

.5 x 1200 W = 600 W

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23
Q

electromagnetic induction

A

the process of generating a voltage by cutting a magnetic field with a conductor or by cutting a conductor with a magnetic field

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24
Q

Inductance formula

A

E = - L times (delta I/delta t)

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25
Q

Inductors are built by

A

winding a conductor into a coil to increase the inductive effect. chokes, coils and reactors

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26
Q

Air-core inductors

A

high-frequency applications and some high voltage protection systems
what is the symbol

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27
Q

Iron-core inductors

A

have a ferrite core often used as smoothing chokes in power supplies

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28
Q

variable inductors

A

Used in welding machines

tuners in radios

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29
Q

factors affecting inductance and formula

A 
L = N^2uA/l
L inductance in henries
N number of turns in the coil
u permeability of the core in webers/amp turn metre
l length of the core in metres
30
Q

Inductance is the property of an electric cct

A

that opposes any change in current in that cct

31
Q

time constant in a RL cct
during each time constant the current increases by 63.2% of the difference between its starting value and its steady state
After 5 time constants reaches 99.33% or steady state

A

t = L/R
t length of one time constant in seconds
L inductance of the cct in henries
R resistance of the cct in ohms

32
Q

mutual induction

A

is the process of inducing a voltage in a coil by changing current in another coil

33
Q

DC current will only induce a voltage in an

Most come use is the automobile ignition coil

A

other coil in the instant it is turned on or off.

must have an alternating magnetic field so that the coil is continually cutting the lines of flux

34
Q

Inductance of inductors in series

A

cct is the sum of the individual inductances

35
Q

Inductance of inductors in parallel

A

cct is the sum of the reciprocals of the individual inductances

36
Q

Coulomb’s law of electrostatic force

A

the electric force between two point electric charges is directly proportional to the product of the two charges and inversely proportional to the square of the spacing between them

37
Q

Coulomb’s law of electrostatic force formula

A

F = k((Q1 x Q2)/d^2)

38
Q

Electric field intensity

A 
E = V/d
E = the intensity of the field expressed in volts per metre
39
Q

dielectric strength

A

the maximum field intensity the dielectric is able to withstand without breakdown

40
Q

Capacitance of the capacitor

Unit for capacitance is farad F

A

is C = Q/V
C the capacitance of the capacitor in farads
Q the charge on either plate in coulombs
V the voltage between the plates

41
Q

Define farad

A

an electric circuit has a capacitance of 1 farad when a charge of 1 coulomb on the plates is required to raise the voltage between the plates by 1 volt

42
Q

A capacitors working voltage is

A

the max usable DC voltage, beyond which damage to the dielectric results

43
Q

Three thing affect the capacitance of a capacitor

A

capacitance is directly proportional to the plate area
inversely proportional to the spacing between the plates
on the type of dielectric between the plates

44
Q

dielectric constant

A

the ratio of the capacitance of a capacitor with a certain dielectric between its plates to one with the same physical dimensions but with only air between its plates

45
Q

permittivity

A

is a measure of the capacitance between opposite ends of a unit length and unit cross-section of a dielectric material

46
Q

formula for calculating the capacitance of a capacitor

A 
C = (8.85 x 10^-12 x K x A)/d
C capacitance of the capacitor in farads
K dielectric constant
A area of each plate in square metres
d  distance between the plates in metres
8.85 x 10^-12 permittivity of free space
47
Q

Types of capacitors

A

fixed
variable
trimmer
please note the curved end of a capacitor symbols is usually connected to common

48
Q

Three types of fixed capacitors

A

Oil-filled
Dry, self-healing
Electrolytic capacitors
Page 90 of D-1

49
Q

special capacitor

A

Electrolytic capacitor

polarity-sensitive, use only in DC circuits to avoid puncturing

50
Q

electrolytic capacitor puncturing

A

when it is hooked up back to front the electrolytic action reverses and aluminum oxide on one plate will break down and begin to form on the other plate as the dielectric diminishes the capacitor will conduct

51
Q

Two ways to vary the capacitance of a variable capacitor

A

vary the effective area of the plates

vary the distance between the plates

52
Q

capacitor problems

A

shorting
leaky dielectric or weak spots in the dielectric
opens
strays when rapid switching is required stray capacitance will limit how fast voltage can change

53
Q

Dielectric absorption

A

if a capacitor is discharged and then let sit for a few min, a charge may build up on the plates again. When a charge is removed from the plates the orbitals of the dielectric try to return to their original shape this takes time, after the initial discharge a charge can be re-established on the plates as the orbitals return to their original shape

54
Q

Safe discharge of capacitors

A

always discharge it and short-cct the leads

CEC requires the capacitors be provided with a means by which can be automatically discharged.

55
Q

time constant for an RC cct

A

t = RC
t the length of one time constant in seconds
C the capacitance of the capacitor in farads
R the resistance of the resistor in ohms

56
Q

Capacitance is the property of an electric cct

A

that opposes any change in the voltage across that cct

57
Q

the total capacitance of capacitors connected in series is always less than that of the

A

smallest individual capacitor. this is because connecting capacitors in series has the same effect as increasing the distance between the plates of one capacitor

58
Q

for capacitors in series

A

the total capacitance is the reciprocal of the sum of each capacitor

59
Q

for capacitors in parallel

A

the total capacitance is the sum of each individual capacitor

60
Q

Inductive reactance

A

Xl = 2(pi)fL

61
Q

reactive (wattless) power

A

is the rate at which energy is transferred back and forth between the source and the inductor
Unit is VARs

62
Q

Active power

A

is transferred to the load and dissipated as energy in some other form
Unit is Watt

63
Q

Q factor is

A

the ratio or inductive reactance to its resistance

64
Q

Saturable reactors

A

use a small DC current to control the flow of a larger AC current

65
Q

capacitive reactance formula

A

Xc = 1/(2pifC)

66
Q

VARs leading

A

in a capacitive cct, the currents leads the voltage, and capacitive VARs is stated as VARs leading

67
Q

VARs lagging

A

In an inductive cct, current lags the voltage, and inductive VARs is stated as being VARs lagging.

68
Q

Reactive power

A

= volts x amps x sin(theta)
when the phase angle is 0 then the reactive power is zero the cct is purely resistive
when the phase angle is 90 then there is no true power the cct is purely reactive

69
Q

Inductive reactance makes VARs

A

lagging

70
Q

Capacitive reactance makes VARs

A

leading

71
Q

Impedance is the term

A

for total opposition to current flow in an AC cct

72
Q

Power factor is

A

Watt/VA

Year 2 Electrical (9 decks)

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