Year 2 D1

Question 1

Faraday’s law of electromagnetic induction

Answer 1

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.

Question 2

Faraday’s formula

Answer 2

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

Question 3

Flux density

Answer 3

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

Question 4

Lenz’s Law

Answer 4

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

Question 5

Induced voltage

Answer 5

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

Question 6

The frequency of the generated emf

Answer 6

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

Question 7

Formula for frequency is

Answer 7

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

Question 8

transformers have efficiencies in

Answer 8

95% to 99% range

Question 9

AC

Answer 9

periodically changes in both magnitude and direction

Question 10

Cycle

Answer 10

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

Question 11

Alternation

Answer 11

one cycle of a sine wave contains two alternations

Question 12

Period

Answer 12

is the time it takes to complete one cyle

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

Question 13

Frequency

Answer 13

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

Question 14

Amplitude

Answer 14

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

Question 15

peak value

Answer 15

max positive or negative value of the waveform

Question 16

Peak to Peak

Answer 16

is twice the peak value

Question 17

Instantaneous value

Answer 17

of the waveform is constantly changing

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

Question 18

Average value

Answer 18

of a waveform is .637 time peak

average value for one complete sine wave is zero

Question 19

Effective value

Answer 19

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

Question 20

in an AC cct effective resistance is made up of

Answer 20

pure ohmic resistance
skin effect losses
hysteresis losses
eddy current losses
dielectric losses
radiation losses

Question 21

opposition to AC current flow is called

Answer 21

impedance

Question 22

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

Answer 22

Peff = .5 x Pm

.5 x 1200 W = 600 W

Question 23

electromagnetic induction

Answer 23

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

Question 24

Inductance formula

Answer 24

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

Question 25

Inductors are built by

Answer 25

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

Question 26

Air-core inductors

Answer 26

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

Question 27

Iron-core inductors

Answer 27

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

Question 28

variable inductors

Answer 28

Used in welding machines | tuners in radios

Question 29

factors affecting inductance and formula

Answer 29

``` 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 ```

Question 30

Inductance is the property of an electric cct

Answer 30

that opposes any change in current in that cct

Question 31

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

Answer 31

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

Question 32

mutual induction

Answer 32

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

Question 33

DC current will only induce a voltage in an Most come use is the automobile ignition coil

Answer 33

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

Question 34

Inductance of inductors in series

Answer 34

cct is the sum of the individual inductances

Question 35

Inductance of inductors in parallel

Answer 35

cct is the sum of the reciprocals of the individual inductances

Question 36

Coulomb's law of electrostatic force

Answer 36

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

Question 37

Coulomb's law of electrostatic force formula

Answer 37

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

Question 38

Electric field intensity

Answer 38

``` E = V/d E = the intensity of the field expressed in volts per metre ```

Question 39

dielectric strength

Answer 39

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

Question 40

Capacitance of the capacitor | Unit for capacitance is farad F

Answer 40

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

Question 41

Define farad

Answer 41

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

Question 42

A capacitors working voltage is

Answer 42

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

Question 43

Three thing affect the capacitance of a capacitor

Answer 43

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

Question 44

dielectric constant

Answer 44

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

Question 45

permittivity

Answer 45

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

Question 46

formula for calculating the capacitance of a capacitor

Answer 46

``` 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 ```

Question 47

Types of capacitors

Answer 47

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

Question 48

Three types of fixed capacitors

Answer 48

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

Question 49

special capacitor

Answer 49

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

Question 50

electrolytic capacitor puncturing

Answer 50

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

Question 51

Two ways to vary the capacitance of a variable capacitor

Answer 51

vary the effective area of the plates | vary the distance between the plates

Question 52

capacitor problems

Answer 52

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

Question 53

Dielectric absorption

Answer 53

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

Question 54

Safe discharge of capacitors

Answer 54

always discharge it and short-cct the leads | CEC requires the capacitors be provided with a means by which can be automatically discharged.

Question 55

time constant for an RC cct

Answer 55

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

Question 56

Capacitance is the property of an electric cct

Answer 56

that opposes any change in the voltage across that cct

Question 57

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

Answer 57

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

Question 58

for capacitors in series

Answer 58

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

Question 59

for capacitors in parallel

Answer 59

the total capacitance is the sum of each individual capacitor

Question 60

Inductive reactance

Answer 60

Xl = 2(pi)fL

Question 61

reactive (wattless) power

Answer 61

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

Question 62

Active power

Answer 62

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

Question 63

Q factor is

Answer 63

the ratio or inductive reactance to its resistance

Question 64

Saturable reactors

Answer 64

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

Question 65

capacitive reactance formula

Answer 65

Xc = 1/(2pifC)

Question 66

VARs leading

Answer 66

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

Question 67

VARs lagging

Answer 67

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

Question 68

Reactive power

Answer 68

= 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

Question 69

Inductive reactance makes VARs

Answer 69

lagging

Question 70

Capacitive reactance makes VARs

Answer 70

leading

Question 71

Impedance is the term

Answer 71

for total opposition to current flow in an AC cct

Question 72

Power factor is

Answer 72

Watt/VA