Study Test Questions D1 Flashcards
If the peak to peak voltage is 588.4 and the heating load is 2000watts of heat what is the resistance of this heat?
Peak to peak voltage / 2 =
peak voltage x 0.707 = E (RMS)
R= E*2/P
588.4/2= 294.2Em x 0.707= 208E
208*2\2000=21.632 ohms
If a source has an instantaneous voltage of 50 at 18• degrees, what would the instantaneous voltage be at 80degrees?
Instant to peak voltage
50E x sine 18• = 161.8Em
First angles max voltage to another angle at max voltage.
161.8Em x 80• = 159.34Em
State three reasons why alternating current is used more than direct current?
Cheaper, smaller, simpler, the step up or step down fiction of a transformer
List five applications where direct current is preferred or required instead of alternating current.
Mining, high intensity light sources that use carbon arcs, electro-plating, battery charging, precise speed, synchronized motors and alternators
What does Epp stand for?
Peak to peak value
2 X the peak
What is another name for RMS OR ROOT MEAN SQUARE?
Effective Value
At what angle in a sine wave does peak value occur?
90° and -270°
With respects to the peak value, what is the average value of an AC sine wave?
0°
With respects to the peak value, what is the average value of one alternation of an AC Sine wave?
.637
What two meters must be use to determine the effective resistance of an AC circuit?
Watt meter and amp meter
Ohm and volt meters might explode
What term is used to refer to the circulating currents in a magnetic core?
Eddy currents
As the frequency of the AC Increases, the effective resistance_____________.
Increases
What are the three formulas for finding Frequency, voltage and number of field poles?
F= PxN/120
N= 120xF/P
P= 120xF/N
P= poles N= speed in rpm F= Frequency
The peak power in an AC lighting circuit is 1200w. What is the effective power in that circuit?
P= 0.5 x Pm
= 0.5x1200w
=600w
What is the instantaneous voltage at 30° into the AC cycle on a sine wave with an effective value of 480E?
E= 0.707x Em
Em= E/0.707
= 480/0.707x VOLTAGE
=679 Em
e = sine 30°x 679V
=0.5x 679v
= 339.5e
If an AC source with a peak voltage of 339.4 Em is placed across a load with a resistance of 288ohms, what power will the resistance dissipate?
E= 0.707 x Em
P= E*2/R
200w
What is the frequency of an alternating voltage produced by a 10-pole alternator turning at a speed of 600rpm?
F= PxN/120
50hz
An turns at a speed of 450 rpm and produces an alternating current of 60 Hz. How many poles does this alternator contain?
P= fx120/ N
16 poles
At what rpm must a prime mover turn a six pole alternator to produce an alternating voltage with a frequency of 50Hz?
N= 120xF/P
1000rpm
What is the effective voltage of a sine wave with a peak value of 679 volts?
E = Em x 0.707
480.053 E
What is the average value of one alternation of a sine wave with an effective value of 277 volts?
Eav= 277
Eav/0.707= Em
Em x .631= E
247.22
What three quantities affect the magnitude of an induced Emf?
Active length, Flux density, velocity
In Flemings left hand rule which direction does the middle finger point?
Direction of induced electron current flow,
Index= direction of field
Thumb= thrust or motion of conductor
What is meant by electromagnetic term “prime mover”?
The device supplying energy to the generator.
A conductor has an active length of 2meters. To induce a voltage of 240 volts, at what velocity must it be pushed through a field of flux density 1.1T (tesla)
V=e/BL
109 m/s
A conductor is to be pushed through a field with a flux density of 1.5Tesla at a velocity of 100 m/a to produce a voltage of 347v. What must the active length of the conductor be?
L= e/BV
2.3m
To increase the field flux density in a generator, would the resistance of the field rheostat have to be increased or decreased?
Decreased
The unit of inductance is the
L for Henry
Increasing the length of the magnetic circuit in an inductor ________ it’s inductance
Decreases
Adding a ferrite core to a coil will cause it’s inductance to ______
Increase
State the equation that relates induced voltages to inductance
E = -L x(^I/^t)
What does the abbreviation cemf stand for?
Counter electromagnetic force
When a conductor is cut by two webers of flux lines in one second, what is the induced voltage?
2 volts
Tripling the number of turns in an inductor increases the inductance _________ times.
Nine
Calculate the total equivalent capacitance in a circuit consisting of four capacitors with the following values connected in series:
20uF, 30uF, 50uF, 60uF?
(1/.00002)+(1/.00003)+(1/.00005)+(1/.00006)=.00000833(8.33uF)
What would the total equivalent capacitance be for the following four capacitors connected in parallel?
5uF, 10uF? 20uF, 60uF
95uF
Add all the sums together.
series you add as followed 1/Ft= 1/Fa+1/Fb+1/Fc
When a capacitor is discharging from a fully charged state, what percentage of the source voltage will exist across the capacitor after one time constant?
- 3%
63. 7-100=36.3
Define capacitance?
Capacitance is the property if an electric circuit which opposes any change in voltage across the circuit
As voltage across the capacitor increases, the current flowing through the circuit _______ and the voltage across the current limiting resistor ______.
Decreases, decreases
What is the formulas for calculating time constants of a circuit in terms of its capacitance and resistance?
T= RC
Where does the capacitor store it’s energy?
The capacitor stores it’s energy in the electric field between it’s played and in the distortion of the orbitals in its dielectric.
What equation that determines maximum current in an RC circuit?
I = Es/R
What is the equation for one coulomb?
- 24 x 10*18=1 coulomb
* = power
What is the formula for Coulomb’s law of electrostatic force?
F= k Q1xQ2/d*2
F= the force of attraction or repulsion between charges in newtons.
K= the constant of proportionality for the type of medium between the two charges.
Q= the magnitude of the charges in coulombs.
d = the distance between the charges in the metres. P
*= power
The forces between two points of electric charges is______ proportional to the square of the distance between them, and ______ proportional to the product of the two charges
Inversely , directly
This is coulombs electrostatic field law
What is a dielectric?
A dielectric is an insulator exposed to an electric field.
What is the unit of electric field intensity?
An electric field may be expressed in either Volts per meter or newtons per coulomb.
State the formula for calculating the capacitance of a capacitor.
C= 8.85 x 10*-12 x Kx A/d
*= power
Name two devices that can be used to discharged a built up charge safely.
Field discharge resistors, free wheeling diodes
List to items that utilize mutual induction
Motors and car ignitions
