Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
43 Cards in this Set
- Front
- Back
|
Define direct current.
|
An electrical current which flows in one direction only.
|
|
|
Define alternating current.
|
An electrical current which is constantly varying in amplitude, and which changes direction at regular intervals.
|
|
|
What is the disadvantage of a direct current system with respect to supply voltage?
|
The DC voltage must be generated at the level required by the load.
|
|
|
What disadvantage of a direct current is due to the resistance of the transmission wires?
|
Power loss is excessive.
|
|
|
What kind of electrical current is used in most modern power distribution systems?
|
Alternating Current.
|
|
|
When placed in the vicinity of a current-carrying conductor, the needle of a compass becomes aligned at what angle to the conductor?
|
The needle aligns itself at right angles to the conductor.
|
|
|
What are the directions of the magnetic field around a vertical conductor for the following:
A) Current flowing upward B) Current flowing downward |
A) Clockwise
B) Counter clockwise |
|
|
Describe the "left hand rule" and how it is applied.
|
It is used to dertermine the relation between the direction of the magnetic lines of force around a conductor and the dirction of the current through the conductor.
|
|
|
A compass is placed in a magnetic field surrounding a wire. In which direction will the compass needle point?
|
The north pole of the compass will point in the direction of the magnetic lines of force.
|
|
|
When two adjacent parallel wires carry current in the same direction, the magentic field about one wire has what effect on the other?
|
It combines with the other field.
|
|
|
When two adjacent parallel conductors carry current in opposite directions, the magnetic field about one conductor has what effect on the magnetic field about the other conductor?
|
It deforms the other field.
|
|
|
What is the shape of the magnetic field that exists around
A) a straight conductor? B) a coil? |
A) The field consists of concentric circles in a plane perpendicular to the wire
B) The field of each turn of wire links witht he fields fo adjacent turns producing a two-pole field similar in shape to that of a simple bar magnet. |
|
|
This happens to the two-pole field of a coil when the current through that coil is reversed.
|
The polarity of the two-pole field is reversed.
|
|
|
What rule is used to determine the polarity of a coil when the direction of the electron current flow iin the coil is known?
|
Use the left hand rule for coils.
|
|
|
State the purpose of the rule used in Q14
|
Grasp the coil in your left hand, with your fingers "wrapped around" in the direction of electron flow. The thumb will point toward the north pole.
|
|
|
When a conductor is rotated in a magnetic field, which points in the cycle are emf
A) at maximum amplitude? B) at minimum amplitude? |
A) When the conductors are cutting directly across the magenetic lines of force at 90 degrees and 270 degrees.
B) When the conductors are moving parallel to the magnetic lines of force at 0, 180, and 360 degrees. |
|
|
One cycle is equal to how many degrees of rotation of a conductor in a magnetic field?
|
360 degrees.
|
|
|
State the left hand rule used to determine the direction of current in a generator.
|
Extend your left hand so that your thumb points in the direction of conductor movement, and your forefinger points in the direction of the magnetic flux (North to South). Now point your middle finger 90 degrees from the forefinger and it will point in the direction of electron current flow in the conductor.
|
|
|
How is an AC voltage produced by a generator?
|
Continuous rotation of the conductor through magnetic lines of force produces a series of cycles of alternating voltage or, in other words, a sine wave of voltage.
|
|
|
Define frequency.
|
Frequency is the number of complete cycles of alternating voltage or current completed each second.
|
|
|
What term is used to indicate the time of one complete cycle of a waveform?
|
Period.
|
|
|
What is a positive alternation?
|
A positive alternation is the positive variation in the voltage or current of a sine curve.
|
|
|
What do period and wavelength of a sine wave measure, respectively?
|
the period measures time and the wavelength measures distance.
|
|
|
Describe the difference between peak and peak-to-peak values of AC.
|
The peak value is the maximum value of oen alternation; the peak-to-peak value is twice the maximum or peak value.
|
|
|
How many times is the maximum or peak value of emf or current reached during one clycle of AC?
|
Twice.
|
|
|
If any point on a sine wave is selected at reandom, and the value of the current or voltage is measured in that moment, what value is being measured?
|
The instantaneous value (E inst or I inst)
|
|
|
What value of current or voltage is computed by averaging all the instantaneous values during the negative alternation of a sine wave?
|
Average value (E avg or I avg).
|
|
|
What is the average value of all the instantaneous currents or voltages that occur during one complete cycle of a sine wave?
|
Zero.
|
|
|
Which mathematical formulas are used to find the average value of current and voltage of a sine wave?
|
I avg= .636 * I max
E avg= .636 * E max |
|
|
If E max is 115 volts, what is E avg?
|
E avg= .636 * 115 volts
E avg= 73.14 volts |
|
|
If I avg is 1.272 Amps, what is I max?
|
2 amps
|
|
|
What is the most convenient basis for comparing alternating and direct voltages and currents?
|
The power (heat) produced in a resistance by a DC voltage is compared to that produced in the same resistance by an AC voltage of the same peak amplitude.
|
|
|
What value of AC is used as a comparison to DC?
|
The effective value
|
|
|
What is the formula for finding the effective value of an alternating current?
|
I eff= .707 * I max
|
|
|
If the peak value of a sine wave is 1,000 volts, what is the effective (E eff) value?
|
E eff= 707 volts
|
|
|
If I eff= 4.25 Amps, what is I max?
|
6 amps
|
|
|
When are voltage and current waves in a circuit considered in phase?
|
When two waves go through their maximum points at the same time and in the same direction.
|
|
|
When are two voltage waves considered out of phase?
|
When the waves do not go through their maximumand minimum points at the same time.
|
|
|
What is the phase relationship between two voltage waves that differ in phase by 360 degrees?
|
They are in phase with each other.
|
|
|
How do you determine the phase difference between two sine waves that are plotted on the same graph?
|
Locate the points on the time axis where the two waves cross traveling inthe same direction. The number of degrees between these two mpoints is the phase difference.
|
|
|
A series circuit consists of three resistors: R1=10Ohms, R2=20Ohms, R3=15Ohms and an alternating voltage source of 100 volts. What is the effective value of current in the circuit?
|
2.22 Amps
|
|
|
If the alternating source in Q41 is changed to 200volts peak-to-peak, what is I avg?
|
1.41 Amps
|
|
|
If E eff is 130 volts and I eff is 3 amps, what is the total resistance (Rt) in the circuit?
|
43.3 Ohms
|
