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71 Cards in this Set
- Front
- Back
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What is a disadvantage of a direct-current system with respect to supply voltage?
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The dc voltage must be generated at the level required by the load.
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What disadvantage of a direct current is due to the resistance of the transmission wires?
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The I 2R power loss is excessive.
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What kind of electrical current is used in most modern power distribution systems?
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Alternating current (ac).
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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
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What is the direction of the magnetic field around a vertical conductor when (a) the current flows
upward and (b) the current flows downward. |
(a) clockwise (b) counterclockwise
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The "left-hand rule" for a conductor is used for what purpose
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It is used to determine the relation between the direction of the magnetic lines of force around a
conductor and the direction of current through the conductor. |
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In what direction will the compass needle point when the compass is placed in the magnetic field
surrounding a wire? |
The north pole of the compass will point in the direction of the magnetic lines of force
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When two adjacent parallel wires carry current in the same direction, the magnetic field about
one wire has what effect on the magnetic field about the other conductor? |
It combines with the other field.
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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.
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What is the shape of the magnetic field that exists around (a) a straight conductor and (b) a coil?
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(a) The field consists of concentric circles in a plane perpendicular to the wire (b) the field of each
turn of wire links with the fields of adjacent turns producing a two-pole field similar in shape to that of a simple bar magnet. |
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What happens to the two-pole field of a coil when the current through the coil is reversed?
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The polarity of the two-pole field reverses.
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What rule is used to determine the polarity of a coil when the direction of the electron current flow
in the coil is known? |
Use the left-hand rule for coils.
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When a conductor is rotated in a magnetic field, at what points in the cycle is emf (a) at maximum
amplitude and (b) at minimum amplitude? |
(a) When the conductors are cutting directly across the magnetic lines of force (at the 90º and
270º points). (b) When the conductors are moving parallel to the magnetic lines of force (at the 0° , 180° , and 360° points). |
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One cycle is equal to how many degrees of rotation of a conductor in a magnetic field?
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360° .
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State the left-hand rule used to determine the direction of current in a generator.
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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° from the forefinger and it will point in the direction of electron current flow in the conductor |
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How is an ac voltage produced by an ac generator?
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Continuous rotation of the conductor through magnetic fines of force produces a series of cycles
of alternating voltage or, in other words, an alternating voltage or a sine wave of voltage. |
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Define Frequency
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Frequency is the number of complete cycles of alternating voltage or current completed each
second. |
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What term is used to indicate the time of one complete cycle of a waveform?
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Period.
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What is a positive alternation?
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A positive alternation is the positive variation in the voltage or current of a sine curve
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What do the period and the wavelength of a sine wave measure, respectively?
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The period measures time and the wavelength measures distance.
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How many times is the maximum or peak value of emf or current reached during one cycle of ac?
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Twice.
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If any point on a sine wave is selected at random and the value of the current or voltage is
measured at that one particular moment, what value is being measured? |
The instantaneous value (Einst or Iinst)
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What value of current or voltage is computed by averaging all of the instantaneous values during
the negative alternation of a sine wave? |
Average value (Eavg or Iavg)
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What is the average value of all of the instantaneous currents or voltages occurring during one
complete cycle of a sine wave? |
Zero
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What mathematical formulas are used to find the average value of current and average value of
voltage of a sine wave? |
I avg= .636(Imax)
E avg= .636(Emax) |
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If Emax is 115 volts, what is Eavg?
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E avg= .636(115)
73.14 volts |
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If Iavg is 1.272 ampere, what is Imax?
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If I avg= I max(.636) then I max= I avg/.636
I max= 1.727/.636 amps 2 amps |
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What is the most convenient basis for comparing alternating and direct voltages and currents?
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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. |
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What value of ac is used as a comparison to dc?
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The effective value.
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What is the formula for finding the effective value of an alternating current?
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I eff= .707(I max)
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If the peak value of a sine wave is 1,000 volts, what is the effective (Eeff) value?
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E eff= .707(E max)
= .707(1000) =707 volts |
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If I eff = 4.25 ampere, what is I max?
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I max= 1.414(I eff)
= 1.414(4.25) = 6 amps |
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A series circuit consists of three resistors (R1 = 10Ω, R2 = 20Ω, R3 = 15Ω) and an alternating
voltage source of 100 volts. What is the effective value of current in the circuit? |
I= E/R
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What is the phase relationship between current and voltage in an inductor?
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Current lags voltage by 90º (ELI).
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What is the term for the opposition an inductor presents to ac?
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Inductive reactance
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What is the formula used to compute the value of this opposition?
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Xl= 2piefL
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What happens to the value of XL as frequency increases?
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XL increases.
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What happens to the value of XL as inductance decreases?
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XL decreases.
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What effect does the capacitor have on a changing voltage?
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The capacitor opposes any change in voltage.
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What is the phase relationship between current and voltage in a capacitor?
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Current leads voltage by 90º (ICE).
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What is the term for the opposition that a capacitor presents to ac?
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Capacitive reactance.
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What happens to the value of XC as frequency decreases?
Xc= Capacitive reactance |
XC increases.
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What happens to the value of XC as capacitance increases?
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XC decreases.
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What term is given to total opposition to ac in a circuit?
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Impedance.
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What is the true power in an ac circuit?
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True power is the power dissipated in the resistance of the circuit or the power actually used in
the circuit. |
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What is the reactive power in an ac circuit?
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Reactive power is the power returned to the source by the reactive components of the circuit.
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What is the unit of measurement for reactive power?
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var.
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What is apparent power?
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The power that appears to the source because of circuit impedance, or the combination of true
power and reactive power. |
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What is the unit of measurement for apparent power?
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VA (volt-amperes).
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What is the power factor of a circuit?
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PF is a number representing the portion of apparent power actually dissipated in a circuit.
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What is meant by "transformer action?"
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The transfer of energy from one circuit to another circuit by electromagnetic induction.
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What are, the three basic parts of a transformer?
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Primary winding; secondary winding; core.
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What are three materials commonly used as the core of a transformer?
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Air; soft iron; steel.
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What are the two main types of cores used in transformers?
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Hollow-core type; shell-core type.
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Which transformer windings are connected to an ac source voltage and to a load, respectively?
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Primary to source; secondary to load.
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A transformer designed for high-voltage applications differs in construction in what way from a
transformer designed for low-voltage applications? |
Additional insulation is provided between the layers of windings in the high-voltage transformer
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What is meant by a "no-load condition" in a transformer circuit?
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A voltage is applied to the primary, but no load is connected to the secondary.
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What is meant by "exciting current" in a transformer?
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Exciting current is the current that flows in the primary of a transformer with the secondary
open (no load attached). |
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What is the name of the emf generated in the primary that opposes the flow of current in the
primary? |
Self-induced or counter emf.
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What causes a voltage to be developed across the secondary winding of a transformer?
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The magnetic lines generated by the current in the primary cut the secondary windings and
induce a voltage into them. |
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What is the phase relationship between the voltage induced in the secondary of an unlike-wound
transformer and the counter emf of the primary winding? |
In phase. Remember, the cemf of the primary is 180 degrees out of phase with the applied
voltage. The induced voltage of the secondary of an unlike-wound transformer is also 180 degrees out of phase with the primary voltage. |
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What is "leakage flux?"
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Lines of flux generated by one winding which do not link the other winding.
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What effect does flux leakage in a transformer have on the coefficient of coupling (K) in the
transformer? |
It causes K to be less than unity (1).
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Does 1:5 indicate a step-up or step-down transformer?
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Step up.
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Name the three power losses in a transformer.
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Copper loss, eddy-current loss, and hysteresis loss.
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Why should a transformer designed for 400 hertz operation not be used for 60 hertz operation?
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The inductive reactance at 60 hertz would be too low. The resulting excessive current would
probably damage the transformer. |
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List five different types of transformers according to their applications.
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a. Power transformer
b. Autotransformer c. Impedance-matching transformer d. Audio-frequency transformer e. Radio-frequency transformer |
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The leads to the primary and to the high-voltage secondary windings of a power transformer
usually are of what color? |
Primary leads-black; secondary leads-red.
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What is the cause of most accidents?
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Carelessness
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Before working on electrical equipment containing capacitors, what should you do to the
capacitors? |
Discharge them by shorting them to ground.
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When working on electrical equipment, why should you use only one hand?
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To minimize the possibility of providing a path for current through your body.
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