AC/DC Final Exam Ch 13-16
wavelength
A ___ is the distance covered by one complete cycle of a given frequency of sound as it passes through the air.
period
A ___ is the time required to produce one complete cycle of a waveform.
120
A conversion factor of ___ is used to convert frequency to cycles per sec.
False
A frequency that is too high can cause electric motors to run slow and draw excessive current.
True
A peak-to-peak value measures from the maximum positive to the maximum negative alternation of a sine wave.
True
A pure inductive circuit is an AC circuit with no resistance.
True
A stranded conductor with the same circular-mil rating of a solid conductor has a greater surface area and flexibility than the solid conductor.
False
A utilization voltage is a primary voltage of generators given to all loads.
False
A vector represents the changing value of a cosine wave.
thyristor
A(n) ___ is a solid-state switching device that switches current ON using a quick pulse of control current.
phasor
A(n) ___ is a vector in which length represents the magnitude of an electrical parameter and direction represents phase angle in electrical degrees.
diode
A(n) ___ is an electronic device that allows current to flow in only one direction.
volt-ampere
A(n) ___ is the unit of measure for apparent power.
True
AC generator output frequency is based on the number of poles and the rotational speed of the armature.
volt-amperes
AC generators are rated in ___.
True
AC is more easily distributed at higher voltages and lower current, with greater efficiency in the delivery of power.
True
AC waveforms cross the 0 A baseline when there is a reversal of current flow.
False
Admittance is the phase difference between the source voltage and resistive voltage.
volt-amperes
Alternators are rated in ___.
True
Although there is little true power in a pure reactive circuit, there is still current in the circuit.
True
An acute angle is one that is less than 90°.
False
An alternation is a complete cycle.
True
An average DC current causes greater heat than an average AC current.
False
An average value refers to a whole cycle of a sine wave.
True
Any vector can be broken into vertical and horizontal components by using the length of the vector and the sine and cosine values of the resultant vector angle.
True
Apparent power represents a load that has both true power and reactive power.
False
As frequency increases in a series inductive-resistive AC circuit, both impedance and current increase as well.
True
Bleeder resistors reduce changes in the ripple voltage when current is delivered to the load.
True
Coordinates on the x-axis that are right of the y-axis are positive, and coordinates on the x-axis that are left of the y-axis are negative.
True
Counter electromotive force (CEMF) is created by a change in current, which is caused by a constantly changing magnetic field that cuts the inductor.
True
Current can be calculated in inductive AC circuits by modifying Ohm's law to substitute inductive reactance (XL) for resistance (R).
False
Current in an inductive AC circuit can be calculated without knowing source voltage.
False
DC flow occasionally reverses in direction, unlike AC.
False
Deflection of an electron beam in a CRT computer monitor is responsive to the average voltage.
360
For a two-pole generator, one cycle is generated for each ___° of mechanical rotation of the rotor.
four
For an eight-pole generator, ___ electrical degrees are produced for each mechanical degree of rotation.
True
For an ignitron to conduct electricity, the anode must be positive.
False
Forward breakover voltage is the voltage needed to switch from a conductive state to a nonconductive state in a silicon-controlled rectifier.
True
Frequency does not affect resistance in a parallel inductive-resistive AC circuit.
True
Harmonics can either be voltage harmonics or current harmonics.
True
High transmission voltages are reduced to lower voltages at primary distribution transformer substations.
True
Ignitrons are useful for their ability to withstand high voltages and conduct high currents.
False
Impedance aids the flow of alternating current.
False
Impedance is measured in joules.
False
In AC circuits, voltage and current are always in phase.
True
In a live circuit, it is not possible to separately measure inductive voltage and inductive resistance because the inductor is the resistive element.
False
In a parallel inductive-resistive AC circuit, the algebraic sum of the two currents is equal to the source current.
True
In an AC resistive circuit, instantaneous current and voltage are in phase while power is always a positive value.
True
In an inductive AC circuit, counter-electromotive force is 180° out of phase with the source voltage.
True
In inductive-resistive AC circuits, voltage is the same across all components connected in parallel.
x
In listing a vector coordinate, the ___-coordinate is listed first.
True
Inductance can be held constant with the frequency varied, or frequency can be held constant with the inductance varied.
False
Inductors have no resistance.
True
Kirchhoff's voltage law states that the algebraic sum of the voltage in a closed-loop circuit is equal to zero.
True
Lower armature rotation speed increases the life of the generator.
True
Mass and length are both examples of scalar quantities.
True
Mathematical methods often provide answers that are more accurate than graphical methods.
180
Maximum out-of-phase condition occurs when signals are ___° out of phase.
True
Only resistance can dissipate power in an electrical circuit.
False
Peak power is equal to effective voltage multiplied by effective current.
True
Phase is the time relationship of a sine wave to a known time period.
True
Poor power factor exists when the power factor in a circuit is less than 90%.
False
Power dissipated in a circuit is always negative.
False
Quality factor is measured in volt-amperes reactive.
False
Resistances connected in series are subtracted from each other in an AC circuit.
True
Semiconductor devices make it easy to switch from AC to DC.
True
Skin effect occurs in AC when more current flows near the outer surface of a conductor at higher frequencies.
True
Steam-powered turbines can be used to drive electric power generators.
True
The Greek letter theta (Θ) is used to designate an angle.
hertz
The ___ is the international unit of frequency equal to 1 cycle per second.
tangent
The ___ of a right triangle represents the ratio of the lengths of the sides opposite and adjacent to an acute angle.
False
The common value of voltage used in homes in the United States is 240 VAC.
False
The difference between a DC generator (dynamo) and an AC generator is that the AC generator has split rings (commutator), whereas the DC generator has slip rings.
True
The difference between the parallelogram calculation method and the vector diagram calculation method is that the parallelogram calculation method operates on two vectors at a time.
True
The difference between true power and reactive power is that true power supplied to a resistive load is used to perform work or is dissipated as heat.
True
The parallelogram calculation method is a graphical method.
two
The parallelogram calculation method operates on ___ vectors at a time.
False
The part of the generator from which electric power is generated is known as the stator.
False
The root-mean-square voltage value is also known as the mathematical factor for the peak value.
False
The standard power source line frequency in Canada is 120 Hz.
True
The strength of the electromagnet for small DC generators can be adjusted externally with fixed wiring to its field.
False
The tangent of a right triangle (sin) represents the ratio of the length of the side opposite an acute angle to the length of the hypotenuse.
False
The value of impedance is always less than that of reactance or resistance.
True
The vector diagram calculation method connects the heads and tails of the individual vectors.
True
The wavelength of a signal and its frequency are proportional.
False
Thyristors are typically used in applications such as power control systems, battery chargers, and protection circuits.
True
Total current is equal to current flow through any component in a series resistive circuit.
False
True power is measured in volt-amperes and apparent power is measured in watts.
False
Vectors can be added only when they are in the same direction.
False
Vectors cannot be used to represent real-life parameters such as voltage.
False
Vectors that are not in phase with each other can still be added directly.
True
Voltage at any given point during the rotation of a vector can be calculated by multiplying the sine of the vector rotation angle by the maximum peak voltage.
False
Wavelength becomes longer as frequency is increased and is measured in meters.
False
When an AC value is given without it being specified, it is assumed to be a peak value.
True
When current flows through a diode, electrons are supplied by the positive plate of the capacitor and flow through the bleeder resistor.
False
When subtracting vectors, the resultant vector magnitude is always a negative number.
Transformers
___ are electrical devices that use electromagnetism to change voltage from one level to another.
Frequency-modulated
___ broadcasting operates at 54 MHz to 216 MHz.
Direct
___ current is current that flows in only one direction.
Trigonometry
___ is the branch of mathematics that uses the relationships between the lengths of the sides of a triangle and the angles to perform calculations.
