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Alternating current can be defined as current that varies in
amplitude and direction
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Before a 120-volt dc source can be used to power a 12-volt load, the voltage must be reduced. Which of the following methods can be used?
A resistor placed in series with the load
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Alternating current has replaced direct current in modern transmission systems because it has which of the following advantages?
Ac can be readily stepped up or down
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A waveform is a graphic plot of what quantities?
Amplitude versus time
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Which of the following properties surrounds a current-carrying conductor?
A magnetic field
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Which of the following statements accurately describes the magnetic field surrounding a current-carrying conductor?
It is perpendicular to and equal along all parts of the conductor
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Which of the following factors determine(s) the intensity of a magnetic field surrounding a coil?
- The amount of current flow through the coil
- The type of core material
- The number of turns in the conductor
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When you grasp a coil in your left hand with your thumb pointing in the direction of the north pole, your fingers will be wrapped around the coil in the direction of the
current flow
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The power consumed in a conductor in realigning the atoms which set up the magnetic field is known as what type of loss?
Hysteresis loss
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The magnetic field surrounding a straight conductor is (a) what shape, and (b) is in what position relative to the conductor?
Concentric circles and Perpendicular
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When the induced voltage in a conductor rotating in a magnetic field is plotted against the degrees of rotation,the plot will take what shape?
A sine curve
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When a loop of wire is rotated through360º in a magnetic field, the induced voltage will be zero at which of the following points?
180
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When a loop of wire is rotated 360º in a magnetic field, at what points will the induced voltage reach its maximum (a)positive, and (b) negative values?
90º and 270
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According to the left-hand rule for generators, when your thumb points in the direction of rotation, your (a)forefinger and (b) your middle finger will indicate the relative directions of what quantities?
a) Magnetic flux, north to south,(b) Current
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Continuous rotation of a conductor through magnetic lines of force will produce what type of (a) voltage and(b) waveform?
(a) Ac, (b) sine wave
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What is the term for the number of complete cycles of ac produced in one second?
Period
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What is the unit of measurement for frequency?
Hertz
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A loop of wire rotating at 60 rpm in a magnetic field will produce an ac voltage of what frequency?
1 Hz f=1/t
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An ac voltage of 250 hertz has a period of
0.004 second 250 = 1/t
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What is the approximate frequency of an ac voltage that has a period of .0006 second?
1667 Hz f = 1 / .0006
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The value of current of an ac waveform taken at any particular moment of time is what type of value?
Instantaneous value
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The total of ten instantaneous values of an alternation divided by ten is equal to what value?
The effective value
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Which of the following mathematical formulas is used to find the average value of voltage for an ac voltage?
Eavg = 0.636 × Emax
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What is the average value of all of the instantaneous voltages occurring during one cycle of an ac waveform with a peak value of 60 volts?
38 volts
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If an ac voltage has an Emax of 220 volts,what is Eavg?
140 volts
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The value of alternating current that will heat a resistor to the same temperature as an equal value of direct current is known as
Ieff
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The rms value for an ac voltage is equal to what other ac value?
Eeff
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What value will result by squaring all values for Einst, averaging these values,and then taking the square root of that average?
Eeff
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If the maximum value for an ac voltage is known, the Eeff can be found by using which of the following formulas?
Eeff = Emax × .707
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If the Ieff of an ac waveform is 3.251. Eavg amperes, what is Imax?
4.6 amperes
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