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55 Cards in this Set
- Front
- Back
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Alternating current can be defined as current that varies in
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1. 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? 1. A resistor placed in parallel with the load 2. A resistor placed in series with the load 3. A step-down transformer placed in series with load 4. A step-down transformer placed in parallel-with the load |
2. 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 what advantage? |
4. Ac can be readily stepped up or down
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A waveform is a graphic plot of what quantities?
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2. Amplitude versus time
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Which of the following properties surrounds a current-carrying conductor?
1. A magnetic field 2. A repulsive force 3. An attractive force 4. An electrostatic field |
1. A magnetic field
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What statement 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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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?
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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?
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(a) Concentric circles
(b) Perpendicular |
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Why is a two-pole magnetic field set up around a coil?
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Because separate lines of magnetic force link and combine their effects
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When a conductor is moving parallel to magnetic lines of force, (a) what relative
number of magnetic lines are cut, and (b) what relative value of emf is induced? |
(a) Minimum
(b) Minimum |
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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 through 360º in a magnetic field, the induced
voltage will be zero at what point? |
180º
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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? |
(a) 90º
(b) 270º |
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When a coil of wire makes eight complete revolutions through a single magnetic field, (a) what total number of alternations of voltage will be generated and, (b) what total number of cycles of
ac will be generated? |
(a) 16
(b) 8 |
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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?
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Frequency
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What is the unit of measurement for frequency?
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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?
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1 HZ
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An ac voltage has a frequency of 350 Hz. In two seconds, what total number of times will the peak value of voltage
be generated? |
1400 times
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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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While the value of an ac voltage may be expressed as one of several values, the
accepted practice is to express it as what type value? |
Effective value
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The total of ten instantaneous values of
an alternation divided by ten is equal to what value? |
The average value
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What mathematical formula is used to find the average value of voltage for an ac voltage?
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Eavg = 0.636 X Emax
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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
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Ieff
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The rms value for an ac voltage is equal to what other ac value?
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Eavg
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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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An ac voltmeter is usually calibrated to read what AC value?
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Effective
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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?
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Eeff = Emax X .707
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What is an important rule to remember when using Ohm's Law to solve ac circuit problems?
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Never mix values
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The property of inductance offers opposition to what quantitie?
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Changes in current
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What is the unit of measurement for inductance?
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Henry
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What physical property is similar to inductance?
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Inertia
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The difference in potential across a resistor, created by current through the resistor is an example of what force?
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Electromotive
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When a magnetic field moves through a stationary conductor, the electrons in orbit are affected in what manner?
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They are dislodged from orbit
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When electrons are moved in a
conductor by a magnetic field, a force known by what term is created? |
Electromotive
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Self-induced emf is also known as what force?
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Counter electromotive force
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According to Lenz’s Law, the induced emf produced by a change in current in an inductive circuit tends to have what effect on the current?
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It opposes either a rise or a fall in current
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The direction of the induced voltage in an inductor may be found by application of what rule?
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The left-hand rule for generators
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When source voltage is removed from a current-carrying conductor, a voltage
will be induced in the conductor by what action? |
The collapsing magnetic field
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How are inductors classified?
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By core type
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Normally, most coils have cores composed of either air or
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soft iron
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The hollow form of nonmagnetic
material found in the center of an aircore coil has what purpose? |
To support the windings
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Why do large diameter coils have greater inductance than smaller diameter coils, all other factors being the same?
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Large diameter coils have more wire and thus more flux
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If the radius of a coil is doubled, its inductance is increased by what factor?
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Four
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If the length of a coil is doubled while the number of turns is kept the same, this will have (a) what effect on
inductance and (b) by what factor? |
(a) Decrease
(b) by 1/2 |
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A soft iron core will increase inductance because it has what characteristics?
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High permeability and low
reluctance |
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An increase in the permeability of the core of a coil will increase what coil characteristic?
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Magnetic flux
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If a coil is wound in layers, its inductance will be greater than that of a similar single-layer coil because of a higher (what)
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Flux linkage
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Regardless of the method used,
inductance of a coil can ONLY be increased by increasing what coil characteristic? |
Flux linkage
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What does the Greek letter Delta signify as in "ΔI" or "Δt"?
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The values are changing
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Inductors experience copper loss for what reason?
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Because the reactance of an inductor is greater than the resistance of an inductor
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What term applies to the power loss in an iron core inductor due to the current
induced in the core? |
Eddy-current loss
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Power consumed by an iron core
inductor in reversing the magnetic field of the core is termed as what type of loss? |
Hysteresis loss
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