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55 Cards in this Set

  • Front
  • Back
Alternating current can be defined as current that varies in
1. amplitude and direction
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
Alternating current has replaced direct current in modern transmission systems
because it has what advantage?
4. Ac can be readily stepped up or down
A waveform is a graphic plot of what quantities?
2. Amplitude versus time
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
What statement accurately describes the magnetic field
surrounding a current-carrying
conductor?
It is perpendicular to and equal along all parts of the conductor
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
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
The magnetic field surrounding a straight conductor is (a) what shape, and (b) is in what position relative to the conductor?
(a) Concentric circles
(b) Perpendicular
Why is a two-pole magnetic field set up around a coil?
Because separate lines of magnetic force link and combine their effects
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
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
When a loop of wire is rotated through 360º in a magnetic field, the induced
voltage will be zero at what point?
180º
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
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
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
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
What is the term for the number of complete cycles of ac produced in one second?
Frequency
What is the unit of measurement for frequency?
Hertz
A loop of wire rotating at 60 rpm in a magnetic field will produce an ac voltage of what frequency?
1 HZ
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
The value of current of an ac waveform taken at any particular moment of time
is what type of value?
Instantaneous value
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
The total of ten instantaneous values of
an alternation divided by ten is equal to what value?
The average value
What mathematical formula is used to find the average value of voltage for an ac voltage?
Eavg = 0.636 X Emax
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
The rms value for an ac voltage is equal to what other ac value?
Eavg
What value will result by squaring all values for Einst, averaging these values,
and then taking the square root of that average?
Eeff
An ac voltmeter is usually calibrated to read what AC value?
Effective
If the maximum value for an ac voltage is known, the Eeff can be found by using which of the following formulas?
Eeff = Emax X .707
What is an important rule to remember when using Ohm's Law to solve ac circuit problems?
Never mix values
The property of inductance offers opposition to what quantitie?
Changes in current
What is the unit of measurement for inductance?
Henry
What physical property is similar to inductance?
Inertia
The difference in potential across a resistor, created by current through the resistor is an example of what force?
Electromotive
When a magnetic field moves through a stationary conductor, the electrons in orbit are affected in what manner?
They are dislodged from orbit
When electrons are moved in a
conductor by a magnetic field, a force known by what term is created?
Electromotive
Self-induced emf is also known as what force?
Counter electromotive force
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?
It opposes either a rise or a fall in current
The direction of the induced voltage in an inductor may be found by application of what rule?
The left-hand rule for generators
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
How are inductors classified?
By core type
Normally, most coils have cores composed of either air or
soft iron
The hollow form of nonmagnetic
material found in the center of an aircore coil has what purpose?
To support the windings
Why do large diameter coils have greater inductance than smaller diameter coils, all other factors being the same?
Large diameter coils have more wire and thus more flux
If the radius of a coil is doubled, its inductance is increased by what factor?
Four
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
A soft iron core will increase inductance because it has what characteristics?
High permeability and low
reluctance
An increase in the permeability of the core of a coil will increase what coil characteristic?
Magnetic flux
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)
Flux linkage
Regardless of the method used,
inductance of a coil can ONLY be increased by increasing what coil characteristic?
Flux linkage
What does the Greek letter Delta signify as in "ΔI" or "Δt"?
The values are changing
Inductors experience copper loss for what reason?
Because the reactance of an inductor is greater than the resistance of an inductor
What term applies to the power loss in an iron core inductor due to the current
induced in the core?
Eddy-current loss
Power consumed by an iron core
inductor in reversing the magnetic field of the core is termed as what type of loss?
Hysteresis loss