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Define direct current.
An electrical current which flows in one direction only.
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Define alternating current.
An electrical current which is constantly varying in amplitude, and which changes direction at regular intervals.
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What is a disadvantage of a direct-current system with respect to supply voltage?
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?
The I^2R power loss is excessive.
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What kind of electrical current is used in most modern power distribution systems?
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?
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?
- (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?
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 90� and 270� 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?
360� .
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State the left-hand rule used to determine the direction of current in a generator.
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?
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.
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?
Period.
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What is a positive alternation?
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?
The period measures time and the wavelength measures distance.
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What is meant by peak and peak-to-peak values of ac?
The peak value is the maximum value of one alternation; the peak-to-peak value is twice the maximum or peak value.
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How many times is the maximum or peak value of emf or current reached during one cycle of ac?
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?
- Iavg = 0.636 x Imax
- Eavg = 0.636 x Emax
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If Emax is 115 volts, what is Eavg?
- Eavg = 73.14 volts
- (Eavg = 0.636 x 115 volts)
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If Iavg is 1.272 ampere, what is Imax?
- Imax = 2 amps
- (I max = 1272 / 0.636)
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What is the most convenient basis for comparing alternating and direct voltages and currents?
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?
The effective value.
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What is the formula for finding the effective value of an alternating current?
Ieff = 0.707 x Imax
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If the peak value of a sine wave is 1,000 volts, what is the effective (Eeff) value?
- Eeff = 707 volts
- (Eff = 0.707 x Emax)
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If Ieff = 4.25 ampere, what is Imax?
- Imax = 6 amps (Imax = 1.414 x Ieff)
- (Remember: Unless specified otherwise, the voltage or current value is always considered to be the effective value.)
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When are the voltage wave and the current wave in a circuit considered to be in phase?
When the two waves go through their maximum and minimum points at the same time and in the same direction.
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When are two voltage waves considered to be out of phase?
- When the waves do not go through their maximum and minimum points at the same time, a PHASE DIFFERENCE exists, and the two waves are said to be out of phase.
- (Two waves are also considered to be out of phase if they differ in phase by 180� and their instantaneous voltages are always of opposite polarity, even though both waves go through their maximum and minimum points at the same time).
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What is the phase relationship between two voltage waves that differ in phase by 360� ?
They are in phase with each other.
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How do you determine the phase difference between two sine waves that are plotted on the same graph?
Locate the points on the time axis where the two waves cross traveling in the same direction. The number of degrees between these two points is the phase difference.
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A series circuit consists of three resistors (R1 = 10 ohms, R2 = 20 ohms, R3 = 15 ohms) and an alternating voltage source of 100 volts. What is the effective value of current in the circuit?
- Ieff = 2.22 amps
- (Ieff = 100 / 45)
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If Eeff is 130 volts and Ieff is 3 amperes, what is the total resistance (RT) in the circuit?
43.3 ohms.
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