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NAVEDTRA 14174A

ALTERNATING CURRENT AND TRANSFORMERS CH 1,2,4,5

QuestionAnswer
Which type of current regularly changes direction? alternating
What is the amplitude of direct current in a wire when a COULOMB of electrons moves past a point in a wire in ONE SECOND and all of the electrons are moving in the same direction? one ampere
Which type of current is practically used by all modern commercial power companies? alternating
Which type of device can step up or down alternating voltages amplitude? transformer
Which type of patterns are plotted on graph paper that result from changes in amplitude with respect to time? waveform
What is the most commonly used means for producing or using electricity? magnetism
What year did Hans Christian Oersted discover a definite relationship between magnetism and electricity? 1819
Which unit indicates one cycle per second? Hertz (Hz)
What is frequency measured and always expressed in? Hertz (Hz)
What is the time required to complete one cycle of a waveform called? Period
What is the distance from zero to the maximum value of each alternation called? Amplitude
What is the distance along the waveform from one point to the same point on the next cycle? Wavelength
How many maximum or AC peak values during each complete cycle are there? two
What are alternating voltage and current usually expressed in? Effective values
Which law states that current is directly proportional to the applied voltage? Ohm's Law
What are two sine waves that are precisely in step with one another said to be? in phase
What is the amount by which one sine wave leads or lags another sine wave measured in? degrees
Which type of elements are resistors, lamps, and heating elements examples of? resistive
What is the effect called when a conductor is in a magnetic field and either the field or the conductor moves, an emf (voltage) is induced in the conductor? Magnetic induction
What is the maximum value reached during one alternation of a sine wave called? peak value
The average value of a sine wave of voltage or current is the average of all the instantaneous values during one alternation. The average value is equal to what amount of the peak value? 0.636
How many times the peak value is the effective value of a sine wave? 0.707
Which electrical circuit characteristic opposes the starting, stopping, or change in value of current? inductance
Which mass characteristic opposes a change in velocity? inertia
What is the difference of potential or voltage which exists between two points in an electrical circuit called? electromotive force
What is the EMF induced in the conductor carrying the current called? self-induced
Which law states that "The induced EMF in any circuit is always in a direction to oppose the effect that produced it"? Lenz's Law
What can a conductor be formed into in order to increase inductance? Loop/coil
What are inductors classified according to? core type
What are the coils said to have if they are so positioned with respect to each other so that all of the flux of one coil cuts all of the turns of the other? Unity coefficient of coupling
What is the requirement of the current in an inductor to increase to 63.2 % of the maximum or to decrease to 36.8 percent of the maximum current known as? time constant
Copper, hysteresis, along with what else are the main power losses in an inductor? Eddy-current
Which type of inductor power loss is due to power that is consumed in reversing the magnetic field of the core each time the current direction changes? Hysteresis
Which type of inductor power loss is due to core heating caused by circulating currents induced in an iron core by the magnetic field of the coil? Eddy-current
The opposition which a capacitor offers to AC is inversely proportional to frequency and capacitance. What is the opposition called? Capacitive reactance
AC circuits containing reactance also contain resistance, the two combine to oppose the flow of current. What is this combined opposition by the resistance and the reactance called? impedance
By how many degrees from that in a purely resistive circuit will the voltage in a purely reactive circuit differ in phase? 90 degrees
When current is out of phase with the voltage, the power indicated by the product of the applied voltage and the total current gives only what is known as what? apparent power
Which symbol is used to represent phase angle? theta
Which type of power appears to the source because of the circuit impedance? apparent power
Which number (represented by a decimal or percentage) represents the portion of the apparent power dissipated in a circuit? power factor
What is the easiest way to find the power factor? Find the power angle cosine
Which formula is used to determine the total reactance of a series AC circuit? X=XL-XC or X=XC -XL
What are the formula's derived from Ohm's Law used in AC? E=IZ and I=E/Z
Which device transfers electrical energy from one circuit to another by electromagnetic induction? transformers
Most power transformers are of what type? iron-core
Generally air-core transformers are used when the voltage source has a high frequency (above what)? 20 khz
What is the most popular and efficient type of transformer core? shell core
What is said to exist when a voltage is applied to the primary, but no load is connected to the secondary? No load condition
What is established around the winding when an alternating current flows through a primary winding? magnetic field
Which type of transformers are those in which the secondary voltage is in phase with the primary ? Likewound
What are the lines of flux generated by one winding which do not link with the other winding called? leakage flux
What is a transformer in which the voltage across the secondary is less than the voltage across the primary called? step down
What is defined as the magnetomotive force developed by one ampere of current flowing in a coil of one turn? ampere-turn
What indicates the amount by which the transformer increases or decreases the voltage applied to the primary? turns ratio
Small power transformers used in electrical equipment have an 80-90 % efficiency range, while large, commercial powerline transformers may have efficiencies exceeding what percent? 98%
The total power loss in a transformer is a combination of how many types of losses? three
Which type of low resistance wire are the primary and secondary windings of a transformer usually made of? copper
What is done to transformer cores to minimize the loss resulting from eddy currents? lamination
Power transformers normally operate from either 60 Hz, or what alternating current? 400 Hz
The power handling capacity of a transformer is measured in either the volt-ampere unit or what other unit? Watt
What are used to supply voltages to the various circuits in electrical equipment? power transformers
Radio-frequency (RF) transformers are used to couple circuits to which frequencies above what are applied? 20 khz
Created by: nng7294