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# NAVEDTRA 14174A

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

Question | Answer |
---|---|

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 |