AC theory Part 2

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  1. Capacitance is measured in what? AC Theory, NJATC 2009 PP 91
    Farads
  2. Capacitors have the characteristic to oppose what? AC Theory, NJATC 2009 PP 91
    To oppose a change in voltage. They are used in motor variable frequency drive applications for this reason.  They are said to block AC and pass DC.  They are also used in AC to DC full wave bridge rectifiers to filter the DC and also on generators for this same reason on the voltage regulator.
  3. Capacitors are used to block and pass what? voltage, current, static, etc. Which one? AC Theory, NJATC 2009 PP 91

    Regarding the above statement, do the electrons pass through the insulating dielectric and/ or exert electrical pressure? AC Theory, NJATC 2009 PP 91
    Block DC and pass AC. Direct current doesn’t flow through a capacitor, in an alternating-current circuit, the electrons in the circuit move back and forth to alternately charge the capacitor, first in one direction, and then in the other. A capacitor permits current to flow because of its ability to store ‘ and then discharge the energy as the alternating current flows the opposite direction. Source: Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 69  

    The electrons do not pass through the dielectric plate but do exert electric pressure.
  4. Using scientific notation, what is Micro, Nano, and Pico? AC Theory, NJATC 2009 PP 91
    • Micro 10>(-6)
    • Nano 10>(-9)
    • Pico 10>(-12)
  5. The symbol for a fixed and variable capacitor is what? AC Theory, NJATC 2009 PP 91
  6. The general formula for capacitance is what? AC Theory, NJATC 2009 PP 92

    This formula is used to do what? AC Theory, NJATC 2009 PP 92
    • C = Q/V
    • where:
    • C= capacitor value in farads
    • Q= charge in coulombs
    • V= voltage applied

    This formula is used to compare the charge a capacitor can hold per volt that is applied to the capacitor. The larger the plates the more the voltage.
  7. Capacitors appear to oppose a change in ______. AC Theory, NJATC 2009 PP 93
    voltage: block dc but pass ac as well.
  8. The formula for calculating the capacitance of a capacitor measured in farads is? AC Theory, NJATC 2009 PP 92
    • C = .225AK/10^6D
    • where:
    • C = capacitance in microfarads
    • A = cross-sectional area of one plate in square inches
    • K = dielectric constant of the material between the plates (for air K is 1)
    • d = the thickness of the dielectric (or the distance between the plates) measured in inches
  9. What is the relationship between the current and voltage in a capacitor? AC Theory, NJATC 2009 PP 97
    The current in the capacitor circuit leads the voltage across the capacitor by 90 electrical degrees. (ELI the ICE man)
  10. Capacitors designed for use in AC circuits are considered what? AC Theory, NJATC 2009 PP 98
    Non-polarized, meaning no specific voltage polarity. Polarized would be indicated WVDC (working volts DC) or a +/- stamp.
  11. What abbreviation means that a capacitor is rated for a maximum voltage in DC terms? AC Theory, NJATC 2009 PP 99
    WVDC: working volts DC
  12. Polarized capacitors are indicated how and used in what type of voltage? AC Theory, NJATC 2009 PP 99
    By a positive or negative marked leads and used only in DC voltage.
  13. The plate with a lack of electrons in a capacitor has a ______charge, where the plate with an excess of electrons has a ______ charge. AC Theory, NJATC 2009 PP 100
    • positive
    • negative
  14. What is the relationship between the voltage rating and the thickness of the dielectric in a capacitor? AC Theory, NJATC 2009 PP 104
    The voltage rating is decreased proportionally to the thickness of the dielectric.
  15. What is a shorted capacitor and how does this happen?
    Shorted: current flows = really low ohmic value….the more the capacitor is charged, the stronger the electrical field. If the capacitor is overcharged, the electrons from the negative plate could be pulled through the insulation to the positive plate- If this happens, the capacitor is said to have broken down (shorted). Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 70.
  16. How do you test a capacitor? AC Theory, NJATC 2009 PP 105

    What are the symptoms of a leaky / shorted capacitor?
    By using an ohm-meter and watching for an ohmic spike as he capacitor charges and current flow decreases, and then as current ceases an infinite ohmic value is given.

    • Leaky: current flows = never reaches infinite ohms
    • Shorted: current flows = really low ohmic value….the more the capacitor is charged, the stronger the electrical field. If the capacitor is overcharged, the electrons from the negative plate could be pulled through the insulation to the positive plate- If this happens, the capacitor is said to have broken down (shorted). Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 70.
  17. Capacitors charge and discharge at an exponential rate when connected with what? AC Theory, NJATC 2009 PP 106

    Proven by the Time Constant formula for capacitors which is what?
    When connected with resistors or in series with resistors

    • T=RxC
    • Versus T=L/R
  18. Is there Capacitive reactance in a D.C. circuit and what does it oppose? AC Theory, NJATC 2009 PP 110

    What is the formula for capacitive reactance?

    What does this formula show?
    NO, capacitive reactance only exists in an AC circuit and it opposes a change in current.

    XC= 1/ (2pieFC)

    That as the frequency increases XC decreases, or that they are inversely proportional to each other.
  19. What is the formula for Capacitance? AC Theory, NJATC 2009 PP 113
    C=1/(2pieFXc)
  20. Inductors store energy in a _________, and capacitors store energy in a__________. AC Theory, NJATC 2009 PP 91
    • magnetic field
    • electrostatic field
  21. It has two leads, the anode and the cathode, which is where it gets its name from. http://www.funtrivia.com
    Diode.  The greek words ana and hodos means up and way, so this is the negative lead.  The greek words katta and hodos means down and way, and therefore is the positive lead, using conventional electron theory.
  22. ______ were used for logic circuits in some of the earliest computers. They were later replaced by modern semiconductors and are used to switch high voltage circuits. http://www.funtrivia.com
    • Relays.  Modern semiconductors such as the PNP transistor or the NPN transistor use a base collector and emitter to switch a circuit.
  23. This is a type of semiconductor in which the flow of current can be controlled though a base or gate terminal it is usually used for electronic switching or amplifiers. http://www.funtrivia.com
    Transistor
  24. This consists of usually two coil windings which usually share a magnetic core. It is used to convert an ac source or signal into another ac source or signal with same power (ideally) but different voltage and current. http://www.funtrivia.com
    transformers
  25. This is a special type of diode that breaks down when a critical reverse voltage is applied. it is most commonly used to regulate voltage. http://www.funtrivia.com
    Zener diode
  26. This is a special type of diode that lights up when a small voltage is applied. They are usually used as indicators or in numerical or alphanumerical displays. http://www.funtrivia.com
    LED. LED's use mili-amps and therefore large amounts of power is displays and therefore LCD's using milliamps are preferred.
  27. A multi-segment rotating connection that is connected to the armature windings defines what term? AC Theory NJATC 2nd Ed. 2009, PP 341
    A Commutator is connected to this.
  28. Impedance is a combination of what 3 components in AC theory?
    • resistance, inductive reactance, and capacitive reactance compose the 3 parts of this.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP63
  29. Alternating current is primarily used because it can be transmitted ________ by transforming it to high-transmission level voltages then converting this voltage back to lower distribution voltage levels. High-voltage transmission lines allow for the transmission of alternating-current at high voltage, and relatively low current. High-voltage transmission, with the outcome of lower current, results reduced voltage drop in the transmission lines. Conductors and electrical equipment are sized based on the current in the lines, so the lower current levels also allow the use of smaller wiring smaller electrical equipment.
    • inexpensively.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP64

    Although it is important to note that the voltage drop calculation for AC requires the use of VD=1.73KLI/CM versus 2KLI/CM for DC systems.  This means that a 120volt DC system versus a 120volt AC system with the same distance and wiring will have a smaller number as the resulting voltage drop in an AC system.  As illustrated by the calculations below for a 100ft long and 15 amp circuit using number 12AWG respectively.

    • VD=2KLI/CM        or.       VD=1.73KLI/CM
    • VD=2x12.9x100x15/6530=5.93 vd
    • VD=1.73x12.9x100x15/6530=5.17 vd
  30. magnitude of the voltage produced depends upon? In AC voltage.
    • The number of turns of wire, the strength of the Magnetic field, and the speed at which the coil rotates determines this. 
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 64
  31. The rotating conductor loop is called a “_____” or “_________.” “_____” or " _______” rings and carbon brushes are used to connect the output voltage from the generator to an external circuit.
    rotor or armature and slip or collector rings accomplish this loop to output voltage. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 64
  32. Small single-phase ac generators have a conductor loop rotating inside a magnetic field. 
    Note: Larger ac generators have a rotating ____________
    magnetic field inside stationary conductors. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 65
  33. Using electromagnets permits the strength of magnetic field (and thus the lines of force) to be modified, thereby controlling __________.
    • the output voltage can be controlled using this for magnetism in a generator.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 65
  34. _____ generators are separately excited. Batteries or small dc generator are typically used with the ac generator.
    • All of these are separately excited. 
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation PP 65
  35. Devices that intentionally introduce capacitance into circuits are called “capacitors.” An older term for these components is “_________.”
    • condensers is another term for this.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, Based on the 2014 NEC PP 69
  36. Be careful when working on a circuit that capacitors; such as, those found in ________. Capacitors can store large amounts to energy a long period of time. They can present a shock hazard after the power has been removed.
    • Such as those found in variable speed drives.  You must be careful when working around these as they can carry a charge after the circuit is off.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP70
  37. Capacitance is inversely proportional to _________. The closer the plates of the capacitor are to each ‘ the greater the capacitance and, conversely, the greater the distance is between the plates, the lower the capacitance.
    • The distance between the capacitor plates is inversely proportional to this.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP 71
  38. the voltage induced within conductor caused by its own expanding and collapsing magnetic field known as “_________”. 
    Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP73
    self-induced voltage is this in regards to the expanding and collapsing field in AC.  This is also known as VAR or Volt Amps Reactive which is the energy stored in the collapsing field.
  39. CEMF aids or opposes current flow?
    • This either opposes or aids conductor current flow.  As alternating current increases, the polarity of the induced volt(CEMF) within the conductor opposes the conductor’s current. As it tries to prevent the current from increasing. However, as alternating current decreases, the polarity of the induced voltage (CEMF) within the conductor aids the conductor’s current and tries to prevent the current from decreasing.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP73

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Author:
johnbowens
ID:
124557
Filename:
AC theory Part 2
Updated:
2017-04-16 21:52:42
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Ac theory JATC IBEW NJATC capacitance
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Ac theory in partnership with the NJATC 2009 copyright
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