Module 2

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Module 2
2012-01-03 11:25:20


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  1. What is the basic unit of inductance and the abbreviation for this unit?
    The henry, H.
  2. An emf is generated in a conductor when the conductor is cut by what type of field?
    Magnetic field
  3. Define inductance.
    Inductance is the property of a coil (or circuit) which opposes any CHANGE in current.
  4. What is meant by induced emf? By counter emf?
    Induced emf is the emf which appears across a conductor when there is relative motion between he conductor and a magnetic field; counter emf is the emf induced in a conductor that opposesthe applied voltage
  5. State Lenz's law
    The induced emf in any circuit is in a direction to oppose the effect that produced it.
  6. What effect does inductance have (a) on steady direct current and (b) on direct current while it is
    changing in amplitude?
    • a. No effect.
    • b. Inductance opposes any change in the amplitude of current.
  7. a. List five factors that affect the inductance of a coil.
    b. Bending a straight piece of wire into a loop or coil has what effect on the inductance of the
    c. Doubling the number of turns in a coil has what effect on the inductance of the coil?
    d. Decreasing the diameter of a coil has what effect on the inductance of the coil?
    e. Inserting a soft-iron core into a coil has what effect on the inductance of the coil?
    f. Increasing the number of layers of windings in a coil has what effect on the inductance of the
    • 1. The numbers of turns in a coil.
    • 2. The type of material used in the core.
    • 3. The diameter of the coil.
    • 4. The coil length.
    • 5. The number of layers of windings in the coil.
    • b. Increases inductance.
    • c. Increases inductance.
    • d. Decreases inductance.
    • e. Increases inductance.
    • f. Increases inductance.
  8. a. When voltage is first applied to a series LR circuit, how much opposition does the inductance
    have to the flow of current compared to that of the circuit resistance?
    b. In a series circuit containing a resistor (R1) and an inductor (L1), what voltage exists across
    R1 when the counter emf is at its maximum value?
    c. What happens to the voltage across the resistance in an LR circuit during current buildup in
    the circuit, and during current decay in the circuit?
    • a. Inductance causes a very large opposition to the flow of current when voltage is first applied
    • to an LR circuit; resistance causes comparatively little opposition to current at that time.
    • b. Zero.
    • c. During current buildup, the voltage across the resistor gradually increases to the same
    • voltage as the source voltage; and during current decay the voltage across the resistor
    • gradually drops to zero
  9. What is the formula for one L/R time constant?
    T= L/R
  10. a. The maximum current applied to an inductor is 1.8 amperes. How much current flowed in the
    inductor 3 time constants after the circuit was first energized?
    b. What is the minimum number of time constants required for the current in an LR circuit to
    increase to its maximum value?
    c. A circuit containing only an inductor and a resistor has a maximum of 12 amperes of applied
    current flowing in it. After 5 L/R time constants the circuit is opened. How many time
    constants is required for the current to decay to 1.625 amperes?
    • a. 1.71 amperes.
    • b. 5 time constants.
    • c. 2 time constants.
  11. State three types of power loss in an inductor
    Copper loss; hysteresis loss; eddy-current loss.
  12. Mutual Inductance?
    • Mutual inductance is the property existing between two coils so positioned that flux from one coil
    • cuts the windings of the other coil.
  13. When are two circuits said to be coupled?
    When they are arranged so that energy from one circuit is transferred to the other circuit.
  14. What is meant by the coefficient of coupling?
    • The ratio of the fines of force produced by one coil to the lines of force that link another coil. It is
    • never greater than one.
  15. Define the terms "capacitor" and "capacitance."
    • a. A capacitor is a device that stores electrical energy in an electrostatic field.
    • b. Capacitance is the property of a circuit which opposes changes in voltage.
  16. State four characteristics of electrostatic lines of force.
    • a. They are polarized from positive to negative.
    • b. They radiate from a charged particle in straight lines and do not form closed loops.
    • c. They have the ability to pass through any known material.d. They have the ability to distort the orbits of electrons circling the nucleus.
  17. An electron moves into the electrostatic field between a positive charge and a negative charge.
    Toward which charge will the electron move?
    Toward the positive charge.
  18. What are the basic parts of a capacitor?
    Two pieces of conducting material separated by an insulator.
  19. Define the term "farad."
    • A farad is the unit of capacitance. A capacitor has a capacitance of 1 farad when a difference of 1
    • volt will charge it with 1 coulomb of electrons.
  20. What is the mathematical relationship between a farad, a microfarad, and a picofarad.
    • a. One microfarad equals 10-6 farad.
    • b. One picofarad equals 10-12 farad.
  21. State three factors that affect the capacitance of a capacitor.
    • a. The area of the plates.
    • b. The distance between the plates.
    • c. The dielectric constant of the material between the plates.
  22. Name two types of power losses associated with a capacitor.
    • a. Hysteresis
    • b. Dielectric leakage
  23. Define the term "working voltage" of a capacitor.
    b. What should be the working voltage of a capacitor in a circuit that is operating at 600 volts?
    • a. It is the maximum voltage the capacitor can work without risk of damage.
    • b. 900 volts.
  24. State what happens to the electrons in a capacitor circuit when (a) the capacitor is charging and
    (b) the capacitor is discharging
    • a. When the capacitor is charging, electrons accumulate on the negative plate and leave the
    • positive plate until the charge on the capacitor is equal to the battery voltage.
    • b. When the capacitor is discharging, electrons flow from the negatively charged plate to thepositively charged plate until the charge on each plate is neutral.
  25. At what instant does the greatest voltage appear across the resistor in a series RC circuit when the
    capacitor is charging?
    At the instant of the initiation of the action.
  26. What is the voltage drop across the resistor in an RC charging circuit when the charge on the
    capacitor is equal to the battery voltage?
  27. An oxide-film dielectric is used in what type of capacitor?b. A screw adjustment is used to vary the distance between the plates of what type of capacitor?
    • a. Electrolytic capacitor
    • b. Trimmer capacitor
  28. What is meant by "transformer action?"
    The transfer of energy from one circuit to another circuit by electromagnetic induction.
  29. What are, the three basic parts of a transformer?
    Primary winding; secondary winding; core.
  30. What are three materials commonly used as the core of a transformer?
    Air; soft iron; steel.
  31. What are the two main types of cores used in transformers?
    Hollow-core type; shell-core type.
  32. Which transformer windings are connected to an ac source voltage and to a load, respectively?
    Primary to source; secondary to load.
  33. A transformer designed for high-voltage applications differs in construction in what way from a transformer designed for low-voltage applications?
    Additional insulation is provided between the layers of windings in the high-voltage transformer.
  34. What is meant by a "no-load condition" in a transformer circuit?
    A voltage is applied to the primary, but no load is connected to the secondary.
  35. What is meant by "exciting current" in a transformer?
    • Exciting current is the current that flows in the primary of a transformer with the secondary
    • open (no load attached).
  36. What is the name of the emf generated in the primary that opposes the flow of current in the primary?
    Self-induced or counter emf.
  37. What causes a voltage to be developed across the secondary winding of a transformer?
    • The magnetic lines generated by the current in the primary cut the secondary windings and
    • induce a voltage into them.
  38. What is the phase relationship between the voltage induced in the secondary of an unlike-wound transformer and the counter emf of the primary winding?
    • In phase. Remember, the cemf of the primary is 180 degrees out of phase with the applied
    • voltage. The induced voltage of the secondary of an unlike-wound transformer is also 180
    • degrees out of phase with the primary voltage.
  39. What is "leakage flux?"
    Lines of flux generated by one winding which do not link the other winding.
  40. What effect does flux leakage in a transformer have on the coefficient of coupling (K) in the
    It causes K to be less than unity (1).
  41. Does 1:5 indicate a step-up or step-down transformer?
    Step up.
  42. Name the three power losses in a transformer.
    Copper loss, eddy-current loss, and hysteresis loss.
  43. Why should a transformer designed for 400 hertz operation not be used for 60 hertz operation?
    The inductive reactance at 60 hertz would be too low. The resulting excessive current would probably damage the transformer.
  44. List five different types of transformers according to their applications.
    • a. Power transformer
    • b. Autotransformer
    • c. Impedance-matching transformer
    • d. Audio-frequency transformere. Radio-frequency transformer
  45. The leads to the primary and to the high-voltage secondary windings of a power transformer usually are of what color?
    Primary leads-black; secondary leads-red.
  46. What is the cause of most accidents?
  47. Before working on electrical equipment containing capacitors, what should you do to the capacitors?
    Discharge them by shorting them to ground.
  48. When working on electrical equipment, why should you use only one hand?
    To minimize the possibility of providing a path for current through your body.