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Subelement A
Principles
Section 2
Magnetism
What determines the strength of the magnetic field around a conductor?
  • The resistance divided by the current.
  • The ratio of the current to the resistance.
  • The diameter of the conductor.
  • Correct Answer
    The amount of current.
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What will produce a magnetic field?
  • A DC source not connected to a circuit.
  • The presence of a voltage across a capacitor.
  • Correct Answer
    A current flowing through a conductor.
  • The force that drives current through a resistor.
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When induced currents produce expanding magnetic fields around conductors in a direction that opposes the original magnetic field, this is known as:
  • Correct Answer
    Lenz’s law.
  • Gilbert's law.
  • Maxwell’s law.
  • Norton’s law.
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The opposition to the creation of magnetic lines of force in a magnetic circuit is known as:
  • Eddy currents.
  • Hysteresis.
  • Permeability.
  • Correct Answer
    Reluctance.
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What is meant by the term “back EMF”?
  • A current equal to the applied EMF.
  • An opposing EMF equal to R times C (RC) percent of the applied EMF.
  • Correct Answer
    A voltage that opposes the applied EMF.
  • A current that opposes the applied EMF.

Back-EMF (Electromotive Force), is most commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the magnetic field from the motor's field magnets, or windings.

Remember "E" stands for voltage.

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Permeability is defined as:
  • The magnetic field created by a conductor wound on a laminated core and carrying current.
  • Correct Answer
    The ratio of magnetic flux density in a substance to the magnetizing force that produces it.
  • Polarized molecular alignment in a ferromagnetic material while under the influence of a magnetizing force.
  • None of these.
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