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Electromagnetics (Difference in self induction in AC and DC)

  1. Nov 9, 2011 #1
    Hello folks.

    I have an electromagnetics quiz tomorrow and I am kind of nervous. I can't seem to grasp the concept of induction, more importantly how it differs in AC and DC circuits.

    So in lay terms: Induction is production of electric current across a conductor which is moving through or being passed by a magnetic field.

    Tell me if I am way off here:

    In DC, when the primary current is turned on, the induced current moves in the opposite direction of the primary current. When the primary current reaches it's peak, no induced current is imparted since the magnetic lines of force are stationary (no electromotive force). When the primary current is switched off, the induced current gradually decreases, moving in the same direction as the primary current.

    In AC, a steady flow of electrons is supplied for half the cycle (since the electrons move back and forth) During the other half of the cycle, the north and south poles of the magnetic field induce an against the incoming supply of electrons, which is called inductive reactance.

    Someone please tell me if I'm on the right track here. This stuff is driving me nuts


  2. jcsd
  3. Nov 9, 2011 #2


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    Staff Emeritus
    Science Advisor

    I THINK the AC circuit will experience a resistance to a change in current for both directions, not just one. Anyone clarify?
  4. Nov 9, 2011 #3


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    Staff: Mentor

    I don't distinguish any difference in the phenomenon, whether using switched DC or sinusoidal AC. When the current is changing, a voltage is induced, and the faster the current changes, the higher the induced voltage. When the current is not changing (i.e., is held temporarily constant), there is no induced voltage. This covers all driving waveforms, switched DC, triangular ramps, sinusoidal, whatever.
    There is not much I can agree with in this sentence. It seems you are are considering a transformer or induction coil. If so, then when the primary current is abruptly interrupted, then the induced voltage will be a tall short-duration spike. I wouldn't picture the secondary current as necessarily "gradually decreasing". The secondary current will be governed by the circuit in the secondary, and the coil's own equivalent C, L and R. Typically, with nothing but measuring instruments and stray capacitance loading on the secondary side, you will see a ringing spike (indicative of an underdamped second-order system) each time the primary current is abruptly interrupted.
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