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current in coil

Discussion in 'Electronic Basics' started by stefan, Jul 11, 2003.

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  1. stefan

    stefan Guest

    When I power up a DC coil the current increases rather slowly, so they
    say. But is this true for the very, very first moment? You do need a
    current to get a counter EMF or???? At atomic level, is the current
    equal everywhere in the coil?
  2. John Larkin

    John Larkin Guest

    I = T*V/L

    When T=0, I=0. So they say.

  3. You need a rate of change of current to get an EMF across a coil.
    This works, even when the current passes through zero as it changes,
    or includes zero as a starting point. From the lumped component
    viewpoint, the current is the same everywhere in the coil. From a
    fields viewpoint, the applied voltage passes through the wire as a
    wave, at the local speed of light. This wave has to charge all the
    stray capacitances as it proceeds, so things get a little complicated
    on the very small and very fast scale.
  4. Yes for ideal coils. No for coils with paracitic capacitance.
    U = L* dI/dT
    When the current changes (dI) in the time (dT), a voltage will be
    enerated depending on the inductance L. Because you place a voltage
    on the coil, a current will be generated, increasing in time (assuming
    an ideal coil with 0 ohm resistance): dI/dT = U/L depending on the
    voltage and inductance. As you place the voltage on the coil, the
    voltage of the counter emf IS the voltage you place on the coil? In
    other words, the current changes so the coil counter emk's up to you
    supplied voltage.

    When there is a resistor in the coil, the counter EMK will drop with
    the current, counter EMK = (Supplied voltage) - I * R.
    Once the highest current is reached (after several L*R times), the
    counter EMK will be almost zero, as the current doesn't change


    Pieter Hoeben
  5. Are you talking about an ideal inductor? Real inductors are
    extremely complicated. They're waveguides and radio antennas
    and tuned circuits and inductors all in one. As soon as you
    stop seeing them as ideal inductors, all the complexity comes
    boiling out.

    In the first few hundred picoseconds, waves of EM energy
    are distributing a voltage-pattern through all the parts
    of the inductor. Yet usually we can't turn the power
    supply on in a matter of picoseconds. If it takes
    hundreds of thousands of picoseconds for the voltage to
    rise, then the energy-waves are too small to notice.

    You need an EMF to cause a current to begin rising.
    Voltage is associated with e-fields and with the
    push/pull upon electrons, so voltage causes current.
    In ideal inductors, yes. In real inductors, no, not
    in the first fractions of nanoseconds while waves are
    bouncing around between different parts of the coil.
    (Imagine sound waves bouncing around inside a long
    plastic tube. Now imagine that you slowly raise the
    speed of air flow in that tube without creating any
    sound waves.)
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