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Inductance -- why does current lag voltage?

  1. Dec 9, 2014 #1
    Could someone give me an intuitive explanation as to why the current lags the voltage in an inductive circuit. I can understand it through the equation E=ldi/dt. But how exactly does the current lag, on a molecular level?
     
  2. jcsd
  3. Dec 9, 2014 #2

    DaveC426913

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  4. Dec 10, 2014 #3

    sophiecentaur

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    EM effects are not "molecular" phenomena so I don't think there exists an answer on those terms. But you could perhaps think of how the emf that is induced across an inductor will be proportional to the rate of change of the magnetic field i.e the current flowing. If the current is sinusoidal then the voltage will have the biggest value when the current waveform passes through zero.
    The Mathematical description takes a lot of beating - horses for courses. . . .
     
  5. Dec 10, 2014 #4

    CWatters

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    Perhaps start by understanding why voltage lags current in a capacitor. When you turn on the current, charge flows into the capacitor and the voltage builds up much like water filling a bucket. You don't expect a bucket to fill up the instant you start pouring in water.

    Inductors are essentially the compliment of a capacitor.
     
  6. Dec 11, 2014 #5

    sophiecentaur

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    What a perfectly splendid Capacitor you have there. :D
    It complements the colours on your resistor stripes well.
     
  7. Dec 11, 2014 #6
    In a typical conductor, electrons carry the current. Current is just the flow of electrons. A voltage is an electric potential, which is generated by an imbalance of charge. The electric field is just the spatial derivative of the potential, ##E = -\nabla \phi##.

    You can break it down into simple Newtonian mechanics. The electric field exerts a force on electrons, which accelerates them. Therefore, the electric field causes a current to form, and the current lags behind the electric field (and voltage) due to the inertia of the electrons.
     
  8. Dec 11, 2014 #7

    sophiecentaur

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    It isn't simple Newtonian mechanics; that's far too simplistic to be of use in understanding what is happening in an inductor. The electrons have such little mass and the KE involved is so low that it plays no significant part in the way circuits work. The macroscopic approach is more than enough to account for what is measured.
     
  9. Dec 11, 2014 #8

    CWatters

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    My spelling is even better than my dress sense ;-(
     
  10. Dec 11, 2014 #9

    sophiecentaur

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    It did make me smile. Only a temporary lapse on your part, I'm sure. :)
     
  11. Dec 11, 2014 #10
    I cannot explain in molecular level, but I hope a physical explanation should suffice.
    Lets say, you apply a voltage = E, across an inductor (which is actually just a coil of wires). Now, since an inductor is actually just wires wound into coil, current will tend to flow. But, the current cannot suddenly rise, because, the change in current would create a changing magnetic field (remember it is a coil), and that changing magnetic field would produce/induce an Emf in the coil, that opposes the applied emf. If the current tends to change too fast, the internal emf will tend to exceed the applied emf, and hence the current will have to reduce. If the current doesn't change fast enough, the applied emf will increase the current and make it change quick. So, only allowed case is if the current change just in the right way. For an inductor with zero resistance, the E_applied = E_internal must be instantaneously be true, otherwise the difference would create infinite current. If you assume the initial condition to be I=0 and E_internal = 0 and apply the E_applied, then I will increase instantly (and would tend to be infinite), but to do that it will need to change, and the change will make E_internal = dI/dt. So, it can neither change faster than dI/dt = E_applied nor can it change slower than that. So, for +ve E_applied current will increase and for -ve E_applied current will decrease, which makes the current tend to follow E.
    Having written all that, I think, it doesn't do better than just saying "its because the inductor follows that equation, and the solution of that equation implies that current lags behind voltage", and which doesn't really answer your question. But anyway, I tried.
     
  12. Dec 12, 2014 #11

    CWatters

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    The energy stored in a capacitor is proportional to the charge in the capacitor and hence the voltage.
    The energy stored in an inductor is proportional to the magnetic field and hence the current.

    It takes time to move energy from one place to another so it takes time to move energy into or out of a capacitor or inductor.

    So..
    In a capacitor it takes time for the voltage to build up following a step change in the current.
    In an inductor it takes time to for the current to build up following a step change in the voltage.

    In both cases it's the fact that it takes time to move energy is the cause of the lag.
     
  13. Dec 12, 2014 #12

    sophiecentaur

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    Or how about this?
    An inductor is just a short circuit to a non changing current. There will be no Volts across it when DC passes. When you change the current, the magnetic field will change and induce a Voltage in the wire (by Lenz's Law, it will be in opposition to its cause), The faster the current changes, the greater the voltage so the peak of the voltage when an Alternating Current happens to be sinusoidal, will be as the current waveform changes at its fastest rate i.e. at the zero crossing.
    Note that when the current waveform is not a sinusoid, the Voltage waveform will be a distorted version of the waveform because each frequency component is shifted by 900, which will not produce the same shaped resultant waveform. So you will not just get a time shifted version.
     
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