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The energy stored in an inductor

  1. Dec 15, 2010 #1
    I knew the text book says the inductor store energy in magnetic field. The typical way they demonstrate this is by setting up a circuit, where a battery is parallel connected to a light bulb and an inductor. At the beginning, the light bulb lights up. Then the battery is cut off and the light bulb doesn't instantly turned off but slowly dimming until it is completely die out. This experiment shows that the inductor store energy.

    Okay I am fine with this demonstration and I also agree inductor store energy.

    But, can inductor store energy like a battery does?

    I think is better if I can illustrate a situation as follow:
    Now, imagine you connect only an inductor to a battery. So now the inductor slowly build up its magnetic field until the current flow reaches maximum value. Then suddenly, you take away the inductor (completely isolating it, i.e. the two terminals not connect with anything), put it at somewhere else, keep it like that for some duration (few seconds, minutes, hours, days, etc). After that, you take that inductor, connect the terminals to a light bulb, will the light bulb emits light?
  2. jcsd
  3. Dec 15, 2010 #2
    What is the energy stored in an inductor? Look at the expression.
  4. Dec 15, 2010 #3
    No, you can't do i that way. Breaking the current path in an inductor will cause a high voltage over the inductor and a spark between the two terminals.

    What you can do is short circuit the inductor, causing the current flow to maintain stable. Then you can store energy in it. But unless you have a superconductor, resistive losses will drain the energy form it pretty fast.
  5. Dec 15, 2010 #4
    Yes, and this is really done like this, in a lab of course. But it is a promising method of storing energy, i.e. from solar cells to use at night.
  6. Dec 15, 2010 #5
    E = 1/2*L*I
    Are you trying to say if there is no current flowing in the inductor (when it is isolated), there will be no energy stored in the inductor, and hence the light bulb will not emit light?

    But energy has already stored in it before it is isolated. Then, after it is isolated, if it is not capable of keeping that energy, where the energy goes (since energy is conserved)?
  7. Dec 15, 2010 #6
    what exactly happens during the spark and after that?
    When there is a spark, I think it means during that split second, the two terminals are considered "connected" and current flow during that split second. Then, does the spark causes all the energy in the inductor transformed into heat and that is the end of the story?

    About the inductor and wire made of superconductor, I think it is a great idea, but I am not exactly understand what is happening.
    Let's assume at the time the current from battery is cut off, the terminals of the inductor is shorted simultaneously. So, no current, magnetic field in solenoid starts to decrease, a change in magnetic flux. A change in magnetic flux causes a current to flow in the shorted inductor circuit, since it is a superconductor, current flows forever. With current keep flowing, magnetic field in solenoid maintains. Hence, can I say that originally before the inductor is shorted, if the magnetic field is B in it, then the magnetic field after it is shorted is B-b , where b is a very small value.
    Or do you think before and after the inductor is shorted, magnetic field doesn't change?
    Last edited: Dec 15, 2010
  8. Dec 15, 2010 #7
    Inductor: U=L*di/dt

    From this you can see that if the current is cut of instantaneous the voltage over the inductor becomes infinitely. The voltage will make the air conducting and you get a spark, maintaining current flow trough the inductor. The energy stored in the coil will dissipate into heat etc. and thats the end of the story.

    If there is no change in current there is no change in magnetic field. Shorting the terminals and a superconducting material will not change the current. And hence the magnetic field stays the same.
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