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tank circuit
tank circuit  
in the lc tank circuit(with no resistance of wires or components),assuming that capacitor is fully charged initially and the process of that circuit is,the capacitor then start discharging. .Now I have two questions
1)why capacitor start discharging?
then after it completely discharged,it charges with opposite polarity ,so question 2 is:
Why it charge with opposite polarity or even how current pass through inductor,the basic work of inductor is to oppose current,so why or how current go through it,so as to charge capacitor oppositely?

Hello Ali,

Setting up this situation required a few steps. An external voltage was applied across the capacitor to charge it up before the inductor was introduced to the situation. Electrons were sent to one plate of the capacitor and the other plate suffered from a shortage of electrons. So one plate had + charge and the other had - charge. The electrons on the - plate were attracted to the + plate but could not get there. The capacitor was disconnected from the voltage but continued to have that voltage across it because it was "charged".

1. Suddenly the inductor was connected across the capacitor and those electrons saw a path to get to the + plate. Remember that all the electrons on the - side were feeling repulsion of each other and at the same time they also felt attraction to the other plate. So they tried to rush through the inductor to get to the other plate (and to get separation from the other electrons).

2. First let me correct one error in what you said. An inductor does not exactly oppose current, that is what a resistor does. An inductor opposes rapid changes in current - fights both increases and decreases in current. However, over a bit of time, the electrons were able to develop some current in their eagerness to get to the other plate. Once the capacitor had a potential difference between the plates of 0V, the electrons were evenly distributed and would have happily stopped where they were and stayed there. But the inductor would not allow the current to immediately go to zero.

Call it momentum, like a pendulum's momentum. When a pendulum rod is vertical, so the bob is at the lowest point, can it stop there? No, momentum of the bob forces it to continue to the other side. Very much the same thing with the current in this circuit. The inductor forces the current to continue in the same direction for a while. After a while the polarity across the capacitor is opposite what it was before, but the electrons again see that the other side of the capacitor has + charge. So they want to again go through the inductor toward that + charge. And the opposite but equivalent thing happens again.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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