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# Physics/Is it possible to charge a grounded capacitor ?

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Question
Hello Steve,

A have a simple question. I have a Signal generator with an output of 0,1 W and would like to charge a capacitor. We know from AC that, it is possible the drive a current through a capacitor. I want to use 500 kHz (0,50 MHz) my capacitor has a capacitance of 0,68 micro Farad, lets say R is 5 ohm, so, I would find Z (impedance) as 5,02 ohm (XC could be neglected !), so the current would be P = IxIxZ, I find i as 141 mA, this would flow (approximately) through the capacitor according the rules of displacement current, I think so far is everything okay. This is the case if I connect the 2 poles of the capacitor to my signal generator !

My question would be, what happens, when I make a one way connection, so, I would connect my capacitor to the SG and pulse one plate of the capacitor, the other plate would be connected into the earth (grounding) I speculate, the same process would occur, between ground and the capacitor, while it is AC, there will be always a change of the charge polarity between the plates, also between the 2nd plate and the ground, would this system
work ?
Would the i value, be same with the first case, or not, why ?
My target is to pulse the first plate, the second plate of the capacitor my be connected into the ground, or maybe to a grounded transformer, would such arrangements work ?

Hmmm, is it needed to ground the SG in such arrangements ?
But be careful, the grounding of the SG and the grounding of second plate of the Capacitor would not have any interactions with each other !!!

My only target is to pulse the first plate with a random frequency, by this way I expect to create a current flow as in the first case. If I ground the second plate of the capacitor, the charges should flow back and forth between the 2nd plate and ground, am I right ?
What can you say about the current value, would this be still 141 mA ?

My electrician friend says, it would not work while the circuit is open, hmmm, this is not DC, so, I hope to be correct with my expectations, would need your guidance !

With my best regards,
Birol

Answer
You can't just ignore Xc in this case, it's 0.468 Ohms at the frequency you mention.  Yes, you have to have the same ground for the other capacitor plate as for the signal generator or this whole thing is meaningless.  I question what value of L you used for your inductor, where'd you get it at all?

Then you go talk about random frequency.  What does that even mean?  No, it won't be the same at all, you went and radically altered the parameters by changing the frequency.  If you start lowering the frequency any further, you'll increase the impedance of the capacitor.  Is 0.1 Watts the maximum power or some sort of setting?  That would make little sense, but it seems essential to your mathematics.  Re-think this.

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#### Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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