# Both plates of capacitor connected to positve voltage

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1. Dec 31, 2014

### arpon

If both the plates of a capacitor are connected to positve voltage, (but has a potential differnce between them) will the capacitor be charged? And how?

2. Dec 31, 2014

### Bystander

And what potential is this particular ground at with respect to another ground? Does the ground potential matter?

3. Dec 31, 2014

### arpon

Actually, by 'ground', I mean something which is electrically neutral. Both of the plates are connected to some conductor which has lack of electrons (and so they are electrically positive). But the voltages of the plates are not the same.

4. Dec 31, 2014

### Drakkith

Staff Emeritus
Well, if one plate is charged to +5v and the other to +3v, is there a potential difference between them?

5. Dec 31, 2014

### arpon

Why not? It's $(+5V) - (+3V) = 2V$

6. Dec 31, 2014

### Drakkith

Staff Emeritus
Okay. So if the potential between the plates is 2 volts, do you think the capacitor is "charged"? Why or why not?

7. Dec 31, 2014

### arpon

In a capacitor, one of the plates is charged positive and the other negative and they are equal in quantity. But in this case, both the plates lack electron. So how can it provide negative charge or, electron?
If I assume, the plate connected to $3V$ is charged negative, then the plate will have a negative electric potential, and so there should be a current flow because it is connected with a voltage generator which will always keep potential to $+3V$.

Last edited: Dec 31, 2014
8. Dec 31, 2014

### Drakkith

Staff Emeritus
Both plates lack a very, very small number of electrons. Over 99.999% of the electrons are still on the plates. The plate charged to +5v simply has a few more electrons missing. A few missing electrons adds up to a large amount of charge.

9. Dec 31, 2014

### arpon

But the point is, if the plate connected to $+3V$ is negatively charged, then it will have a negative electric potential. As, it is connected to a voltage generator which always keeps the voltage to $+3V$, there should be a current flow. So, the plate will keep losing electrons, won't it?

10. Dec 31, 2014

### Drakkith

Staff Emeritus
Not at all. The plate will lose electrons until it reaches +3 volts. It will then stop losing them and remain at +3v.

11. Dec 31, 2014

### arpon

How can the plate have negative charge, but positive electric potential?

12. Dec 31, 2014

### Drakkith

Staff Emeritus
It doesn't have a "negative charge", it has -2 volts of potential relative to ground compared to the +5v plate. Remember that electric potential is measured between two points. Measuring the voltage between the ground and each plate gives you +3v and +5v, but measuring from one plate to the other gives you either +2v or -2v. In other words, if you measure the voltage between the plates with a volt meter, you'll get either +2v or -2v depending on how you place the leads. Switching them around will cause current to flow the other way through the voltmeter and it will read the voltage a being the opposite sign.

13. Dec 31, 2014

### arpon

So, you are saying, both the plates will have positive charges?

14. Dec 31, 2014

### Drakkith

Staff Emeritus
Relative to ground, yes.

15. Dec 31, 2014

### Staff: Mentor

arpon,

Absolute potentials don't matter, only relative potentials. You could label both plates as +5 +3 or +1005 +1003 or -1005 -1003, and the result is the same.

Zero volts, is an arbitrary choice. We normally choose "ground" as zero volts becasuse it is natural and convenient. But other choices for zero work just as well.

An "uncharged object" has the same number of electrons as protons. But even an uncharged object can have a plus or minus potential difference between it and some other object.

Think relative, not absolute voltage. Balls roll down a ramp the same if the ramp is at the top of a mountain or at the bottom of a valley, as long as one end of the ramp is at a height higher relative to the other end.

16. Dec 31, 2014

### CWatters

Stop press. Scientists discover the earth isn't at 0V. The "ground" we have all being relying on is actually at -1000V wrt the sun today. Would it make any difference to your circuit?

17. Dec 31, 2014

### arpon

In the typical situation (which is most commonly described in text books), one of the plate is charged $+Q$ and the other $-Q$; If the potential difference is $V$, then the capacitance is defined as $C = \frac{Q}{V}$.
Then, what it would be in this case as I mentioned in post #1. My question is in the point that Drakkith said both the charges are positive.

18. Dec 31, 2014

### Staff: Mentor

Relative to ground yes, but also relative to any common point (of which ground is a convenient but not mandatory point).

Look at all the responses so far. They are all telling you the same thing.

19. Dec 31, 2014

### Drakkith

Staff Emeritus
Use 2 volts as the voltage in the equation and you'll come out with the right answer.