# Why do capacitors 'in series' each carry the same charge?

1. Feb 11, 2010

### jrdelacruz

1. The problem statement, all variables and given/known data
Exactly from my homework:
"It is assumed that for capacitors 'in series' that each capacitor carries the same charge Q. Give a physical argument for this. Why does this make sense?"

2. Relevant equations

3. The attempt at a solution
I've tried searching for the answer, and none of it is making sense to me, perhaps someone can explain it in a simpler way? This is what I have come up with so far, please help me finish/correct it:

"Let us assume that there are 2 capacitors connected in series by a battery, capacitor 1 consists of plates A and B of size X and capacitor 2 consists of plates C and D of size Y. The plates are lined up in series alphabetically. Going to plate A is a positive charge Q. This means the charge –Q will be on D."
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 11, 2010

### Redbelly98

Staff Emeritus
Welcome to Physics Forums.

You have a conductor comprising the connection between the capacitors, and one plate from each capacitor. This conductor is insulated from anything else. What must the net (i.e. total) charge on this conductor be?

3. Feb 11, 2010

### vela

Staff Emeritus
You're just restating what they asked you to explain. You haven't said why -Q has to be on plate D.

When you place charge +Q on plate A, where does the charge -Q on plate B come from?

4. Feb 11, 2010

### jrdelacruz

The net charge on the conductor should be 0. But how does that relate to the problem?
The charge -Q on plate B comes from the charge +Q of plate A. All of the electrons from the conductor (plate B and C) will move to plate B because of the +Q of plate A right?

5. Feb 11, 2010

### vela

Staff Emeritus
Now put it all together. You said the net charge on plates B and C is zero and that the charge of +Q on plate A will attract a charge -Q onto plate B. Charge is conserved, so how much charge must be on plate C?

6. Feb 11, 2010

### jrdelacruz

So that means there must be a charge Q on plate C! So, now I know each capacitor has a charge of Q, is it safe to say that each plate has a charge of magnitude Q?

7. Feb 11, 2010

### Redbelly98

Staff Emeritus
Yes, correct.