# Homework Help: Capacitor Question

1. Oct 12, 2012

### schaefera

When you use a battery to charge a capacitor, say to charge Q, does the battery provide any charge, or does it only provide energy to move charges? Should we think of it like pulling positive charge from one plate and placing it on the other, or some other way?

2. Oct 13, 2012

### rude man

The battery provides charge equal to the charge deposited on the capacitor. The charge is generated electrochemically and is a source of emf. A capacitor is not a source of emf.

3. Oct 13, 2012

### schaefera

So by conservation of charge, you would argue that the battery becomes negatively charged?

4. Oct 13, 2012

### rude man

There is no 'conservation of charge'. Charge is produced by the battery. It's called an oxidation-reduction reaction in which electrons are released from one electrode, flow thru the external circuit, & deposit on the other electrode.

I suggest reading up on the subject further on the hyperphysics link. I am not a chemist.

5. Oct 13, 2012

### dipole

A battery does not squirt out electrons. The charge on a capacitor comes from the electrons already inside the metal.

The battery sets up a potential difference which drives electrons to build up on one of the capacitor plates, and an opposite charge is induced on the other plate because of the electrostatic repulsion. The net charge in the circuit is always conserved.

6. Oct 13, 2012

### vela

Staff Emeritus
It provides the energy to move the charges.

7. Oct 13, 2012

### SammyS

Staff Emeritus
Say what?

'Conservation of charge' is the basis of Kirchhoff's current law (KCL).

8. Oct 13, 2012

### schaefera

Then the capacitor must do nothing more than provide energy to move charges, but provide no charge itself?

9. Oct 13, 2012

### frogjg2003

The battery is a voltage source, meaning it has a constant voltage and can provide any current to maintain that voltage. In terms of charges, it will push positive charges out of its positive terminal and negative charges out of its negative terminal (knowledge of the sign of the electron doesn't matter). The capacitor is a barrier to their movement so excess charge builds up on the plates. Positive charge accumulates on the plate connected to the positive terminal of the battery, and negative charge accumulates on the plate connected to the negative terminal.

What's actually happening is that chemical reactions strip electrons from some atoms on the negative side, push them out of the battery, through the circuit, and add electrons on the positive side. Charges are created, but for every negative charge created, there is also a positive charge, so net charge is conserved. When the electrons come to the capacitor, they accumulate on one plate, making it negatively charged. This negative charge pushes the electrons on the other plate away, making that plate positively charged, and those electrons are the ones absorbed by the battery on its positive side.

10. Oct 13, 2012

### rude man

Mea culpa. I apologize for that foolish statement.