# Capacitors and batteries

1. Aug 19, 2004

### daveed

really, whats the difference between a capacitor and a battery? and on a circuit diagram, do capacitors have current induced to keep a current? I've only taken first year physics in high school, and we havent covered them yet, though when i came across some problems with capacitors, neighboring resistors behaved as if the capacitors had no current-although my friend says there is current because of induction if the capacitor is fully charged?
im confused -_-

sorry if this seems dumb

2. Aug 19, 2004

### Integral

Staff Emeritus
A capacitor stores electric charge on conductive parallel plates. The plates are insulated from each other so there is no direct flow of current through the device. The symbol for a cap on a schematic diagram is quite accurate, though the actual device may be rolled up.

A battery stores energy in chemicals, a chemical reaction creates free electrons which are available to do work in a circuit. In most batteries the reaction only runs in one direction so when all reactants are consumed the battery is dead.

The while the chemical reaction in a battery is a key difference there is more to it. The fact that the capacitor can store energy along with the charge makes it a very useful circuit device. When DC current is switch on to a capacitor, initially current will flow as if the cap were not there, but as the charge builds up on one of the plates the current will rapidly decrease and finally stop all together, this is called "charging" the cap.

AC current on the other hand will alternately charge then discharge the cap, thus to AC a cap is nearly invisible. So in circuits a capacitor will be used to block DC and pass AC currents.

This is just a light skim of the surface of this subject, I am sure others will bring up other facets.

3. Aug 19, 2004

### chroot

Staff Emeritus
Capacitors can store energy, as can batteries. The biggest difference you need to be concerned with is that the voltage across a capacitor necessarily decreases as current is drawn from it, while a battery can maintain its output voltage, at the same current, for a long time.

Also, as Integral said, capacitors act as shorts to AC currents, and open circuits to DC currents. Batteries act as shorts to DC currents, and, depending upon their design, may or may not also act as a short to AC currents.

- Warren