# Not sure what a capacitator is

1. Mar 19, 2005

### Gale

ook, so i didn't go to my last physics class before vacation, and i'm looking at the homework, and its on capacitators.

so first off, i'm not sure what a capacitator is, and what sort of units its measured in. I'm looking at ohm's law, which i remember from lab, but i forget how we used it. And i have a probablem where C=5.40 µF and i have no idea what that means. Lets see... i also know that capacitors in series have the same charge. Capacitors in parallel have the same voltage. so... i have a setup with a bunch of capacitators in series and parallel and i have to find the charges on the capacitators, and potential across them. I think if i had a better idea of what a capacitator was, i could manage that.

is the ratio of the energy stored at a given voltage in a capacitor with a
paper dielectric (K = 3.78) to the energy stored in a capacitor with the same
dimensions containing a polystyrene dielectric (K = 2.07)?

thats another question, and i see nothing similar to that in my notes. then i have a few more questions about "dialectric" stuff. All i remember from lab was stuff about resistance and capacitators, and i totally don't remember the relation between the two. I'm pretty sure that with a little guidance, i can probably work out the problems. I just have no idea where to start. thanks....

2. Mar 19, 2005

### Integral

Staff Emeritus
Essentially a capacitor is two thin sheets of conducting material separated by a thin layer of insulator. They act as a open circuit to DC and a short circuit to AC. When a DC voltage is applied to a capacitor current will flow until a certain amount of charge is stored on the plates. The amount of charge stored depends on the area of the plates, the distance between and the applied Voltage, the bigger the area and the voltage the greater the stored charge the further apart the less charge. The property's of the material separating the plates effects the apparent distance between them.

Capacitance with air or free space between the plates is given by:

$$C = \frac {\epsilon_0 A}D$$

The charge stored on a cap is given by

q= CV

A Cap appears to be a short to AC because the reversing current first charges one plate, then the other. There is a quantity called impedance that is used in place of Resistance to describe how a cap reacts to different frequencies.

Last edited: Mar 19, 2005
3. Mar 19, 2005

### cepheid

Staff Emeritus
Hi Gale17,

I'm just wondering: do you have a physics textbook, or are you relying solely upon the notes? If you have a book...I'd recommend perusing it. The definitions are usually pretty good. Simply put when you have two conductors separated by a certain distance, and they are charged (by transferring a certain number of electrons from one to the other), the quantity called capacitance, C, is a measure of the ratio of the charge on Q one of those conductors (they have equal and opposite charge) to the potential difference V, between them:

C = Q/V

and the unit of capacitance is therefore 1 coulomb/volt = 1 farad

A capacitor (it's spelled and pronounced a little differently than you were first thinking, btw) is any such arrangement of conductors, although it usually refers to a manmade device designed to have specific capacitance. It's used in electric circuits for a variety of purposes that I won't go into since you will learn about them, though I think you can see right away that one of them is to store electrical energy. The capacitance depends only on the geometry and separation of the conductors...not on what charge you put on it. I hope this helps.

edit: and this post is now completely redundant. Oh well...