# Electrostatically charging a capacitor

By rubbing a cotton cloth along a PVC pole, static electricity is generated. How can this be used to charge a capacitor?

http://home.earthlink.net/~lenyr/stat-gen.htm is what I'd like to do, but it doesn't explain how it should be wired up.

To get the charge from the PVC into a capacitor, you could ground one side of the cap, and attach a wire to the other side of the cap. The end of the wire should preferably end in a fine point or collection of fine points, and you sweep this over the charged surface of the PVC to collect what charge you can.

If you use a smaller capacitor you will tend to get more voltage, but even a small capactior (e.g., 100pF) is probably only going to get as far as a couple of volts, even off a PVC surface charged to 10's of kV, because the capacitance of the PVC surface probably isn't even 0.1pF.

• Tie one to ground, and just pop current in the other. There is not really a better method. Well done JustJeff. – Kortuk Oct 31 '10 at 22:26
• Sorry if that was confusing. Your post was the first post I came across as I read down the list of posts that actually answered the question. Most people are telling how to build a capacitor or how capacitors work. He asked how to use it to charge a cap, that is what you answered. – Kortuk Oct 31 '10 at 22:50

Sounds like you want a Leyden Jar (an early type of capacitor), here's a tutorial:

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p049.shtml

This is a good way to explain the behavior of capacitors, actually. People talk about "charging up" a capacitor, but they don't actually store charge. They store energy in the form of a displacement of charge. The electric charge of an empty capacitor and a full capacitor are both 0.

If you charge up a piece of PVC and touch it to a floating capacitor, it won't accept any more charge than any other piece of metal of the same size. The reason capacitors can store so "much" is because you're removing charge from one plate and depositing it on the other. If you connect one terminal to ground, you should be able to add more charge to the other terminal, since an equal amount can "escape" to the earth from the other side.

http://amasci.com/emotor/cap1.html

• They can get charge without loosing charge on the other plate with this method. If you charge something to a voltage and then touch one side of a cap with the other side held at a reference you will add extra charge to that side. The physical effects are not measurably different though. – Kortuk Oct 31 '10 at 22:26
• The reason a capacitor stores so much is because Capacitance is the measure of amount of charge moved for 1 V of difference. As you increase epsilon, A or decrease D it takes a much larger amount of charge to get 1V. – Kortuk Oct 31 '10 at 22:27

My favorite demo of charging a Leyden jar is by Makezine's Collin Cunningham, it's a very basic video that looks at capacitors in general, but it does show how to charge a simple Leyden using the PVC pipe and cloth method.

Don't forget Charge Conservation. (It's sorta like forgetting energy conservation!)

You cannot create charge by rubbing with a cloth. You can only separate the opposite charges which were already there. Which means, the negative charge on the plastic has an exactly equal positive charge on the (slightly conductive) cloth. This is commonly lost through your hand and to ground.

So, to 'charge' a capacitor, stop dumping the opposite charge into ground. Instead wear thick rubber gloves, so the cotton cloth becomes one terminal, the plastic pipe the other.

One method for transferring charge from an insulating surface is by induction, via "Faraday's Ice Pail." You'd need two steel mixing bowls. Place them on insulators (glass or plastic.) Connect them to corresponding plates of your Leyden jar. Charge your two objects and place them in the bowls. Disconnect one capacitor terminal, then the other, et voila!