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Determining Electrostatic Charge Polarity
2005 W. Beaty


Julia, 10-12 Grade, Auckland new zealand
Physics 1108445317.Ph 

> When you have a rod and a cloth and you start rubbing them together, how 
> do you know which one is positive or negative?




Hi Julia!

A historical clue about polarities: way back in the 1700s the two types of charge were not called positive and negative. Instead, they were called "vitreous" and "resinous." Ben Franklin is usually credited with choosing the modern terms. He re-named "resinous" charge to be "negative" charge. And "vitreous" of course became the positive. Nobody knows why he chose the polarities this way; he could have done the reverse. But these became the charge polarities widely used throughout modern science.

This gives us a big clue. If you want to reliably produce negative or "resinous" charge, just rub some fur on some resinous material such as amber or rubber. The rubber will aquire the negative charge. For example, rub a rubber balloon on your hair to charge the balloon negative. (NOTE: The fur will become positive at the same time, but usually this positive charge is lost because humidity makes the fur slightly conductive, and the excess positive will spread to your hand and eventually to the Earth.) Or, to produce positive or "vitreous" charge, rub some silk cloth on some glass. The glass will become positive.

But how can we measure the polarity of other charged materials? One way to do this is to hold them near a known polarity of charge and see if there is attraction or repulsion. Here's a simple way to do this:

- Partly inflate a small rubber balloon and hang if from a string (perhaps attaching the top of the string to a door jamb.)
- Charge the balloon negatively by rubbing it all over some clean dry hair or fur.
- Now bring your charged object near the balloon.
- If your charged object is negative, it will repel the alike-charged balloon.
- If the charged object is positive, it will STRONGLY attract the oppositely-charged balloon.
- And if the charged object is not charged after all, but is neutral, it will WEAKLY attract the charged balloon.

If you want a more complicated project appropriate for a science fair, you can build a "Two-dollar electrometer" using a transistor from Radio Shack:

Sensitive charge detector
http://amasci.com/emotor/char gdet.html
To test for charge polarity, move your charged object towards the electrometer antenna. A negative charge will make the LED go dark, while a positive charge will make it become temporarily brighter.

The transistor in the above device is an "N-type FET" and will always respond to charge polarity the same way. If you don't use the MPF-102 transistor, and instead buy a "P-type FET" from some other electronics store, then the LED will go dark when a positively-charged object is brought near.

If you can borrow a digital voltmeter, you can test the polarity of a charged object via induction. WARNING: IT IS POSSIBLE TO DAMAGE A VOLTMETER IF YOU SUDDENLY TOUCH A HIGHLY CHARGED METAL OBJECT TO ITS TERMINALS. You'll need an insulating object such as a glass or plastic cup, and a metal object such as a metal bowl or a foil-covered square of cardboard. The metal object will function as a polarity- detecting "antenna." Connect the voltmeter's negative terminal to ground (such as a metal water spigot in your sink, or use the metal screw on a wall switch.) Securely connect the voltmeter's positive terminal to your metal bowl or foil. Support your metal object on the insulating cup, and don't touch the metal object while performing the following measurements. To determine polarity, turn on the voltmeter, set it to a sensitive range, then move your charged object toward the metal bowl or foil. The voltmeter will indicate the object's polarity. Now move your charged object away from the metal bowl or foil, and the meter will indicate the opposite of the charged object's polarity. (The meter only indicates the correct polarity while the charged object is approaching the metal "antenna.")

Here's another exotic technique. Buy a small neon pilot light such as one of these:

Bare neon pilot lamp

Turn off the lights in the room, grip one of the neon bulb wires in your fingers (it doesn't matter which one,) then drag the other wire across the charged surface of your object. Closely observe the two metal electrodes inside the neon, since one of these will glow as you drag the wire. If your charged object is positive, then the electrode connected to your fingers will glow orange. But if the charged object is negative, then the other electrode, the one connected to the dragged wire, will glow instead.

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