Determining Electrostatic Charge Polarity
Julia, 10-12 Grade, Auckland new zealand
> When you have a rod and a cloth and you start rubbing them together, how
> do you know which one is positive or negative?
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
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"
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
- Now bring your charged object near the balloon.
- If your charged object is negative, it will repel the alike-charged
- If the charged object is positive, it will STRONGLY attract the
- 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
Sensitive charge detector
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
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
Here's another exotic technique. Buy a small neon pilot light such as one
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
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