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You have three separate glass rods, and you know one is positive, one is negative and one is neutral. You also have an electroscope that is positively charged.

How can you determine which rod is positive, negative and neutral?

I think you will be able to determine which rod is negatively charged, as that rod will cause the electroscope leaf to rise. I'm not sure how to differentiate between the positive+neutral though, as both will cause the leaf to collapse. Could you place it next to the negative rod and see which repels?

Essentially it would be great if somebody could explain the actual physics behind the situation. Why will the negative rod cause the leaf to rise and vice versa?


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The answer is found in any elementary textbook on electrostatics. – Georg Apr 13 '11 at 13:23
I don't think your description of the electroscope is accurate anyway, the leafs will separate upon either negative or positive charge. Like charges repel and both leafs get the same charge. – BjornW Apr 13 '11 at 13:48
see which makes the the leaves collapse more – Approximist Apr 13 '11 at 15:31
up vote 1 down vote accepted

I did a google search and found that: When a glass rod is rubbed with a paper towel, the glass becomes positively charged.


1.)ground the electroscope through touching it with you fingers (removing any excess charge); one can assume it is neutral/has no net charged at this point.

2.) touch one of the charged rods to the electroscope. The leaves should rise with an unknown charge.

3.) Vigorously rub the uncharged glass rod with a paper towel to give it a positive charge.

4.) bring the positively charged rod close to the Electroscope. If the leaves are repulsed, then you know that the glass rod of unknown charge was/is of the same charge, and therefore is positively charged. Opposite charges attract. Therfore if the leaves attract one knows that the glass rod of unknown charge was negatively charged.

5.) [check] repeat steps 1-4 to verify that that last rod is of opposite charge.

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Electrons are able to accelerate more freely then protons within a conductor. Therefore when a leaf electroscope is negatively induced the charged will move outwards while in a positively charged electroscope the leafs will stay hanging.

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I think I have a simple, yet correct way for doing this.

Suppose the three rods are A, B and C (with unknown charges.)

Suppose now rod A is positive, If you bring Rod A to the positively charged electroscope, the electrons on the leaves of the electroscope will be attracted by the positively charged rod and thus will go to the knob. So the leaves will repel more.

Suppose rod B is negative. If you bring Rod B to the positively charged electroscope, the electrons on the knob will repel the negatively charged rod and go on to the leaves. so the leaves will come closer.

Suppose rod C is neutral. If you bring Rod C to the positively charged electroscope, nothing will happen and the leaves will stay as they are (will not move).

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protected by Qmechanic Jan 15 '15 at 13:34

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