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In introductory electrostatics one often sees an experiment where the instructor rubs a rod (for example a PVC or a glass rod) with a cloth (for example made of wool, cotton or silk). And then strips of the charge on the rod on an electroscope.

Now if you charge the electroscope using the rod and then put the cloth onto the electroscope, it should completely discharge, since the cloth should contain the same but opposite charge as the rod (the polarity depends on the materials). You may not press it on the electroscope with your hand since then you may ground it through your body.

Alternatively you should be able to charge the electroscope using the cloth to the same amount of charge (but opposite sign) as you charge it with the rod.

I tried this using different material combinations (for example rods of PVC or PMMA, cloths of wool, cooton).

But it didn't work. The cloth seems to be not charged at all (or just having a minmal amount of charge).

Then I repeated the experiment using an electrometer amplifier (to measure the charge) and a large faraday cage where I dropped in the rod or the cloth.

Same result: The cloth contains almost no charge. Only if I drop it into the cage immediately after rubbing, it shows a certain degree of charge, but not the same amount as on the rod.

Now I have two questions about this:

  1. Why this "null result"? I guess its because the cloth has many fibers, which lead to strong electric fields on the tips of the fibers and thus to a very fast electric discharge to the air, which is significantly increased by the large effective surface area due to the fibers. So the cloth would hold the charge only for a very short time. Is this correct? If not, what would be the correct explanation?

  2. How can I modify the experiment to show conclusively that one body gets just the same amount of charge (but opposite sign) as the other body after rubbing? Maybe by using other materials (which one) or by changing the experiment in a different way.

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  • $\begingroup$ I remember my high school physics teacher doing this experiment. He ordered rabbit fur from Amazon and showed us that rubbing a rod with rabbit fur gave a noticeable amount of charge and charge conservation almost worked. $\endgroup$
    – user261609
    Commented Feb 9, 2022 at 13:09
  • $\begingroup$ How exactly did he show that the charge conservation worked? Did he put the fur on the electroscope after it was charged by the rod? If so, did he touch the fur or did he just drop the fur? $\endgroup$
    – dp21
    Commented Feb 9, 2022 at 13:17

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I think that charge leakage from the soft material (cloth or fur) is indeed the culprit. A film of moisture is often blamed, though this might inhibit charge separation in the first place rather than cause subsequent charge leakage, provided that you avoid contact between the soft material and conductors, e.g. hands. The soft material, though, has a large true surface area so even random collisions from air molecules (occasionally having electrons pulled off them or foisted on them) will probably cause charge loss. And your hypothesis of strong electric fields at the fibre tips is plausible.

I suggest you do the experiment the other way round (as suggested by Faraday's experiments). Line your faraday cage with fur (no elaborate cutting or securing of fur needed), and rub the rod on it, the rod being more or less enclosed by the fur. There should be no deflection. Take the rod away and the leaf should deflect. Take the fur away, discharge the electroscope and put the rod in the cage instead. Equal deflection is predicted. Good luck!

If you want more success with your original version you might try thoroughly drying the soft material with a hair dryer just before you do the experiment. It might also be worth trying an artificial, polymer-based fur.

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  • $\begingroup$ Thanks for your suggestions. I will try it and report if it worked. Just out of curiosity: Do you have a link to the original source where Faraday describes the mentioned experiment? $\endgroup$
    – dp21
    Commented Feb 9, 2022 at 20:10
  • $\begingroup$ Sorry: I have to let you down. Nor am I sure that Faraday did precisely this experiment, though he may well have done. His famous 'ice pail' experiment is similar in its use of a can or cage, but not the same. I await your report with interest. $\endgroup$ Commented Feb 9, 2022 at 20:40
  • $\begingroup$ Seems to work very well. Thanks! $\endgroup$
    – dp21
    Commented Feb 15, 2022 at 12:09
  • $\begingroup$ Good. Did you use leave the fur in the can or use the hair-dryer or both? $\endgroup$ Commented Feb 15, 2022 at 12:12
  • $\begingroup$ I did leave fur in the can. $\endgroup$
    – dp21
    Commented Feb 15, 2022 at 12:16

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