I got a question. In Cavendish experiment was used lead in both of spheres. Can we repeat that experiment with balls made from stone or anything else. Because lead could be a byproduct of radioactive decay and by that emit ionizing radiation. And by ionizing lead it can create positive charge and to be attracted.
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2 Answers
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Naturally found lead is not radioactive. Its longest lived isotope has a half-life of approx. 15 million years, which means that over the course of geological time this isotope would have disappeared. Unless it gets contaminated by radioactive isotopes of other elements or its produced by decay, as you mentioned, it does not radiate.
Having said that, you are correct that one has to be very careful about these kinds of error sources in precision experiments, especially when the error, as in this case, would have the exact same functional dependence as the effect that we want to measure! In case of the Cavendish experiment this is easy: you make the entire apparatus conductive so that all electrical charges are shorted out and the electric field it produces is zero.
The experiment, more exactly, it's modern equivalents, have been repeated with many types of materials to test the equivalence principle. That is one of the most important experiments in all of physics because general relativity and with that the entirety of modern cosmology depends on the equivalence principle being valid.
answered Dec 26, 2015 at 20:49
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$\begingroup$ Can you give me link to video, with that experiment done with anything but lead. $\endgroup$– ArturCommented Dec 28, 2015 at 11:26
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1$\begingroup$ What would a video prove? The Cavendish experiment is only being done in high schools and on the college level these days, there are far more precise experiments of its kind. Some of the best versions are being done at the University of Washington: npl.washington.edu/eotwash $\endgroup$ Commented Dec 28, 2015 at 15:17
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$\begingroup$ "Naturally found lead is not radioactive." OK, let's have a trivial moment. For the purposes a very low background experiments recently mined lead is radioactive enough to be a problem because the lead-210 daughter of the radon chain is generally present. This makes lead from classical roman shipwrecks (known to have been above ground for many half-lives) valuable for such specialized purposes. $\endgroup$ Commented Jan 24, 2018 at 3:37
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You can, of course, do the experiment with any kind of weights. However, heavier weights give a stronger force and thus a better signal-to-noise ratio. Charge non-neutrality due to radioactive losses are totally negligible, since macroscopic matter tends to neutralize very swiftly. I have actually done the Cavendish experiment with much more radioactive depleted Uranium and the improvement in sensitivity over less dense lead was substantial.
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$\begingroup$ I have a feeling that you didn't quite understand, though, why the experiment you did with Uranium wasn't sensitive to charge effects. It wasn't because those charges got neutralized quickly on their own. Somebody made an effort to make sure that there was no charge buildup on the experimental hardware. $\endgroup$ Commented Dec 26, 2015 at 21:10
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$\begingroup$ No, there was no grounding apparatus or anything. Had we been trying to do a state-of-the-art measurement, more carry would have been needed, but our experiment was precise enough for what we wanted. $\endgroup$– Buzz ♦Commented Dec 26, 2015 at 21:58
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$\begingroup$ What, exactly, did you want? The Cavendish experiment is pretty much obsolete except as a high school or college level physics lab experiment. Even there it's controlled for charge, even if you didn't notice. $\endgroup$ Commented Dec 26, 2015 at 21:59
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$\begingroup$ The apparatus was for pedagogical purposes, for college students, but it was capable of 1% precision without electrical contacts. I've never seen a version for students for which anything more than touch grounding was used (and we did not touch ground the U-238). $\endgroup$– Buzz ♦Commented Dec 26, 2015 at 22:06
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$\begingroup$ One does not need electrical contacts to control weak charges and one would not use any for something like this. It's all about selecting the right materials for the experiment. $\endgroup$ Commented Dec 26, 2015 at 22:07