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I am an amateur physics enthusiast (during the day I am a police officer), and recently, I learned how to build an electroscope. I regret that I am learning how wonderful our universe is so late in my life, and I suppose it is hard to teach an old dog new tricks ( I do not know how to properly research a question, or really how to locate a correct answer surrounded by a lot of other information).

My question is this: On a simple electroscope, when one applies a charged object, the thin pieces of foil will separate, due to a separation of charges. However, I am curious as what factor balances the electric force of repulsion, so that the leaves do not continue to separate any further.

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2 Answers 2

up vote 4 down vote accepted

I am glad you are interested in physics.

As you know, the leaves of an electroscope are attached together by their bases, which generates a kind of force that we call tension.

The horizontal component of this tension force balances the electric force of repulsion.

In addition, it is worth mentioning that the vertical component of the tension force balances the weight of the electroscope's leaves $\big($or, the mass of the leaves multiplied by the acceleration due to the earth's gravity $9.8\frac{\text{meters}}{\text{second}^2}\big)$.

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please don't pointlessly use TeX (it's a massive JavaScript call for every block of text) when it just looks like you want bold or maybe italics. –  Nick T Aug 11 '14 at 4:58
I like the way it looks. Is there something that I don't know about, where it costs the site money? –  Gödel Aug 11 '14 at 6:32
I apologize though. –  Gödel Aug 11 '14 at 6:32
I don't think I mentioned any cost, but it is costly in processing time, mostly evident on a mobile device. As far as aesthetics, I'd agree that TeX can look better, but it looks ugly when juxtaposed with normal text on a page. –  Nick T Aug 11 '14 at 6:36
Right. Well, my conjecture was based upon the word "pointlessly," as it seemed to insinuate that I was squandering Javascript (which didn't make sense to me, so consequently, I asked). However, the juxtaposition that you mentioned seems to be much more subjective in my eyes. Nevertheless, I will keep it in mind from now on, as the mobile device issue that you mentioned is just cause in my opinion. –  Gödel Aug 11 '14 at 7:09

Distance. Electric fields weaken as the distance between them increases, so the force applied on the other leaf shrinks as the leaves separate. Eventually, the force from the electric field balances the force of gravity trying to pull the leaves back to their normal rest position, so the leaves cease to accelerate upwards.

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