McCrunchy Posted November 16, 2009 Share Posted November 16, 2009 Hi, does anyone have an idea of the amount of static charge (in Coulombs) that is transferred to a balloon ( 20 cm in diameter) upon rubbing it with wool ? I know it depends on all sorts of factors, how I rub, etc., but I'm just looking for an order of magnitude. Thanks, McCrunch Link to comment Share on other sites More sharing options...
swansont Posted November 16, 2009 Share Posted November 16, 2009 This is actually not that easy. You could look at the repulsion of two balloons to get a very rough idea of how much, or even build your own electroscope. mg = kQ1Q2/r^2 Because of the nonlinear nature of the force, any error in estimating distance will give a larger error in the estimation of charge. A balloon sticking to the ceiling is even trickier, since the balloon induces a charge, and the force should then go as r^3 Link to comment Share on other sites More sharing options...
McCrunchy Posted November 16, 2009 Author Share Posted November 16, 2009 Hi, measuring the force would be possible, however you always need two balloons and you never really know whether they're identically charged (except for saying " hey , I rubbed it the same number of times with the same sweater, it must be equally charged !" . Plus it musn't be all that easy to measure. I thought of using a Faraday cup. Basically you collect all of the charge of the balloon on a initially neutral , isolated conductor, for example an aluminium cup, and measure the potential difference that results between the cup conductor and the ground. Whatever the shape of your cup, you have Q = C * V Q : charge - unknown C : capacitance, unknown, but constant V : voltage measured I'll try calibrating the device to figure out the C with a charged capacitor of known capacity ( I can't think of any other device that would provide a known amount of charge) McCrunch Link to comment Share on other sites More sharing options...
cadwaller Posted December 5, 2009 Share Posted December 5, 2009 The static charge is directly proportional to the difference in electrical affinities between the two objects when separated. Latex has an electrical affinity of -105, wool, 0. I don't really understand the units though. The charge is also directly proportional to the contact area. I know barely anything about static but I got the affinity info from http://www.trifield.com/triboelectric.htm hope this helps. 1 Link to comment Share on other sites More sharing options...
McCrunchy Posted January 23, 2010 Author Share Posted January 23, 2010 So I've found an easy method to get an approximate value for the charge on ballons, here it goes (see picture in attachment): - take two ballons, attach them with a thread of length l (in my case 80 cm) - anchor the center of the thread round an axis, a broom might do. The uncharged ballons hang from the thread touching each other. - rub each one of the balloons with wool or whatever material you are interested in. Be careful that the first balloon doesn't stick/discharge to your positive skin/sweater, etc. while charging the second one. - let the ballons loose, they're now at some equilibrium distance from each other (in my case ~5 cm) - measure the distance between them, deduce the angle alpha the thread makes with the vertical axis. - balance the three forces to deduce the formula : f_electrostatic = tan (alpha) * mass_ballon * g - get the charge from Coulomb's law. In my case no matter how hard I rubbed the ballons with my woolen sweater the distance between the balloons would be about 3-6 cm, which makes for about 10-7 C on each of them, or a 100 nC. Yours, McCrunchy Link to comment Share on other sites More sharing options...
swaha Posted February 5, 2010 Share Posted February 5, 2010 i read about this experiment somewhere i forgot who did it 1st. pls tell me that before closing the thread .cant find it now. Link to comment Share on other sites More sharing options...
JoeyFuzz Posted March 15, 2019 Share Posted March 15, 2019 MrCrunchy, I'm developing a lab for my AP physics class and came across your post looking to verify my results. I also got approximately 100 nC of charge on each balloon. Link to comment Share on other sites More sharing options...
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