Measuring Coulomb's Forces with Torsion Balance

[Versus]

New to this forum as you can probably tell, if this is in the wrong place please move this. I'm doing AS Physics in a school in London and we are doing coursework for Physics. I'm trying to measure Coulomb's forces and am having a bit of trouble.

The way I'm setting it up is using a torsion balance and two balls coated in a conducting substance.

The problem I'm encountering is I need to induce a charge in Coulomb's into the two balls and have limiting factors as high school equipment. How can I do this and accurately measure the distance traveled of the ball?

There is a template here if anybody needs reminding of how this is supposed to work.

http://www.education.com/science-fair/article/static-charges/

This template is very basic, and am looking to include more.

 

[Delphi51]

Wow, the instructions neglected to mention you must run the thread through the ball and hang it by both ends of the thread (V shape with the ball at the bottom). This limits the ball to movement in one direction. It will swing all over the place if you don't do this.

Try for a dry day. If you can, turn up the heat in the room for an hour to reduce humidity which causes rapid loss of charge. You must do the measurements quickly so the charge doesn't change too much in the process. Projecting the shadow on a chalkboard or large piece of paper let's you quickly mark the positions of the ball shadows, then adjust the distance for the next position. Include the position of the pivot where the threads are tied so the angle can be measured. Move the "fixed" ball, not the hanging one, so the pivot is always in the same place. If you repeat the first position at the end of the sequence, you'll have an estimate of the error due to charge loss. Then you can take your time and measure the marks. If the board or paper is much closer to the balls than the light the shadow distances will not be significantly larger than the real distances (and the angle will not be affected).

It does work if you are careful and the humidity is not too high. I found it a most satisfying experiment - you actually find Coulomb's Law.

 

[Versus]

Thank you very much for that, helped a lot. One more thing though, how can I induce the charge into the balls (while knowing the actual charge)? The ways I have been doing it haven't been working.

The equation F= k times q1q2/r2 requires me to know the charge in Coulombs (q).

Those weren't my instructions by the way, just a picture incase people didn't exactly remember how it went.

 

[Delphi51]

There is no way to measure the charge without more sophisticated instruments. But you can find Coulomb's law - all but the value of the constant - without knowing charge. Using the two force equations from the horizontal and vertical F = ma equations on the hanging charge (which has a=0), you can find the coulomb force at each distance. When you graph Fc vs charge separation distance, you'll see the inverse square curve. Guessing that it is an inverse square and graphing Fc vs 1/d², you'll get a straight line to within experimental error (estimated from comparing the repeated first and last measurements). The Q*Q part of the Coulomb law is fairly obvious from theoretical reasoning - the force will obviously double if you have double the charge. The error will be quite significant, so the experiment will not be very satisfying unless you have error estimate bars on the graph and see that you can draw a straight line through them.

Lots of calculations even with less than 10 sets of data, so quite an advantage to use a spreadsheet to do them - and draw the graphs.