Amount of charge on a rubbed ballon ?

[McCrunchy]

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

[swansont]

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

[McCrunchy]

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

[cadwaller]

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.

[McCrunchy]

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

[swaha]

i read about this experiment somewhere i forgot who did it 1st. pls tell me that before closing the thread .cant find it now.

[JoeyFuzz]

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.