Homework Help: Opposite Charged Plates with Sphere

1. The problem statement, all variables and given/known data
A small sphere with mass 2.90 g hangs by a thread between two large parallel vertical plates 0.05 m apart. The plates are insulating and have uniform surface charge densities + \sigma and - \sigma . The charge on the sphere is q = 9.70×10^6 C.

What potential difference between the plates will cause the thread to assume an angle of 30.0 deg with the vertical

2. Relevant equations

[tex] Delta(V) = Ed = \frac {F_{elec} * d} {q} = \frac {(mg)*(d)*tan(30)} {q} [/tex]

3. The attempt at a solution

[tex] \frac {(2.90*8.90)*(0.05)*tan(30)} {9.70*10^{-6}} [/tex]

The online program complains that its wrong, I'm wondering is the equation correct?

 

That was actually a typo, I used 9.8 and still no luck.

The computation comes out to:

[tex] -9.38*10^{-7} [/tex]

Can anyone confirm this?

 

The mass was what threw it off, actually Q was 10^-6, I guess copying it over from the flash page removed the negative, though I had that in my solution. Also I figured out the answer. THanks!