# Mass of charged sphere suspended between charged plates

1. Jun 27, 2016

### krbs

1. The problem statement, all variables and given/known data
A small sphere with charge 2.4 micro coulombs is suspended from a thread between 2 charged plates. The plates have a voltage of 62 and the distance between the plates is 3.1 cm. The sphere hangs at 18 degrees to the vertical.

2. Relevant equations
E = V/r
FE = Eq
Fnetx = FE - FT
Fnety = FTy - Fg

3. The attempt at a solution

E = 62V/0.031m = 2000 N/C

FE = Eq = (2000 N/C)(2.3 x 10^-6 C)

FTx = FE = 4.6 x 10^-3 N

FTy = FE * tan18 degrees
= 4.6 x 10^-3 N * tan18
= 1.5 x 10^-3 N

Fg = mg
1.5 x 10^-3 N = m(9.8 m/s^2)
m = 1.5 x 10^-4 kg

2. Jun 27, 2016

### TSny

Hello. In the problem you state that the charge is 2.4 μC but you use 2.3 μC in the calculation.

You wrote FTy = FE * tan18o . Is this correct (keeping in mind that the 18o is measured from the vertical)?

3. Jun 27, 2016

### krbs

I just realised the problem statement says 2.4 μC but the diagram says 2.3 μC hmmm. Assuming 2.4 is correct..

FTy = FE*tan72
= 1.5 * 10^-2 N

Fg = mg
1.5 * 10^-2 = m(9.8)
m = 1.5 ^ 10^-3 kg

4. Jun 27, 2016

### TSny

That looks right.

5. Jun 27, 2016

Ok than you!