Attempting to find tension force with electric fields

1. Feb 18, 2013

mfgrape123

1. The problem statement, all variables and given/known data
A small conducting sphere of mass 5.0x10^-3 kg attached to a string of length 20 cm, is at rest in a uniform electric field E. There is a charge of -5.0x10^-6 coulomb on the sphere. The string makes an angle of 30° with the vertical. Calculate the tension in the string. Calculate the magnitude of the electric field.

2. Relevant equations
Probably have to use Fm = BIlsin° Not sure what else, though.

3. The attempt at a solution
Honestly, I have no idea where to start. We haven't really covered this in class yet, and I don't know what to Google to figure it out that way... Help, please?

2. Feb 18, 2013

haruspex

Can you draw the free body diagram? What are all the forces on the mass? What do you know about the magnitude and direction of each?

3. Feb 18, 2013

mfgrape123

Okay, I think this is the right free-body diagram. I could be wrong, though. But this is the apparatus set-up we're given.

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• Problem set 60, Problems 1-4.jpg
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4. Feb 19, 2013

haruspex

The OP doesn't say the field is horizontal (so doesn't provide enough info for a solution), but maybe you left that out. So your diagram looks ok. Can you write down the equilibrium equations?