Electro-Magnetism question

[Caligar]

A negatively charged, graphite coated sphere is suspended from the ceiling on an insulating string in the region between opositely charged parallel plates.

The plates are 20cm apart, and are maintained at an electrical potential difference of 3.1*10^2. The charged sphere experiences an electric force of 8.4*10^-7N.

 

What is the charge on the graphite coated shpere?

My teacher is a jerk and won't help me, even though its the first question I've asked him all year.

Walk me through it if you could, people.

[timo]

It's not exactly the idea of the homework section to walk you through the question. You should at least state where your problems with the question lie, what ideas you have, what ideas for solving the problem you have (and why you think they will not work) and what equations/symmetries/properties/... you think might be applicable/relevant here.

As a starter: Do you know how potential, potential energy, electric field and force on a charge in an electric field relate to each other? Do you know what the electric field for "the region between oppositely charged parallel plates" looks like?

[Caligar]

It's not exactly the idea of the homework section to walk you through the question. You should at least state where your problems with the question lie, what ideas you have, what ideas for solving the problem you have (and why you think they will not work) and what equations/symmetries/properties/... you think might be applicable/relevant here.

As a starter: Do you know how potential, potential energy, electric field and force on a charge in an electric field relate to each other? Do you know what the electric field for "the region between oppositely charged parallel plates" looks like?

Fair enough.

 

Here what I did:

Took e=f/q and e=v/d (F is electric force, q is charge, e is energy in field, v is volts, and d is distance, but Im sure you guys know these formulas)

 

Got f/q=v/d

Got fd/v=q

Then plugged it in and got 5.4*10^-9, if memory serves.

Problem is the group I was in didn't get consistent answers, and this makes me want to check my answer, but my teacher wouldn't help me, so I came to you guys to see if you had the same procedure, or at least a proceduce that resulted in the same answer.

Hope that helps, and thanks in advance to any answers I get.

[timo]

The equation q=Fd/V is correct in this case. The reasoning is a bit thin (equations not justified or explained) but I suppose it is sufficient for school level - and well, as I said, they are correct. For the actual result: You did not provide any units for your charge and the voltage. Assuming the voltage is 310 V then with your equation I'd get a charge of approximately

q = Fd/V = 8.4*10^(-7) N * 0.2 m / 310 V = 1.68 * 10^(-7) J / 310 V ~= 1.68 * 10^(-9) C / 3 = 5.6*10^(-10) C.

Except for the exponent that's pretty close to your answer. Never omit the units!

[Caligar]

The equation q=Fd/V is correct in this case. The reasoning is a bit thin (equations not justified or explained) but I suppose it is sufficient for school level - and well, as I said, they are correct. For the actual result: You did not provide any units for your charge and the voltage. Assuming the voltage is 310 V then with your equation I'd get a charge of approximately

q = Fd/V = 8.4*10^(-7) N * 0.2 m / 310 V = 1.68 * 10^(-7) J / 310 V ~= 1.68 * 10^(-9) C / 3 = 5.6*10^(-10) C.

Except for the exponent that's pretty close to your answer. Never omit the units!

 

Thanks for the help.