Charge on a ball

1. Jan 29, 2009

1517279

1. The problem statement, all variables and given/known data
An electric field E=141000.0i N/C causes the 4.70 g ball in the picture to hang at a α=22.9° angle. What is the charge on the ball?

2. Relevant equations
E=(1/4pieE)*(q/r^2)

3. The attempt at a solution
I tried solving this question first by drawing a force diagram on the charged particle. On the diagram:
W = T cos α -1
F = T sin α -2

I divided equation 2 by 1
I get tan α = F/mg ; since F = 1/4pie * q/r^2; i subbed it in
therefore: mgtanα4pie*r^2 = q

but i am having trouble in solving for r; i tried solving for r by using the Pythagorus theorem, but i didnt workout... i'm kinda stuck, i guess its because the known value for E field is in a component form...

can someone please guide me through this question

picture is attatched

Attached Files:

• Picture 41.png
File size:
948 bytes
Views:
59
Last edited: Jan 29, 2009
2. Jan 29, 2009

LowlyPion

What picture?
How long is the string?

3. Jan 29, 2009

1517279

length of the string is not given

4. Jan 30, 2009

Staff: Mentor

That's the field of a point charge--not relevant here. You are given the field.

This is fine.
This is not. What force does a field E exert on a charge q?

5. Feb 3, 2009

1517279

k it should be f=1/4pieEo *q/r^2 then right?

6. Feb 4, 2009

Staff: Mentor

No. Again, that's the field surrounding a point charge--not relevant here. You have a uniform field given; you need to find the force that given field exerts on the charge. (What's the definition of electric field?)