Charge on a ball

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  • #1
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Homework Statement


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?


Homework Equations


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

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
thanks in advance.
 

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Answers and Replies

  • #2
LowlyPion
Homework Helper
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What picture?
How long is the string?
 
  • #3
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length of the string is not given
 
  • #4
Doc Al
Mentor
45,010
1,286

Homework Equations


E=(1/4pieE)*(q/r^2)
That's the field of a point charge--not relevant here. You are given the field.

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 ;
This is fine.
since F = 1/4pie * q/r^2;
This is not. What force does a field E exert on a charge q?
 
  • #5
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k it should be f=1/4pieEo *q/r^2 then right?
 
  • #6
Doc Al
Mentor
45,010
1,286
k it should be f=1/4pieEo *q/r^2 then right?
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?)
 

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