Electric Field Due to a Point Charge

  • Thread starter Wables
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  • #1
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Homework Statement


Two particles are fixed to an x axis: particle 1 of charge q1= 2.1 * 10^-8 and particle 2 of charge q2= -4.00q1 at x=70 cm. At what coordinate on the axis is the net electric field produced by the particles equal to zero?

Homework Equations



Enet = E1 + E2
E=F/q
F=k q1q/r2
Let q = point charge in the above formulas

The Attempt at a Solution



Enet = E1 + E2 = 0
E1 = -E2

k q1/r12 = -k q2/r22
q1/r12 = q2/r22

At this point I thought I should try to isolate for one of the r values and plug it into a second equation to solve for one of the r values. But there is no other equation I can use to set up a system of equations... or is there? Im stuck here.

All help appreciated,
Thanks!
 
Last edited:

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
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Welcome to PF!

Hi Wables! Welcome to PF! :smile:

(try using the X2 and X2 icons just above the Reply box :wink:)
At this point I thought I should try to isolate for one of the r values and plug it into a second equation to solve for one of the r values. But there is no other equation I can use to set up a system of equations... or is there?
you need an equation relating r1 and r2 :wink:
 
  • #3
supratim1
Gold Member
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use r1 + r2 = x = 70 cm
 
  • #4
5
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Thanks for the welcome.

r1+x=20
r2+x=70

I can use these!! Ooooh!!

Okay, so:


q1/r12=q2/r22
=> (q1/q2)(1/2)r2=r1

Let (q1/q2)(1/2) = z

r2+x=70
=> r2=70-x

r1+x=20
=> zr2 + x = 20
=> z(70-x) + x = 20
=> x-zx = 20 - 70z

But z is a root of a negative number...?
 
  • #5
tiny-tim
Science Advisor
Homework Helper
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Hi Wables! :smile:
q1/r12=q2/r22
No.

Did you draw a diagram?

You need a minus in there, don't you? :wink:
 
  • #6
5
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OH! Haha thanks for catching that. Okay. Thanks for your help, this question was really bugging me! :)
 

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