# Coulombs Law find q (charge)

1. Sep 23, 2006

### th77

I attached my message and the problem in a Word document because I don't know how to enter equations in this text box.

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2. Sep 23, 2006

### big man

Yeah your working is generally fine.

You can assign q4 a value, but it doesn't matter since it is a common factor throughout the whole equation. So what you do is you divide the LHS and RHS of your equation by q4. This leaves:

$$\frac {kq_4q_1} {q_4 (r_1_4)^2} - \frac {kq_4q_2} {q_4 (r_2_4)^2} + \frac {kq_4q_3} {q_4(r_3_4)^2} = 0/q4$$

ie $$\frac {kq_1} {(r_1_4)^2} - \frac {kq_2} {(r_2_4)^2} + \frac {kq_3} {(r_3_4)^2} = 0$$

Anyway even doing it your way you should have got the same value as me, but you didn't. I got something closer to -4*10^-5 C (this isn't the answer). The actual answer is slighty more than -4*10^-5 C. So you must have made a simple calculating error.