# Coulomb's Law problem

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1. Jan 31, 2017

### lydia_y620

1. The problem statement, all variables and given/known data
Figure (a) shows charged particles 1 and 2 that are fixed in place on an x axis. Particle 1 has a charge with a magnitude of |q1| = 19e. Particle 3 of charge q3 = +16e is initially on the x axis near particle 2.Then particle 3 is gradually moved in the positive direction of the x axis. As a result, the magnitude of the net electrostatic force on particle 2 due to particles 1 and 3 changes. Figure (b) gives the x component of that net force as a function of the position x of particle 3. The scale of the x axis is set by xs = 1.70 m. The plot has an asymptote of F2,net = 0.8688 × 10-25 N as x → ∞. As a multiple of e and including the sign, what is the charge q2 of particle 2?

2. Relevant equations
F = kq1q2/r2

3. The attempt at a solution
As x --> ∞, the force on particle 2 comes just from particle 1, so the force from particle 1 is always 0.8688 x 10^-25 N because particles 1 and 2 are stationary.
When x = 0.85 for particle 3, the net force on particle 2 is 0, so the force from particle 1 must equal the force from particle 3.
This means that 0.8688 x 10^-25 = k(q2*16e)/0.85^2
Is this correct? Apparently my answer is incorrect after I tried solving for q2 in terms of e.

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2. Jan 31, 2017

### haruspex

I note that it asks for the charge as a multiple of e, but your last equation would naturally give a multiple of 1/e.

3. Jan 31, 2017

### lydia_y620

My final answer was (4.359 x 10^-37)/e, which, like you've said, doesn't make sense because it's a multiple of 1/e.

4. Jan 31, 2017

### haruspex

It does make sense. It arises naturally from q1=Fr2/(kq2).
So express it as constant x e instead. e is a known value.