Where is the point when the potential is zero?

[asz304]

Homework Statement

A 2.96 uC and a -1.85 uC charge are placed 4.48 cm apart. At what points along the line joining them is the potential zero? Let V = 0 at r = infinity and enter the smaller distance first.

Homework Equations

V = k*q/r

The Attempt at a Solution

V(total) = 0 = kq1/x + kq2/(0.0448 - x)

x = q1*d/(q1 - q2)

x = 2.76*10^-2 m


Is there something that I'm missing? The computer says that I'm missing another value..

Thanks

 

[ehild]

"r" means distance, not difference between coordinates in the formula for the potential. If q1 is placed in the origin and q2 is on the positive x-axis at d, You should have written your equation as q1/|x|+q2/|x-d|=0. There are three possibilities to discuss: x>d, (the point is on the right from q2) 0<x<d (the point is between q1 and q2) and x<0 (the point is on the left from q1).

ehild

 

[asz304]

I came with a second answer of 0.0403 m

x(2) = -q1d/(-q1-q2)

but the computer says I'm giving it the wrong answer. Is there something that I'm doing wrong?

 

[ehild]

x(2)=-2.96/(-2.96+1.85) *4.48. It is not 4.03 cm!

ehild

 

[asz304]

Thanks. I knew I'll mess something up with the denominator because of "-q2" and q2 = 1.85*10^-6 C.