# HELP! 1251 HW1 Question 11 Thanks!!

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Two charges q1 = 6 µC, q2 = -28 µC, are L = 14 cm apart. A third charge is to be placed on the line between the two charges. How far from q1 should the third charge be placed so that the net electric force on the third charge is minimized?

Here is what I think: set the distance from q1 is x and set the charge of q3 is Q, according to Coulomb equation F=kQ1Q2/r^2; therefore, minimize:

k*Q{[28/[(0.14-x)^2]]-6/x^2}.

I got [(22x^2+1.68x-0.1176)/(x^4-0.28x^3+0.0196x^2)]

So when the numerator is 0, the Fnet is minimized. Than I got x=0.0443 in meters.

However, it is wrong. So is there anything wrong? Thanks!!!!

asked Jan 12, 2014 in Others