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HELP! 1251 HW1 Question 11 Thanks!!

0 votes
634 views

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 by Will_White (1,010 points)

1 Answer

+1 vote
Q1/x^2=q2/(L-x)^2

That's the equation I got and you have to find the derivative for each side (I used WolframAlpha) and then solve for x, which can also be solved using wolframalpha.
answered Jan 12, 2014 by Pavel V (200 points)
also the answer is in cm not m. Watch your units!!
do you mean, take derivative for both side and let them equal, then, solve for x? Thanks for your answer.
yes and it's easiest to use wolframalpha
im using it...take derivative for both side of the equation which you gave me..but still wrong..only 2 tries left:(
I got 5.24 cm. If that doesn't work just try 5.2.
THANKS!!!!  I tried 5.2 in my final trial and that's correct!!
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