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The following question appears to be extremely straight forward, but I can't seem to be able to obtain the correct answer.

The question is:

A charge Q1 = 1.3uC is at rest and is located 2.3 cm away from another fixed charge Q2 = 1.6uC. The first charge is then released. Calculate the kinetic energy of charge Q1 when it is 5.7 cm away from charge Q2.

My solution is as follows:

Let r = 0.023 m r' = 0.057 m

PE + KE = PE' + KE'

But, KE = 0, so:

KE' = PE' - PE

KE' = (Q1$\times$V1) - (Q1$\times$V1')

KE' = (Q1 $\times \frac{k \times Q2}{r}$) - (Q1 $\times \frac{k \times Q2}{r'}$)

KE' = 0.49 J

I've tried many times to check my substitutions and unit conversions and the answer still remains incorrect. It may be possible that I'm completely using the wrong method. Can anyone please make any suggestions or corrections?

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How do you know your answer is incorrect? –  David Z Jan 28 '13 at 2:31
I'm working on an on-line practice worksheet. It tells me if the answer is correct or incorrect. –  Vanessa Jan 28 '13 at 2:35
What's its tolerance? That is, how close do you have to get to the actual correct answer for the computer to call it correct? –  David Z Jan 28 '13 at 2:41
I'm not sure about tolerance, but I know that it gives a warning or won't take an answer if the sig figs isn't considered. –  Vanessa Jan 28 '13 at 2:44
Yea I just calculated it myself and got 0.48482049... J Perhaps the problem doesn't intend for you to keep charge Q2 fixed after Q1 is released? –  joshphysics Jan 28 '13 at 2:44
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1 Answer

up vote 2 down vote accepted

Your answer is just a little bit off in the last digit, probably due to incorrect rounding. So I would suggest going back and checking the calculation, perhaps including some more significant figures in certain values.

If that doesn't make a difference, then check with your instructor. It's possible that the computer system has the wrong answer programmed into it. This is quite rare but not entirely unprecedented.

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Yep! You were right. The computer was expecting 3 significant figures even though the values given in the question had only 2. Thanks so much for everything! :D –  Vanessa Jan 28 '13 at 3:51
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