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BruceW
#3
Feb27-12, 06:55 AM
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Quote Quote by Genericcoder View Post
I reasoned through this as follows I assumed the charge is already 10 charge is 5 meters away,since the potential from infinity is zero and this would approximate result.
So I got the force that they were putting on each other distance 5 meters away as:

F = 9 * 10^9 * 10 * 10^-6 * -2 * 10^-6 / 5^2 = -7.2 * 10^-3;
So work done should be w = -7.2 * 10^-3 * 5 = -0.036J;
W = -3.6 * 10^-2

The answer exists in the multiple choice but I don't know if its correct please let me know if it is.
I think you have got the right answer. But I think the way you calculated it and your reasoning are not right. You've used the force on the particle at 5 meters, and multiplied by 5 meters, but what would that actually mean? That the particle was being acted on by a constant force over 5 meters? The actual situation is that the particle is acted on by a varying force from infinity to the 5 meter point. I think it is just coincidence that you got the right answer. There's two possible ways to answer this question. One involves using the potential, and the other way involves integrating the force over the path. Are you familiar with these methods?