Electrostatics - find the work done

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Homework Help Overview

The discussion revolves around electrostatics, specifically focusing on the relationship between charge distribution and Coulombic repulsion, as well as the calculation of work done when the distance between charges is altered.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to derive the relationship between the charges for maximum repulsion and is exploring the calculation of work done when the distance is halved. Other participants question the relationship between potential energy and work, and there is confusion regarding the signs of work in relation to potential energy changes.

Discussion Status

Participants are actively engaging with the problem, with some providing insights into the relationship between work and potential energy. There is an acknowledgment of differing forms of the answer and some confusion regarding the signs of work, indicating a productive exploration of the topic.

Contextual Notes

There is mention of a textbook reference that presents the answer differently, which may contribute to the confusion regarding the signs of work done. The original poster expresses uncertainty about the calculation of work in the context of changing distances between charges.

exuberant.me
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A certain charge 'Q' is to be divided into two parts, q and Q-q. What is the relationship of 'Q' to 'q' if the two parts, placed at a given distance 'r' apart, are to have maximum Coulombic repulsion? What is the work done in reducing the distance between them to half its value?

It went easy for the first part
as f(q) = k(q)(Q-q)/r2
and, f'(q) = 0 gives q = Q/2 which provides max repulsion (as f''(q) < 0)

But it is getting a bit "confusing" to calculate the work done for the second part..

someone please provide the idea.
 
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What is the relationship between potential energy and work?
 
God.. i m kind of "fool"...
i actually got the answer.. but in the book its given in the form of 'Q' and i got it in terms of 'q'
but could not check the difference that time... got it thanks...
 
one more thing at last
W = -(U2 - U1) = U1 - U2
So in this question
applying W = (U1 - U2) gives me a negative answer
but in the book its +ve
what of that? :P
 
exuberant.me said:
W = -(U2 - U1) = U1 - U2
That would be the work done by the system in going from U1 to U2. You want the work done on the system.
 

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