Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

How is the boxed step , physically as well as mathematically justified and correct ?

Source:Wiki http://en.wikipedia.org/wiki/Electric_potential_energy

As work done = $- \Delta U $. for Conservative force and it shouldn't matter whether we take $ds$ or $-dr$ ?

And when $dr$ is just a notation to specify the variable and the real thing behind it , is a limit , why is it that $dr$ is so important here .

enter image description here Image : http://www.artofproblemsolving.com/Forum/download/file.php?id=43358&mode=view

share|improve this question
    
Kindly edit my post a bit to prevent linkrot,thank you as I am not able to figure out how to upload images on stack exchange –  nonagon Apr 25 '13 at 21:25
2  
Alright , thank you for editing –  nonagon Apr 26 '13 at 6:27
    
and how much reputation you require for uploading images ? –  nonagon Apr 26 '13 at 8:48
1  
Cross-posted to physics.stackexchange.com/q/62317/2451 –  Qmechanic Apr 28 '13 at 14:29
add comment

1 Answer

up vote 1 down vote accepted
+100

Basically you $dr$ is negative , so $|dr|=-dr.$ like $|-2|=-(-2)$ , evaluate using this , the sign ambiguity will dissolve .

Also displacements are measured w.r.t. some origin and here , origin can't be placed at $\infty$. So you base your origin at the point where the charge '$Q$' is located .

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.