Electrodynamics- energy required for 2 charges approaching

[Mokha75]

Hi guys,

I have a quick q about solving questions where there are two charges q1 and q2 approaching each other (in this textbook problem it is from 0.1 m to 0.01m). I understand that you have to keep one of the charges stationary and use the formula:

W=q(Vf-Vi)
= q1[ (k*q2)/r2 - (k*q2) /r1]

where r2= 0.01m and r1 = 0.01m

however, I don't understand this conceptually, why are we plugging one of the charges in for q1 and how come we get the same answer regardless of which charge we decide to keep constant.

Hope that makes sense.

Thanks in advance!

 

[Hia Hasmut]

It's for convinience make s the problem easy to solve.
As such it is your choice to set the point of reference.

 

[Mokha75]

nope