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two charges 4q and −q are placed a distance r apart.A charge Q is placed exactly mid−way between them.What willbe the value of Q so that charge 4q experiences no net force?

two charges 4q and −q are placed a distance r apart.A charge Q is placed exactly mid−way  between them.What willbe the value of Q so that charge 4q experiences no net force?

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2 Answers

Sher Mohammad IIT Delhi
askIITians Faculty 174 Points
9 years ago
first thing given charges are opposite si they will attract each other Q should be positive so it will repel 4q so force will be balanced
now 4q*Q/(r/2)^2=4q*q/r^2
=>16Q=4q
=>Q=q/4


Sher mohammad, IIT delhi
Askiitians Faculty
Faiz
36 Points
9 years ago
If there is no net charge on Q (it is in equilibrium)
then the two forces should be equal in magnitude but opposite in direction.
Therefore,
4Q*q/(r/2)^2 should be 4q^2/r^2.
So, Q will be q/4. 

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