ElectrostaticsHard
Question
Three charges +4q, Q and q are placed in a straight line of length l at points at distance 0, l/2 and l respectively from one end of line. What should be the value of Q in order to make the net force on q to be zero?
Options
A.-q
B.-2q
C.-q/2
D.4q
Solution
Charges are placed as shown on time line AC.
For net force on q to be zero, Q must be of -ve sign. If F1 is force on q due to 4q & F2 due to Q
Then, F1 = F2 (magnitudewise)
or
= 
∴ 4q = 4Q
or Q = q (in magnitude)
∴ Q = −q (with sign)
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