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Question

Three charges +4q,-q and +4q are kept on a straight line at position (0,0,0),(a,0,0) and (2a,0,0) respectively. Considering that they are free to move along the x-axis only


A
All the charges are in stable equilibrium
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B
All the charges are in unstable equilibrium.
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C
Only the middle charge is in stable equilibrium.
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D
Only middle charge is in unstable equilibrium.
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Solution

The correct option is A All the charges are in stable equilibrium
Configuration of the charges : [Ref. image]
From symmetry and by using Columb's law (F=kQqR2)
one can show net force on each charge is zero.______(i)
also, potential energy of system,
u=k[(4q)(q)a+(4q)(q)a+(4q)(4q)2a]

u=0_____(ii)
(i) and (ii) imply ,
system is in stable equilibrium.

1048957_1019849_ans_632f737ee4c14f02a9c59d1eb3456913.png

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