derive an expression for the torque experienced by electric dipole placed in a uniform electric field. what is the net force acting on this dipole ?

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Consider a dipole of charge q and separation 2l . When it is placed in a uniform EF , an equal and opposite force ( of magnitude Eq ) acts on them  . The net force is 0 .
These equal and opposite forces constitute a couple and a torque is produced .
Torque = Any one force x perpendicular distance between forces
                = Eq x 2lsinϴ  
[Perpendicular distance is 2lsinϴ where ϴ is the angle between dipole moment and direction of EF ]
                =Epsinϴ  , p = 2ql , the dipole moment
This is the magnitude of torque.
So τ = pEsinϴ
Vectorically , τ = p x E
Hope that helps.
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