Question

If the net force acting on a closed loop is zero then how can a torque act on it??? Moving coil galvanometer has a closed rectangular loop ...still it rotates in uniform magnetic field

Answer 1

Apply two forces in such a way that it is, equal and opposite but not acting on the same line. (These two forces form what is called a couple.).Then there will be torque produced.
So,Couple produces torque, not net force.
Imagine yourself pushing one side of an object while pulling the other (like turning a steering wheel with both hands); the net force is zero but it will rotate (assuming no other forces act to prevent it).


Galvanometer works on the principle that current carrying loop placed in a uniform magnetic field experience a torque.
This torque is given by ,
T=NIAB sin@
or
T=MB sin@
where M=NIA called Magnetic dipole moment and B is magnetic field produced.