A conducting ring of mass 2 kg and radius 0.5 m is placed on a smooth horizontal plane as shown.The ring carries a current of 4A.A horizontal magnetic field B=10T is switched on at time t=0.The initial angular acceleration of the ring is:
a)40Pie rad/s2 b)20Pie rad/s2 c)5Pie rad/s2 d)15Pie rad/s2
If I use tau=MB & M=nIA I get 40Pie rad/s2 (option a).
But if I use F=I(lxB) then I get Net tau=Ir (2r x B)(total torque on left part of ring)Ir (2r x B)(total torque on right part of ring)=2Ir(2r x B)=40 here
tau=I (alpha) =40
==>mr2(alpha)=40
==>(alpha)=80
Which method is wrong and why?