A non conducting ring of radius r has acharge Q uniformily distriubuted over it. a magnetic field perpendicularto the plane of the ring changes at the rate dB/dT. the torqueexperienced by the ring = ??. will there be any difference if thering is conducting?whyor why not?

Dear golu ,

Case I : Apply following equation -   ∫_line E.dl = - dØ/dt   i.e line integral of electric field over a surface is the rate change of flux through that surface. note that bold letters are vectors.

  now , varying magnetic field produces electric field , direction given by right hand thumb rule. we can see that electric field is in circular direction along the ring , assume it to be E (magnitude) . it will be constant as ring is symmetrical.

Therefore , E.2∏r  = ∏r²  * dB/dt  ;  E =  (1/2)*dB/dt.  now we can easily see torque = EQr  = (1/2)*dB/dt*Qr.

 Case II: in first case there was no conduction so ring moved with charges but in this case charges shall move inside the ring because ring allows conduction . so a current will flow and ther will not b any torque on ring.

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