Three large plates are arranged as shown. How much charge will flow through the key k if it is closed?

Explanation

Applying Energy conservation, initially kinetic energy =0 gravitational P.E.=0 (say) & Magnetic P.E.=`muB`
where, `mu`=magnetic moment of the loop =i.`((sqrt3a^(2))/4)`
Finally when the loop becomes horizontal, Kinetic energy=0 gravitational P.E.=`mg(a/sqrt3)` (because mg acts on the centre of mass) magnetic P.E.=0
`rArr 0+0+muB=0+((mga)/sqrt3)+0 rArr B=((mga)/(sqrt3 mu))=(4mg)/(3ia)`
using values, `B=400 mT`