Q: When a bar magnet is placed at 90° to a uniform magnetic field, it is acted upon by a couple which is maximum. For the couple to be half of the maximum value, at what angle should the magnet be inclined to the magnetic field B ?
Sol: We know that, τ = MB sin θ
If θ = 90° then τmax = MB ….(i)
τmax/2 = M B sin θ …..(ii)
From equations (i) and (ii)
2 = 1/sinθ
or sin θ = 1/2
or θ = 30°