A bar magnet of magnetic moment 6 J / T is aligned at 60 ° with a uniform external magnetic field of 0 · 44 T . Calculate the torque on the magnet when its magnetic moment is aligned opposite to the magnetic field.

We are given Magnetic moment, m = 6 J / T Magnetic field, B = 0 . 44 T Torque on the magnet, τ = m → × B → The torque on the magnet when its aligned opposite to the field, τ = m . B sin θ Substitute, θ = 180 ° τ = 6 × 0 . 44 s i n 180 ° τ = 0

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A bar magnet suspended by a horse hair lies in the magnetic meridian when there is no twist in the hair, On turning the upper end of the hair through $150 °$ from the meridian the magnet is deflected through $30 °$ form the meridian. Then the angle through which the upper end of the hair has to be twisted to deflect the magnet through $90 °$ from the meridian is ,

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