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www.GOALias.blogspot.comMagnetism andMatterExample 5.2 A short bar magnet placed with its axis at 30º with anexternal field of 800 G experiences a torque of 0.016 Nm. (a) What isthe magnetic moment of the magnet? (b) What is the work done inmoving it from its most stable to most unstable position? (c) The barmagnet is replaced by a solenoid of cross-sectional area 2 × 10 –4 m 2and 1000 turns, but of the same magnetic moment. Determine thecurrent flowing through the solenoid.Solution(a) From Eq. (5.3), τ = m B sin θ, θ = 30º, hence sinθ =1/2.Thus, 0.016 = m × (800 × 10 –4 T) × (1/2)m = 160 × 2/800 = 0.40 A m 2(b) From Eq. (5.6), the most stable position is θ = 0º and the mostunstable position is θ = 180º. Work done is given byW = U ( θ = 180 ° ) − U ( θ = 0 ° )mm= 2 m B = 2 × 0.40 × 800 × 10 –4 = 0.064 J(c) From Eq. (4.30), m s= NIA. From part (a), m s= 0.40 A m 20.40 = 1000 × I × 2 × 10 –4I = 0.40 × 10 4 /(1000 × 2) = 2AEXAMPLE 5.2Example 5.3(a) What happens if a bar magnet is cut into two pieces: (i) transverseto its length, (ii) along its length?(b) A magnetised needle in a uniform magnetic field experiences atorque but no net force. An iron nail near a bar magnet, however,experiences a force of attraction in addition to a torque. Why?(c) Must every magnetic configuration have a north pole and a southpole? What about the field due to a toroid?(d) Two identical looking iron bars A and B are given, one of which isdefinitely known to be magnetised. (We do not know which one.)How would one ascertain whether or not both are magnetised? Ifonly one is magnetised, how does one ascertain which one? [Usenothing else but the bars A and B.]Solution(a) In either case, one gets two magnets, each with a north and southpole.(b) No force if the field is uniform. The iron nail experiences a nonuniformfield due to the bar magnet. There is induced magneticmoment in the nail, therefore, it experiences both force and torque.The net force is attractive because the induced south pole (say) inthe nail is closer to the north pole of magnet than induced northpole.(c) Not necessarily. True only if the source of the field has a net nonzeromagnetic moment. This is not so for a toroid or even for astraight infinite conductor.(d) Try to bring different ends of the bars closer. A repulsive force insome situation establishes that both are magnetised. If it is alwaysattractive, then one of them is not magnetised. In a bar magnetthe intensity of the magnetic field is the strongest at the two ends(poles) and weakest at the central region. This fact may be used todetermine whether A or B is the magnet. In this case, to see whichEXAMPLE 5.3179