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Question
Answer the following question regarding earth’s magnetism:
The earth’s field, it is claimed, roughly approximates the field due to a dipole of magnetic moment 8 × 1022 J T−1 located at its centre. Check the order of magnitude of this number in some way.
Solution
Magnetic moment, M = 8 × 1022 J T−1
Radius of earth, r = 6.4 × 106 m
Magnetic field strength, B = `(μ_0 "M")/(2pi"r"^3)`
Where,
μ0 = Permeability of free space = 4π × 10−7 T mA−1
∴ B = `(4pi xx 10^-7 xx 8 xx 10^22)/(4pi xx (6.4 xx 10^6)^3) = 0.3 "G"`
This quantity is of the order of magnitude of the observed field on earth.
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