The earth's field, it is claimed, roughly approximates the field due to a dipole to magnetic moment `8 xx 10^(22) J T^(-1)` located at its centre. Check the order of magnitude of this number in some way.

Explanation

Here, `M = 8 xx 10^(22) J T^(-1)`.
Let us calculate magnetic field intensity at magnetic line of short magnetic dipole for which, d = R = radius of earth `= 6,400 km = 6.4 xx 10^(6) m`.
`B = (mu_(0))/(4pi) (M)/(d^(3)) = 10^(-7) xx (8 xx 10^(22))/((6.4 xx 10^(6))^(3)) = 0.31 xx 10^(-4)T`.
= 0.31 gauss.
This value is in good approximation with observed values of earth's magnetic field.