# 5.1 e)  The earth’s field, it is claimed, roughly approximates the field due to a dipole of magnetic moment $8 \times 10 ^ { 22} J T ^{-1}$located at its centre. Check the order of magnitude of this number in some way.

Magnetic field

$B=\frac{\mu_o M}{4\pi R^3}$

substituting the values

$R=6.4\times10^6\ m$

$\mu_0=4\pi \times 10^{-7}$

$M=8 \times 10 ^ { 22} J T ^{-1}$

then

$B=0.3G$

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