→ In particular, we can write down the equatorial field ( B E ) of a bar magnet at a distance r , for r >> l , where l is the size of the magnet: E µ = − B ( . ) Likewise, the axial field ( B A ) of a bar magnet for r >> l is: A µ B ( . ) Equation ( . ) is just Eq.
( . ) in the vector form. Table . summarises the analogy between electric and magnetic dipoles.
Electrostatics Magnetism / ε µ Dipole moment p Equatorial Field for a short dipole –p / πε r – µ m / π r Axial Field for a short dipole p / πε r µ m / π r External Field: torque p × E m × B External Field: Energy –p . E –m . B T ABLE . T HE DIPOLE ANALOGY Example .
What is the magnitude of the equatorial and axial fields due to a bar magnet of length . cm at a distance of cm from its mid-point? The magnetic moment of the bar magnet is . A m , the same as in Example .
. Solution From Eq. ( . ) E B µ ( .