Magnetic dipole magnetic dipole as the distance from the source increases. In classical physics, the magnetic field of a dipole is calculated as the limit of either a current loop or a pair of charges as the source shrinks to a point while keeping the magnetic moment constant. For the current loop, this limit is most easily derived for the vector potential. Outside of the source region, this potential is (in SI units) with 4"π r" being the surface of a sphere of radius "r"; and the magnetic flux density (strength of the B-field) in teslas is In spherical coordinates with the
Spectral flux density is the zero vector, while the scalar or hemispheric flux density at in every direction in both senses takes the constant scalar value . The reason for the value is that the hemispheric integral is half the full spherical integral, and the integrated effect of the angles of incidence of the radiation on the detector requires a halving of the energy flux according to Lambert's cosine law; the solid angle of a sphere is . The vector definition is suitable for the study of general radiative fields. The scalar or hemispheric spectral flux density is convenient for discussions in terms