# magnetic flux density (*B*)

The magnetic flux density (*B*) is the magnetic flux per unit area of
a magnetic field at right angles
to the magnetic force. Magnetic flux density (*B*), also known as
the magnetic induction, is a vector quantity.
It is one of two quantities that specifies both the strength and direction
of the magnetic field; the other is magnetic
field strength (*H*). *B* and *H* are related as
follows:

*B* = *μH*

where* μ* is the permeability.

This corresponds to the electrical analog:

*D* = *εE*

where *D* is the electric flux density (also known as the electric
displacement, *E* is the electric field strength (electric intensity),
and *ε* is the permittivity.

This correspondence, however, a legacy from magnetostatics, is misleading.
In a dielectric medium the electrical force exerted upon a free charge is
proportional to *E*, not *D*. On the other hand, in a magnetic
material, the magnetic force that is exerted upon a moving charge is proportional
to *B*, not *H*. Thus, in the sense that *E* is the
electric field for free charges, *B* is the magnetic "field" for
moving charges. Unfortunately, *H* bears the title that describes *B*'s function.

The magnetic flux density can also be defined in terms of the effects the
field has, for example, by *B* = *F */ *qv*sin*θ*,
where *F* is the force a moving charge *q* would experience
if it was traveling at a velocity *v* in a direction making an angle *θ* with that of the field.

The SI unit of magnetic flux density is
the tesla, which is equal to one weber
per square meter (W m^{–2}).