Categories: Electromagnetism, Physics.

# Magnetic field

The **magnetic field** $\vb{B}$ is a vector field
that describes magnetic effects,
and is defined as the field that correctly predicts
the Lorentz force
on a particle with electric charge $q$:

If an object is placed in a magnetic field $\vb{B}$,
and wants to rotate to align itself with the field,
then its **magnetic dipole moment** $\vb{m}$
is defined from the aligning torque $\vb{\tau}$:

Where $\vb{m}$ has units of $\mathrm{J / T}$.
From this, the **magnetization** $\vb{M}$ is defined as follows,
and roughly represents the moments per unit volume:

If $\vb{M}$ has the same magnitude and orientation throughout the body, then $\vb{m} = \vb{M} V$, where $V$ is the volume. Therefore, $\vb{M}$ has units of $\mathrm{A / m}$.

A nonzero $\vb{M}$ complicates things,
since it contributes to the field
and hence modifies $\vb{B}$.
We thus define
the “free” **auxiliary field** $\vb{H}$
from the “bound” field $\vb{M}$
and the “net” field $\vb{B}$:

Where the **magnetic permeability of free space** $\mu_0$ is a known constant.
It is important to point out some inconsistencies here:
$\vb{B}$ contains a factor of $\mu_0$, and thus measures **flux density**,
while $\vb{H}$ and $\vb{M}$ do not contain $\mu_0$,
and therefore measure **field intensity**.
Note that this convention is the opposite of the analogous
electric fields
$\vb{E}$, $\vb{D}$ and $\vb{P}$.
Also note that $\vb{P}$ has the opposite sign convention of $\vb{M}$.

Some objects, called **ferromagnets** or **permanent magnets**,
have an inherently nonzero $\vb{M}$.
Others objects, when placed in a $\vb{B}$-field,
may instead gain an induced $\vb{M}$.

When $\vb{M}$ is induced, its magnitude is usually proportional to the applied field strength $\vb{H}$:

$\begin{aligned} \vb{B} = \mu_0(\vb{H} + \vb{M}) = \mu_0 (\vb{H} + \chi_m \vb{H}) = \mu_0 \mu_r \vb{H} = \mu \vb{H} \end{aligned}$Where $\chi_m$ is the **volume magnetic susceptibility**,
and $\mu_r \equiv 1 + \chi_m$ and $\mu \equiv \mu_r \mu_0$ are
the **relative permeability** and **absolute permeability**
of the medium, respectively.
Materials with intrinsic magnetization, i.e. ferromagnets,
do not have a well-defined $\chi_m$.

If $\chi_m > 0$, the medium is **paramagnetic**,
meaning it strengthens the net field $\vb{B}$.
Otherwise, if $\chi_m < 0$, the medium is **diamagnetic**,
meaning it counteracts the applied field $\vb{H}$.

For $|\chi_m| \ll 1$, as is often the case, the magnetization $\vb{M}$ can be approximated by:

$\begin{aligned} \vb{M} = \chi_m \vb{H} \approx \chi_m \vb{B} / \mu_0 \end{aligned}$