- In electrostatic the basic building block is the monopole electric charge
- In magnetostatics there are no magnetic monopoles, the building block is a magnetic dipole
- From the equation of a current in a loop we know $B \propto z^{-3}$
- Consider the vector magnetic moment $\vec \mu$
- Has magnitude $IA$ (where $A$ is loop area)
- $\perp$ to loop
$$
\vec \mu=I\vec A
$$
Bar magnets
- The electrons in atoms have magnetic moments, when many of these point in the same direction and stay together, a permanent magnet is formed
Dipoles in external B-field
- Magnetic dipoles will not feel a resultant force in a uniform external $B$-field
- We can find the following properties
$$
\begin{aligned}
\vec \tau&= \vec \mu \times \vec B
\\ E_p&= -\vec \mu \cdot \vec B
\end{aligned}
$$