# Contracdicting qualitative statements regarding magnetic polarization

Consider a cube of metalic material (either it is paramagnetic or diamagnetic can be both discussed). An external magnetic field $$\textbf{B}$$ is applied along the +x axis.

We expect $$\textbf{B}$$ to be affected by the material. There are several ways to explain such mechanism. One of them is to consider every atom as having a small loop of current $$I$$, creating a magnetic moment $$\textbf{m}=I \textbf{S}$$. When an external magnetic field is applied, these magnetic moments will line up, making the magnetic field stronger.

Another perspective is related to magnetic charges, or, monopoles. When an external magnetic field $$\textbf{B}$$ is applied, these charges behave like electric charges in an electric field. Hence, they have a polarizing effect. "Positive" (North) charges will accumulate at the +x side and "negative" ones will be at the -x side of the material. By creating an induced magnetic field $$\textbf{B'}$$ in the opposite direction of $$\textbf{B}$$, this will weaken the magnetic field.

The above two statements seems contradicting. Which one is right?

Your first paragraph is wrong. Because of Lenz's law, applying a magnetic field to a current loop reduces the magnetic field. This is called a diamagnetic material with $$\mu<1$$. The second paragraph is wrong because, as the first answer says, there are no magnetic poles. In paramagnetic and ferromagnetic materials, the atoms have permanent magnetic moments due to the intrinsic spin magnetic moments of electrons. These magnetic moments line up with the magnetic field, so that $$\mu>1$$.
• So to summarize, the answer depends on whether the material is diamagnetic, paramagnetic or ferromagentic, right? If it is the former one then the magnetic field is decreased, while for the latter two $\textbf{B}$ will increase. Jun 10 at 6:20
• In general, you need both, but there are special cases when the geometry allows you to infer one immediately or easier than the other. One such example is a toroid with a high permeability and a wire wound around it. The current gets you the $H$ field but you need the $B$ and its flux to calculate any induced voltage. A small gap in the yoke will let you get the B first and from that the H. The same problem shows up in E v. D. Macroscopic physics and engineering need both fields. Jun 7 at 0:54