Ranking magnetic materials

Problem

Four materials are immersed in the same external field: (A) soft iron ($\mu_r \sim 5000$); (B) aluminium ($\mu_r \approx 1{,}00002$); (C) water ($\mu_r \approx 0{,}999991$); (D) bismuth ($\mu_r \approx 0{,}99983$). Rank them by intensity of induced magnetisation, from most ferromagnetic to most diamagnetic.

Solution

The induced magnetisation scales with $\mu_r-1$: positive for para/ferromagnetic materials, negative for diamagnetic ones.

$\begin{aligned} \text{A (soft iron):}\; & \mu_r-1\approx4999 \quad(\text{ferromagnetic, very strong}) \\ \text{B (aluminium):}\; & \mu_r-1\approx+2\cdot10^{-5} \quad(\text{weak paramagnetic}) \\ \text{C (water):}\; & \mu_r-1\approx-9\cdot10^{-6} \quad(\text{weak diamagnetic}) \\ \text{D (bismuth):}\; & \mu_r-1\approx-1{,}7\cdot10^{-4} \quad(\text{more marked diamagnetic}) \end{aligned}$

Order from most ferromagnetic to most diamagnetic:

$\ev{A \gg B > C > D}$

Links

Topics: Magnetismo Concepts: Campo magnetico Skills: Ragionamento di ranking