Para and Diamagnetism

What it shows: 

The behavior of a substance in a non-uniform magnetic field will depend upon whether it is ferromagnetic, paramagnetic or diamagnetic. Here we test different substances to see how they are influenced by a magnetic field.

How it works: 

We have a collection of samples (listed in table 1) that exhibit well the three magnetic properties. Diamagnetic substances have a negative relative permeability (susceptibility); paramagnetic substances have positive.

Ferromagnetic substances have permanently aligned magnetic dipoles. In the presence of the external field the sample moves toward the strong field, attaching itself to the pointed pole.

Paramagnetic substances have permanent but unaligned magnetic dipoles (thermal random motion de-aligns them, that's why the effect is stronger at low temperatures). An external magnetic field does align the dipoles however, and the substance will move toward the strong field and attach itself to the pointed pole.

Diamagnetic substances have no intrinsic magnetic dipole. However, a strong external field can induce a dipole, but in the opposite direction to the applied field (Lenz's Law). A diamagnetic substance will therefore move away from the strong field. Diamagnetic effects are weak (all substances are diamagnetic, but it is swamped by any para or ferromagnetic property), so its movement towards the flat pole is subtle.

Table 1. Relative permeabilities of test substances

substance composition molar susceptibility (cm3 mol-1) volume susceptibility (emu)
zinc chloride ZnCl2 -55 x 10-6 -1.17 x 10-6
bismuth Bi -280 x 10-6 -13.2 x 10-6
aluminum sulfate Al2(SO4)3. 18H2O -323 x 10-6 -2.52 x 10-6
copper sulfate CuSO4 1330 x 10-6 30 x 10-6
manganese sulfate MnSO4. H2O 14200 x 10-6 174 x 10-6
iron Fe ferromagnetic

Setting it up: 

To provide a non-uniform field, select a flat and a pointed pole piece for the electromagnet. The pole separation should be 2 to 3 cm. The samples are contained in identical small test tubes and are dangled by a cotton loop through their bungs from a clamp stand cross-bar above the magnet (all the samples can be hanging, and moved across when it's their turn). A TV camera showing just the sample and the poles is needed for the effect to be seen.

Figure 1. Test substance between poles of electromagnet.

Para and Diamagnetism


Good accompaniments to this demo would be the Paramagnetism of Oxygen and the Curie Point demos, where iron becomes paramagnetic above a critical temperature.