Diamagnetic, paramagnetic and ferromagnetic substances
Diamagnets are weakly repelled, paramagnets weakly attracted and ferromagnets strongly attracted by a magnet. Their χ is small negative, small positive and very large.
The three classes compared
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Near a strong magnet | Weakly repelled | Weakly attracted | Strongly attracted |
| In a non-uniform field, moves | From stronger to weaker field | From weaker to stronger field | Strongly towards the stronger field |
| Susceptibility χ | Small and negative (typically about −10⁻⁵) | Small and positive (typically about +10⁻⁵) | Very large and positive (thousands) |
| Relative permeability μr | Slightly less than 1 | Slightly more than 1 | Much greater than 1 |
| M compared with H | Small, opposite to H | Small, along H | Very large, along H |
| Field lines inside | Pushed out a little | Drawn in a little | Strongly drawn in |
| Atoms | No net moment of their own | Permanent moments, randomly arranged | Moments lined up in domains |
Why each type behaves as it does
Diamagnetism: the atoms have no net magnetic moment of their own. An applied field slightly changes the motion of their electrons. This induces a small moment opposite to the field, so the sample is pushed towards weaker field. Diamagnetism is present in all materials, but it is weak and is hidden in para- and ferromagnets. Paramagnetism: each atom has a permanent magnetic moment. Thermal motion scatters these moments in all directions. A field lines up only a few of them, giving a small M along the field. Ferromagnetism: neighbouring atomic moments line up by themselves in small regions called domains. A typical domain is about 1 mm across and holds about 10¹¹ atoms. In an unmagnetised sample, the domains point in random directions. A field makes the domains along it grow and the others turn, which gives a huge M.
Telling them apart in the lab
- Hang a small rod of the material freely between the poles of a strong magnet. A paramagnetic or ferromagnetic rod sets along the field. A diamagnetic rod sets across (perpendicular to) the field.
- Pour a liquid into a U-tube and place one arm between the poles. A paramagnetic liquid rises in that arm. A diamagnetic liquid falls a little.
- Bring a strong magnet near a piece of each. Iron jumps to the magnet, aluminium is hardly affected, and bismuth is very slightly pushed away.
Hard vs soft ferromagnets
Examples to learn
- Diamagnetic: bismuth, copper, lead, silicon, nitrogen (at STP), water, sodium chloride.
- Paramagnetic: aluminium, sodium, calcium, oxygen (at STP), copper chloride.
- Ferromagnetic: iron, cobalt, nickel, gadolinium, and alloys such as Alnico.
- Superconductors (some metals and alloys cooled to very low temperatures) are perfect diamagnets. They push the field out completely (χ = −1). This is the Meissner effect, which can make a magnet levitate (float) above them.
A fridge magnet sticks to a steel almirah because the steel is ferromagnetic, but not to an aluminium kadai, which is only weakly paramagnetic. MRI scanners use coils of superconductors, which carry current without resistance when very cold.
3 marks: “Distinguish between diamagnetic, paramagnetic and ferromagnetic substances on the basis of (a) susceptibility, (b) behaviour in a non-uniform field and (c) relative permeability. Give one example of each.” (a) χ small negative / small positive / very large. (b) Moves to weaker field / to stronger field / strongly to stronger field. (c) μr just below 1 / just above 1 / much greater than 1. Examples: bismuth; aluminium; iron.
2 marks: “A small rod hung freely between the poles of a strong magnet sets itself at right angles to the field. What kind of material is it? How would a paramagnetic rod set?” It is diamagnetic. A paramagnetic rod would set along the field.
1 marks: “Why is a superconductor called a perfect diamagnet?” It expels the magnetic field completely from inside itself: χ = −1 (the Meissner effect).
Dia departs, Para pulls a little, Ferro firmly grabs.