Un-Understand Physics : How Attractive Are You ?

Attractive people are considered to have what is called a "magnetic personality". Have you ever wondered what makes something magnetic? What are the properties of a substance which make it magnetic? To understand we need to learn about a concept called ‘spin’.

The Stern-Gerlach Experiment[1] was conducted in 1920, which completely revolutionized our understanding of subatomic particles. In the experiment, German physicist Walther Gerlach passed a beam of Silver atoms through a magnetic field. Silver atoms are electrically neutral as they have an equal number of protons and electrons. We know that charged particles show deflection in a magnetic field. We also know that silver is neutral, thus the expected result is that - we do not see any deflection of Silver atoms when they pass through the magnetic field.

However, it was observed, that the atoms deflected, some of them in the upward direction and some in the downward direction, no atom passed through undeflected, which was not expected. We know the cause for the deflection was not electric charge, the only other explanation is that Silver must be magnetic. That is indeed true, Silver is magnetic (However, it is not magnetic enough to be relavant in everyday life). Silver is magnetic due to an intrinsic property of all subatomic particles (electrons in this case) called spin[2], which gives the atoms a magnetic moment[3].

Let us understand spin a little better. Spin is short for spin angular momentum. It is analogous to rotational angular momentum in classical mechanics. Now, you may be thinking that since all subatomic particles have a spin, all atoms must have a magnetic moment. That would mean that all substances would exhibit magnetic properties. Well, we all know, that is not the case.

Not all atoms possess a magnetic moment. Only those atoms which have a large number of unpaired electrons in their orbits have this property. Electrons usually exist in pairs with the electrons in a pair having opposite spin due to which their spins cancel out. (Another property of spin is that it is quantized. The magnitude of the spins of all the electrons is same only the direction is different. Opposite spin directions, cancel each other out). However, there may also be unpaired electrons present in the atom. The spins of the unpaired electrons give the substance a magnetic moment. Atoms which have unfilled orbits have more unpaired electrons and hence, are magnetic in nature. This depends on the orbital spdf configuration of the atoms, determined by the elements position in the periodic table. Substances which contain many unpaired electrons in their orbits like Iron, Nickel and Cobalt are very magnetic.

In the Stern-Gerlach experiment, we saw the atoms deflected in opposite directions, but the extent of deflection was always the same, this is proof that spin is quantized. Electrons act as mini magnets, which all have the same strength, but different polarities depending upon the direction of their spin (up or down). These mini magnets give a substance its magnetic properties, as we know it.

Spin is considered as an intrinsic or fundamental property of all subatomic particles, just like charge and mass. Spin plays an important role in many quantum phenomena (as you will see in later posts) and of course in magnets as we know them. These are the spins of some subatomic particles.