Intro-electron SPIN

…atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but  repelling upon being squeezed into one another…

Richard P. Feynman

An electron is a sub-atomic particle. It means to say that it is one of the constituents of an atom. Like every particle, it has a few characteristics. It has mass, electrical charge and spin. To most people, the idea of mass and charge is clear. The mass of the electron is about 9.1 x 10^-31 kg. It’s charge is about 1.6 x 10^-19 C. In this article, it is intended to have a clearer idea of another property of a electron, the electron SPIN.

Let us do an experiment. We bring a piece of Iron next to a magnet(for example, a piece of lodestone). The piece of Iron gets attracted to the magnet. Next, we bring another piece of Iron near the first piece of Iron as it is still stuck (by magnetic force of attraction) to the magnet. We see that the second piece of Iron is also attracted to the first piece of Iron.

A second experiment. We bring a piece of Graphite near the magnet. It does not get attracted. We dont even feel any force (not felt by human senses).

A third experiment. We keep the piece of Iron in contact with the magnet for a long time (say a few months). Alternatively, we rub the magnet (in one direction only) on the piece of Iron. Now we see that the piece of Iron attracts other pieces of Iron even though there is no other magnet around.

Now, we ask the question “what is it that is happening to the piece of Iron when it gets in contact with the magnet?”. As an answer, we are taught in elementary school,is that all pieces of Iron contain tiny magnets which will get aligned (unlike poles attracting each other and like poles repelling each other), making the piece of Iron act like the bigger magnet. We are also told in school that this is not the case with the piece of graphite. But we are taught in school that all matter is made of atoms and all atoms consist of electrons revolving around a nucleus that is positively charged. That is fair enough. This means that the piece of Iron and the piece of Graphite are both made of atoms containing electrons and nuclei. What is so different in the piece of Iron that makes it magically become a magnet? What causes the tiny magnets in the piece of Iron?

There is a property of an electron, like mass and charge which causes these effects. That property is called the electron SPIN. So, electrons have not just two but actually three fundamental properties: mass, charge and SPIN. The charge of an electron is negative. But the peculiarity of the property spin is that an electron could have either a “Positive” spin or a “Negative” spin. Therefore, depending on the number of electrons with “Positive” and number of electrons with “Negative” spins, an atom may or may not have a total spin. Atoms of Iron have a net SPIN which leads them to behave like tiny magnets.

There are several WRONG ideas about the electron spin, seemingly used to simplify the concept. We will begin the description by stating those wrong ideas.

1. The electron can be visualised as a charged sphere.
2. The electron rotates about it’s own axis.
3. Spin of an electron is determined by the direction of rotation of the electron or how fast the electron rotates.
4. All other descriptions and analogies originating from Classical Physics.

The first two of the above list are wrong because the electron is conceptualised to be a point-charge. It cannot be a sphere or a cube or anything else. There is no rotation of a point. To understand Spin, we need Quantum mechanics. There is simply no classical analogue of a spin. You cannot picturise it classically in any way.

A lot of confusion probably originates because of the term spin. It is just the name given, for historical reasons, to a fundamental property of the electron. Mass and charge are also names given to fundamental properties. The name spin does not mean that there is something spinning about some axis. Just as mass can be felt only in a gravitational field (more generally, in the presence of any acceleration) and electric charge needs an electric field to show up some physical effects, spin of an electron needs a magnetic field to show physical effects. It has properties similar to angular momentum in Quantum mechanics. A peculiarity of the electron spin is that it can take two values, +½ or -½ of a unit. A more detailed description needs some Quantum mechanics.

So, to explain our experiments, in an atom of Graphite, the electrons are arranged so that there are as many electrons with spin of +½ as there are electrons of spin -½. Therefore, the net spin is zero on each atom. Whereas in an atom of Iron, there is an excess two electrons with spin +½ than electrons with spin -½. Therefore, just as a net electrical charge shows a response in an electrical field, atoms with a net spin show a response in a magnetic field. The atoms in a piece of Iron therefore behave like tiny magnets which tend to get aligned in a magnetic field.

Advertisement