# What Dictates the Spin Direction of an Electron?

• Ryan Reed
In summary, spin is an intrinsic property of particles that is determined by the convention we choose for + and - values. Nature doesn't have a preference for one over the other. The difference between +1/2 and -1/2 spin can be observed through the Stern-Gerlach experiment, where an external magnetic field interacts with the particle's magnetic dipole. The spin of a particle can also be flipped or changed through interactions, and in the quantum picture, there is always a probability for the spin to flip. The dipole moment of a particle is proportional to its spin vector, and this can be observed through experiments like Stern-Gerlach. Ultimately, the choice of coordinate system dictates whether a particle has +1/
Ryan Reed
Spin is an intrinsic property of particles meaning that they have it naturally. Electrons can have either +1/2 spin or -1/2, what dictates that it is one instead of the other?

Ryan Reed said:
Spin is an intrinsic property of particles meaning that they have it naturally. Electrons can have either +1/2 spin or -1/2, what dictates that it is one instead of the other?

We do when we choose the convention for + vs -. Nature doesn't care which is which, it only knows that they are different.

Then what is the difference? How could one tell that the electron has a +1/2 or a -1/2?

Ryan Reed said:
Then what is the difference? How could one tell that the electron has a +1/2 or a -1/2?

Google for Stern-Gerlach experiment. Briefly, in an external magnetic field a magnetic dipole has an interaction energy ## H_d = - \vec{\mu}\cdot \vec{B}##. Spin up and down have opposite signs in this formula (if all else is equal), so you could conspire to make a measurement that would distinguish them.

And is the spin of the particle static, as in it doesn't change? Or can particle spin be flipped and changed?

Ryan Reed said:
And is the spin of the particle static, as in it doesn't change? Or can particle spin be flipped and changed?

If we have a particle that we know to be in a definite spin state, then the spin can be flipped if we interact with it. From the same interaction, we can imagine that we can apply a nonconstant field that will rotate the dipole. If the dipole has rotated enough to now have the negative projection onto what we have decided is the axis we are using to measure spin, then we would say that the spin has flipped. This is really a classical point of view though. In the quantum picture, we are adding a small bit of the spin down state to the quantum state when we turn on the field. So there is always a nonzero probability to find that the spin has flipped due to the applied field. Tuning the field can have the effect of increasing the probability that the spin flips.

How would you find these dipoles? I'm guessing that there is some way to tell since the magnetic field isn't distributed in the same way on the equator as on the poles.

Ryan Reed said:
How would you find these dipoles? I'm guessing that there is some way to tell since the magnetic field isn't distributed in the same way on the equator as on the poles.

A particle with intrinsic spin is already a magnetic dipole, with dipole moment proportional to the spin vector. One learns this from the Stern-Gerlach type experiments that I've already mentioned.

Thanks! This is exactly the kind of answers I was searching for!

Ryan Reed said:
Electrons can have either +1/2 spin or -1/2, what dictates that it is one instead of the other?

Choice of coordinate system. Stand on your head, and everything flips sign, but the observable quantities remain unchanged.

bhobba

## 1. What is electron spin?

Electron spin is a fundamental property of an electron that determines its angular momentum and magnetic moment. It is a quantum mechanical property that describes the inherent rotation of electrons around their own axis.

## 2. How is electron spin determined?

Electron spin is determined by the intrinsic angular momentum of the electron, which is an unchangeable property. It is also influenced by the presence of magnetic fields and the interaction with other particles.

## 3. What dictates the direction of electron spin?

The direction of electron spin is dictated by the laws of quantum mechanics. The spin of an electron can only have two possible orientations - up or down - and these orientations cannot be precisely measured at the same time.

## 4. Can electron spin be changed?

No, the spin of an electron cannot be changed as it is an intrinsic property. However, the direction of electron spin can be manipulated by applying external magnetic fields or by interacting with other particles.

## 5. How does electron spin affect chemical reactions?

Electron spin plays a crucial role in determining the stability and reactivity of atoms and molecules in chemical reactions. The arrangement of electrons and their spin states can influence the strength of chemical bonds and the behavior of molecules in various environments.

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