# Spin question

1. Sep 8, 2009

### likephysics

My quantum knowledge is very minimum.
If spin is +/-(1/2), then the proton has 2 energy levels. (for ex: hydrogen)

So, if the spin is just 1/2, there is only one energy level, correct?
Is this possible?

2. Sep 8, 2009

### Staff: Mentor

You're referring to two different quantum numbers, which are associated with different aspects of intrinsic angular momentum ("spin").

In general, angular momentum is a vector quantity: it has both magnitude and direction (orientation). The quantum number $s$ is associated with the magnitude of the spin, which is always positive. For a proton (or neutron or electron, for that matter), $s = 1/2$. The quantum number $m_s$ is associated with the direction (orientation) of the spin. With $s = 1/2$, we can have either $m_s = +1/2$ or $m_s = -1/2$ (often called "spin up" and "spin down"). When a particle is in a magnetic field, its energy depends on the value of $m_s$.