Spin 1/2 particle in |z+> state hits a magnetic field

[StarTiger]

Homework Statement

"A spin 1/2 electron in a hydrogen atom is placed in a time-independent magnetic field B=B0(k)
The electron's spin starts in the |z+> state at t=0.
Calculate probability o ffinding the electron in the |y-> state as a function of time for t>0.

Homework Equations

The Attempt at a Solution

Basically I'm just confused about what to do about there being a z and y direction. I know how to calculate probabilities for spin states.

 

[wotanub]

If you know all these things, you can do it.
1. Can you write down the Hamiltonian for the system?
2. Using the Hamiltonian, can you write the time-dependent state of the particle?
3. Back to basics. What is the projection of the time-dependent state onto |-y>?
4. If you know the projection, what's the probability?