# Homework Help: Magnetic field and spin

1. Feb 2, 2012

### v_pino

1. The problem statement, all variables and given/known data

A beam of neutrons traveling in the x direction initially has all its spins up along the z direction. The neutrons pass through a region in which there is a uniform 1 tesla magnetic field in the y direction. What is the minimum time t the neutrons need to remain in the field to exit it with their spins along -z? The magnetic moment of the neutron is u=-0.966x10^-26 J/T.

2. Relevant equations

Is this question to do with time-dependent Hamiltonian? If so, does this equation help?

$$B=B_0+B_1=B_0 \hat z + b_1(\hat x cos \omega t - \hat y sin \omega t)$$

3. The attempt at a solution