# I don't understand why -1/2 is the higher energy level and +1/2 is the

1. Feb 24, 2013

### duggielanger

This question is off my forum , and I was jsut wondering if anybody could help.

I don't understand why -1/2 is the higher energy level and +1/2 is the lower energy level.

For spin state -1/2, if the external magentic field B0 and magnetic moment uZ oppose each other, surely the energy level will be lower?

Likewise for spin state +1/2: if the external magentic field B0 and magnetic moment uZ are parallel with each other, surely the energy level will be higher?

My main moan is that there doesn't appear to be anything in the text explaining why spin state -1/2 has a higher energy than that of +1/2...

Can anyone help?

Thanks...

2. Feb 24, 2013

### Staff: Mentor

Re: I don't understand why -1/2 is the higher energy level and +1/2 is

If the fields oppose each other, the energy is higher. This is similar to classical magnets: If you try to hold a magnet in the "wrong" direction, it tries to turn around.