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

[duggielanger]

This question is off my forum , and I was just 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...

 

[mfb]

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.