# Angular momentum

1. Oct 13, 2014

### jamie.j1989

Hi, In the principles of quantum mechanics by P.A.M Dirac it says on page 149,

For dealing with an angular momentum whose magnitude is 0.5(h_bar), it is convenient to put
m=0.5(h_bar)a
how is this if (m.m)^0.5= (3/4)^0.5*(h_bar)? Thanks

2. Oct 18, 2014

### Greg Bernhardt

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Oct 18, 2014

### Simon Bridge

"the magnitude of $\vec m$ is 0.5" (in units of hbar) means $\vec m\cdot \vec m = 1/4$ - so I think the answer to the question is "it cannot be".

Probably the passage has been misread, those two expressions appear to be for different states - but there is not enough of the book quoted to make the context clear.

4. Oct 20, 2014

### Jilang

Agree. The second expression looks like the value sqrt(m(m+1)) with m =1/2.

5. Oct 20, 2014

### Simon Bridge

The book being used was published in 1930 - I think the best advise is to get hold of a more recent publication.
They did a lot of things back then, like the above, that can be confusing to the modern student. The recent texts take advantage of advances in education, as well as understanding of QM, made in the last 80-odd years.