# Derivative of Lx^2+Ly^2+Lz^2 =?

1. Feb 19, 2014

### rasi

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

2. Relevant equations

3. The attempt at a solution

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2. Feb 19, 2014

### Staff: Mentor

In $L_x$, it should be $\cos \phi$, not $\cot \phi$. $L_z$ has the wrong sign.

3. Feb 19, 2014

### rasi

So how can i go on. In no way i coulnd't tackled it. Thanks for your help....

4. Feb 19, 2014

### Staff: Mentor

Not sure what you mean here.

Can you describe the problem you want to solve? The title of the thread is not very clear, do you mean you need to write $L_x^2 + L_y^2 + L_z^2$ in spherical coordinates $(\theta, \phi)$?

5. Feb 19, 2014

### rasi

yes. just as you said.

6. Feb 20, 2014

### Staff: Mentor

Then you need to calculate each term by applying it to itself, e.g.,
$$L_z^2 = L_z L_z = -i \hbar \frac{\partial}{\partial \phi} \left( -i \hbar \frac{\partial}{\partial \phi} \right)$$