Answer to Question #141737 in Physics for Sneha

Question

Write down expectation value of r^2for hydrogen atom.

Answer 1

Expectation value of $r^2$:

$$\big<r^2\big>=\int^\infty_0\psi_{1s}(r)r^2\psi_{1s}(r)4\pi r^2\text{d}r=3.$$