Q.
The relation between the orbit radius and the electron velocity for a dynamically stable orbit in a hydrogen atom is (where, all notations have their usual meanings)
In hydrogen atom electrostatic force of attraction (Fe) between the revolving electrons and the nucleus provides the requisite centripetal force (Fc) to keep them in their orbits. Thus, Fe=Fc ∴rmv2=4πε01r2e2
or v2=4πε0mre2 ⇒v=4πε0mre2