The electrostatic potential energy of the electron in an orbit of hydrogen is -6.8 eV . The speed of the electron in this orbit is ( $c$ is speed of light in vacuum)

The surface areas of two nuclei are in the ratio $9: 25$. The mass number of the nuclei are in the ratio

The ground state energy of hydrogen atom is -13.6 eV . The potential energy of the electron in this state is

If the energy released per fission of a ${ }_{92}^{235} \mathrm{U}$ nucleus is 200 MeV . The energy released in the fission of 0.1 kg d ${ }_{92}^{235} \mathrm{U}$ in kilowatt - hour is