The kinetic energy of an electron in the first Bohr orbit of the hydrogen atom is $...... eV$.

The $21\, cm$ radio wave emitted by hydrogen in interstellar space is due to the interaction called the hyperfine interaction in atomic hydrogen. The energy of the emitted wave is nearly

An electron moves in a Bohr orbit. The magnetic field at the centre is proportional to

The number of revolutions per second made by an electron in the first Bohr orbit of a hydrogen atom is ($h$ = Planck's constant, $m$ = mass of electron, $r$ = radius of the orbit).

An electron in the hydrogen atom jumps from the first excited state to the ground state. What will be the percentage change in the speed of the electron (in $\%$)?

Kinetic energy of the electron in a hydrogen atom is $\frac{e^{2}}{8 \pi \varepsilon_{0} r}$. Then its potential energy is . . . . . . .