Considering the Bohr's model of hydrogen atom, the ratio of velocities of electrons orbiting in the 4th orbit to that in the 9 th orbit is

What is the mass number of the nucleus having radius equal to $\frac{1}{3}$ of that of ${ }^{189} \mathrm{Os}$ ?

The number of silicon atoms per $\mathrm{m}^3$ is $5 \times 10^{28}$. This is doped with $4.5 \times 10^{21}$ atoms $/ \mathrm{m}^3$ of arsenic. The ratio of number of electrons to number of holes after doping is (take $n_i=$ number of thermally generated electrons $=1.5 \times 10^{16} / \mathrm{m}^3$ )

The output of the following circuit is equivalent to $.......$ gate