In the $C.G.S.$ system,the magnitude of the force is $100 \ dynes$. In another system where the fundamental physical quantities are $kilogram$,$metre$,and $minute$,the magnitude of the force is:

$A$ thin metallic spherical shell contains a charge $Q$ on it. $A$ point charge $+q$ is placed at the centre of the shell and another charge $q'$ is placed outside it as shown in the figure. All the three charges are positive. The force on the charge at the centre is:

Two radioactive materials $X_1$ and $X_2$ have decay constants $10 \lambda$ and $\lambda$ respectively. If initially they have the same number of nuclei,then the ratio of the number of nuclei of $X_1$ to that of $X_2$ will be $1 / e$ after a time:

When electromagnetic radiation of wavelength $300 \ nm$ falls on the surface of a metal,electrons are emitted with the kinetic energy of $1.68 \times 10^5 \ J \ mol^{-1}$. What is the minimum energy needed to remove an electron from the metal? $(h = 6.626 \times 10^{-34} \ J \ s, c = 3 \times 10^8 \ m \ s^{-1}, N_A = 6.022 \times 10^{23} \ mol^{-1})$

What is obtained when one mole of calcium phosphide is reacted with excess water?

To distinguish between $A$ and $B$,it is observed that both elements form oxides. When burnt in air,$B$ also forms a nitride. What are $A$ and $B$?