When open pipe is closed from one end then third overtone of closed pipe is higher in frequency by 150Hz than second overtone of open pipe. The fundamental frequency of open end pipe will be (Neglect end correction)

A person observes two moving trains. 'A' reaching the station and 'B' leaving the station with equal speed of 30m/s. If both trains emit sounds with frequency 300 Hz, (speed of sound = 330 m/s) difference of frequencies heard by the person will be

Two spherical hollow spheres of radii '$R_1$' and '$R_2$' are charged with same charge. Let '$\sigma_1$' and '$\sigma_2$' are their respective charge densities, then the ratio $\sigma_1$ to $\sigma_2$ is

An electric dipole of length $0.5\ \mu\text{m}$ is placed with its axis making an angle of $30^\circ$ with uniform electric field $10^4\ \text{V/m}$. If it experiences a torque of $5 \times 10^{-9}\ \text{Nm}$, the magnitude of the charge on the dipole is ($\sin 30^\circ = 0.5$)