In hydrogen atom, the energy of electron in first and third orbit is ' $E_1$ ' and' $E_3$ ' respectively. If $E_3=x E_1$ then the value of $x$ will be

Figure shows the north pole of a magnet moving away from a thick conducting loop containing a capacitor. The excess positive charge will arrive on

An observer moves towards a stationary source of sound with a velocity of one fifth of the velocity of sound. The percentage increase in the apparent frequency is

An a. c. source is connected across a pure inductor. Which one of the following figures shows the correct phase relation between the current and e.m.f.?