If ' $R$ ' is the radius of earth \& ' $g$ ' is acceleration due to gravity on earth's surface, then mean density of earth is

In LCR series circuit if the frequency is increased, the impedance of the circuit

A potentiometer wire of length 1 m is connected in series with $495 \Omega$ resistance and 2 V battery. If $0.2 \mathrm{mV} / \mathrm{cm}$ is the potential gradient, then the resistance of the potentiometer wire is

A carnot engine, whose efficiency is $40 \%$ takes heat from a source maintained at temperature 600 K . It is desired to have an efficiency $60 \%$, then the intake temperature for the same exhaust (sink) temperature should be