Space between two concentric conducting spheres of radii a and b (b > a) is filled with a medium of resistivity $$\rho $$. The resistance between the two spheres will be :

A metal coin of mass 5 g and radius 1 cm is fixed to a thin stick AB of negligible mass as shown in the figure. The system is initially at rest. The constant torque, that will make the system rotate about AB at 25 rotations per second in 5s, is close to :

A coil of self inductance 10 mH and resistance 0.1 $$\Omega $$ is connected through a switch to a battery of internal resistance 0.9 $$\Omega $$. After the switch is closed, the time taken for the current to attain 80% of the saturation value is: [take ln 5 = 1.6]

The figure represents a voltage regulator circuit using a Zener diode. The breakdown voltage of the Zener diode is 6 V and the load resistance is, R_L = 4k$$\Omega $$. The series resistance of the circuit is R_i = 1 k$$\Omega $$. If the battery voltage V_B varies from 8 V to 16 V, what are the minimum and maximum values of the current through Zener diode?