Consider a cylindrical element as shown in the figure. The current that flows through the element is I and resistivity of material of the cylinder is $\rho$. Choose the correct option out the following

In the given diagram, a line of force of a particular force field is shown. Out of the following options, it can never represent

The electrostatic potential $\left(\phi_r\right)$ of a spherical symmetric system, kept at origin, is shown in the adjacent figure, and given as

$$ \begin{array}{ll} \phi_r=\frac{q}{4 \pi \epsilon_0 r} & \left(r \geq \mathrm{R}_0\right) \\ \phi_r=\frac{q}{4 \pi \epsilon_0 \mathrm{R}_0} & \left(r \leq \mathrm{R}_0\right) \end{array} $$

A solid cylinder of mass m and radius $r$ is rolling on rough inclined plane of inclination $\theta$. The coefficient of friction between the cylinder and incline is $\mu$. then