$A$ beam contains $2 \times 10^8$ doubly charged positive ions per cubic centimeter,all of which are moving with a speed of $10^5 \,m/s$. The current density is ............. $A/m^2$.

In the given hollow cylindrical conductor,the current density is $J = \frac{J_0}{r^2}$,where $J_0$ is a constant and $r$ is the distance from the axis of the cylinder. If the radius of the inner surface is $a$ and the radius of the outer surface is $2a$,find the current passed through the conductor.

Choose the correct option with respect to the statements $A$ and $B$:
$(A)$: When no electric field is applied across a conductor,the path of free electrons between two successive collisions in it is straight.
$(B)$: When an electric field is applied across a conductor,the drift velocity of electrons is independent of time.

The lengths of two wires made of the same material are in the ratio $2:3$ and their radii are in the ratio $1:2$. If the two wires are connected in parallel to a battery,then the ratio of the drift velocities of free electrons in the two wires is

Two identical conductors are kept at the same temperature. If the ratio of the potential difference across them is $1 : 2$,what will be the ratio of their drift velocities?

Assertion: The current density $\vec J$ at any point in an ohmic resistor is in the direction of the electric field $\vec E$ at that point.
Reason: $A$ point charge when released from rest in a region having only an electrostatic field always moves along electric lines of force.