Electric current is due to the drift of electrons in

In a wire of cross-section radius $r$,free electrons travel with drift velocity $v$ when a current $I$ flows through the wire. What is the current in another wire of half the radius and of the same material when the drift velocity is $2v$?

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.

$A$ $1 \mu A$ beam of protons with a cross-sectional area of $0.5 \text{ mm}^2$ is moving with a velocity of $3 \times 10^4 \text{ m/s}$. The charge density of the beam is:

Write Ohm's law in the form of current density (vector form).

The drift velocity of free electrons in a conductor is $v$ when a current $i$ is flowing in it. If both the radius and current are doubled,then the drift velocity will be