$A$ cylindrical resistor of radius $7.0 \, mm$ and length $4.0 \, cm$ is made of a material that has a resistivity of $10^{-6} \, \Omega \cdot m$. If the energy is dissipated at a rate of $1.54 \, W$ in the resistor, then the current density is:

If the ratio of the radii of two wires of the same material is $1 : 2$ and the ratio of the current flowing through them is $4 : 1$,what is the ratio of their drift velocities?

$A$ potential difference of $V$ is applied at the ends of a copper wire of length $l$ and diameter $d$. On doubling only $d$,drift velocity

An insulating pipe of cross-section area $A$ contains an electrolyte which has two types of ions with charges $-e$ and $+2e$. $A$ potential difference applied between the ends of the pipe results in the drifting of the two types of ions,having drift speeds $v$ (for $-e$ ion) and $v/4$ (for $+2e$ ion). Both ions have the same number density $n$. The current flowing through the pipe is:

$A$ conductor of non-uniform cross-section is connected to a source of constant potential difference as shown in the figure. Then:

$A$ 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 ............. $A/m^2$.