Which of the following quantities does not change when a resistor connected to a battery is heated due to the current?

$A$ wire has a non-uniform cross-section as shown in the figure. $A$ steady current flows through it. The drift speed of electrons at points $P$ and $Q$ is $v_P$ and $v_Q$ respectively.

Explain the drift of electrons and drift velocity. Derive the equation for electric current in terms of the cross-sectional area of a conductor.

Two wires $A$ and $B$ of the same material have lengths $L_A, L_B$ and radii $R_A, R_B$ and drift velocities $v_A, v_B$ respectively. Both wires carry the same current. If $L_A = L_B$ and $R_A = 2R_B$,then the value of $\left(\frac{v_A}{v_B}\right)$ is:

In a metal, the charge carrier density is $9.1 \times 10^{28} \,m^{-3}$ and its electrical conductivity is $6.4 \times 10^7 \,S \,m^{-1}$. When an electric field of $10 \,N C^{-1}$ is applied to the metal, then the average time between two successive collisions of electrons in the metal is (Mass of electron $= 9.1 \times 10^{-31} \,kg$; charge of electron $= 1.6 \times 10^{-19} \,C$)

The unit of mobility in terms of fundamental units is . . . . . . .