$A$ copper wire of length $1\, m$ and radius $1\, mm$ is joined in series with an iron wire of length $2\, m$ and radius $3\, mm$ and a current is passed through the wires. The ratio of the current density in the copper and iron wires is

For which of the following dependencies of drift velocity $v_d$ on electric field $E$ is Ohm's law obeyed?

An electric current flows along an insulated strip $PQ$ of a metallic conductor. The current density in the strip varies as shown in the graph. Which one of the following statements could explain this variation?

$(a)$ The electron drift speed is estimated to be only a few $mm\; s^{-1}$ for currents in the range of a few amperes. How then is current established almost the instant a circuit is closed?
$(b)$ The electron drift arises due to the force experienced by electrons in the electric field inside the conductor. But force should cause acceleration. Why then do the electrons acquire a steady average drift speed?
$(c)$ If the electron drift speed is so small,and the electron's charge is small,how can we still obtain large amounts of current in a conductor?
$(d)$ When electrons drift in a metal from lower to higher potential,does it mean that all the 'free' electrons of the metal are moving in the same direction?
$(e)$ Are the paths of electrons straight lines between successive collisions (with the positive ions of the metal) in the $(i)$ absence of electric field,$(ii)$ presence of electric field?

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 $= 1.6 \times 10^{-19} \,C$)

Electric current is due to the drift of electrons in