Copper and silicon are cooled from $300 \ K$ to $60 \ K$. The specific resistance:

Conductivity increases in the order of

Resistivity of which of the following substances decreases on increasing the temperature?

$A$ cylindrical metallic wire is stretched to increase its length. If the resistance of the wire is increased by $4 \%$, then the percentage increase in its length is (in $\%$)

An electric wire of length $l$ and area of cross-section $a$ has a resistance $R \ \Omega$. Another wire of the same material having the same length and area of cross-section $4a$ has a resistance of:

Consider a block of conducting material of resistivity $\rho$ shown in the figure. Current $I$ enters at $A$ and leaves from $D$. We apply the superposition principle to find the voltage $\Delta V$ developed between $B$ and $C$. The calculation is done in the following steps:
$(i)$ Take current $I$ entering from $A$ and assume it to spread over a hemispherical surface in the block.
$(ii)$ Calculate field $E(r)$ at distance $r$ from $A$ by using Ohm's law $E = \rho j$,where $j$ is the current per unit area at $r$.
$(iii)$ From the $r$ dependence of $E(r)$,obtain the potential $V(r)$ at $r$.
$(iv)$ Repeat $(i)$,$(ii)$ and $(iii)$ for current $I$ leaving $D$ and superpose results for $A$ and $D$.
For current entering at $A$,the electric field at a distance $r$ from $A$ is