Which particles are responsible for the conductivity of metals?

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

When the current $i$ is flowing through a conductor,the drift velocity is $v$. If $2i$ current is flowed through the same metal but having double the area of cross-section,then the drift velocity will be

In a wire of circular cross-section with radius $r$,free electrons travel with a 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 $2V$?

$A$ wire of non-uniform cross-section is carrying a steady current. Along the wire,

$A$ 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