Final answer:
When the current in a wire is doubled, the current density and electron drift speed double, while the conduction-electron density and mean time between collisions remain unchanged.
Step-by-step explanation:
When the current in a wire is doubled:
(a) The current density doubles because it is directly proportional to the current assuming the cross-sectional area of the wire remains constant.
(b) The conduction-electron density remains unchanged as it depends on the material properties and not the current.
(c) The mean time between collisions remains unchanged since it is a characteristic of the material and the temperature, which are assumed to be constant.
(d) The electron drift speed doubles, given that drift velocity is proportional to current for a given conductor with constant charge carrier density and cross-sectional area.
For the Practice Problems: Doubling the voltage across an ohmic resistor also doubles the current through the resistor, according to Ohm's law.