Which of the following materials has the highest number of free electrons?

Why is heat produced when a current is passed through a conductor?

$A$ thick wire and a thin wire of the same material are successively connected to the same circuit to find their respective resistance. Which one will have lower resistance? Give reason.

What is the unit of $Watt-second$?

$(a)$ Why are copper or aluminium wires generally used for electrical transmission and distribution purposes?
$(b)$ Two wires,one of copper and the other of manganin,have equal lengths and equal resistances. Which wire is thicker? Given that the resistivity of copper is lower than that of manganin.

$(a)$ For the circuit shown in the diagram,calculate:
$(i)$ The value of current through the $30\, \Omega$ resistor.
$(ii)$ The total resistance of the circuit.
$(b)$ Give two advantages of connecting electrical devices in parallel with a battery.