why is it that in a normal circuit, there is no Pd across a
wire.
1) The wires in a normal circuit are too short to have any
significant effect on the resistance in the circuit.
2) In O levels, and probably A levels, the wires are also
assumed to be of zero resistance. Actual reason is in (1).
In sensitive circuits on circuit boards (those green colour
boards), resistance of wires will have a very huge effect. Hence
the usage of gold (very good electrical conductor, very low
resistance, lower than copper) for electrical contacts to such
circuits. Even in your handphone, small amounts of gold are used as
'wires'.