temperature is a measure of how much movement there is among molecules/atoms.
When current meets no resistance in wire, it causes no thermal excitation in the wire, just bumps electrons around and no heat is produced.
In resistors, however, it does not flow as smoothly and excites the atoms/molecules.
Wires have varying amounts of resistance (except certain ones when supercooled) so get varying amount of heat excitation from current flow.
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