Transfer of thermal energy — practice question

One end of a copper rod is heated. By what one process is thermal energy passed through the copper rod?

• AFree electrons transfer energy from the cooler end to the hotter end.
• BFree electrons transfer energy from the hotter end to the cooler end.
• CMolecules of copper move from the cooler end to the hotter end.
• DMolecules of copper move from the hotter end to the cooler end.