What is conduction defined as in terms of heat energy?

Conduction is defined as the transfer of heat energy through a solid substance. This occurs at the molecular level, where heat is transferred from one molecule to another without the movement of the substance as a whole. When a solid is heated, its particles vibrate more vigorously and collide with adjacent particles, transferring the kinetic energy that constitutes heat. This process is most efficient in solids due to the closely packed arrangement of their molecules, which facilitates energy transfer.

Understanding conduction is essential in the context of refrigeration and heat transfer applications, as solid materials often play critical roles in the efficiency of cooling systems. The other options refer to different methods of heat transfer: liquids and gases conduct heat but also allow for convection, and a vacuum does not facilitate conduction since there are no molecules present to transfer energy.