In thermodynamics, temperature and heat are categorically different quantities: temperature is an absolute state property defined as the average kinetic energy of the molecules of a substance, whereas heat (denoted q) is not a property a body possesses but a quantity of energy transferred across a boundary in response to a temperature difference. A temperature gradient constitutes stored potential energy, and in the absence of an opposing force the system evolves toward its lowest energy state by dissipating that gradient — this is the mechanism of heat transfer. Because transfer is defined between a **system** and its **surroundings**, sign convention encodes direction (negative q for energy leaving the system, positive q for energy entering), and conservation requires that the heat lost or gained by the system be equal in magnitude and opposite in sign to that gained or lost by the surroundings.
In thermodynamics, temperature and heat are categorically different quantities: temperature is an absolute state property defined as the average kinetic energy of the molecules of a substance, wherea…