Empirical Law: Heat Transfer
Two physical systems which are not at thermal equilibrium will interact with each other and exchange kinetic energy until such an equilibrium is reached.
This process is called heat transfer - we say that heat flows from the system with higher temperature into the system with lower temperature.
- When heat flows out of a physical system, the system loses kinetic energy and so heat is considered negative - from the point of view of the system with higher temperature, heat is negative, since the total kinetic energy of the system decreases during heat transfer.
- When heat flows into a physical system, the system gains kinetic energy and so heat is considered positive - from the point of view of the system with lower temperature, heat is positive, since the total kinetic energy of the system increases during heat transfer.