What is true about heat as a measure in thermodynamics?

Heat is defined in thermodynamics as energy in transit due to a temperature difference between systems or within a system. This means that heat is not contained within the system but rather flows from one system to another, like when hot coffee cools down by transferring heat to the surrounding air. This definition is crucial, as it distinguishes heat from other forms of energy that may be stored or transformed in a system.

The right understanding of heat energy also clarifies why the other statements do not hold true. For instance, measurements related to heat can occur even when systems are not at rest; there can be heat transfer during processes such as phase changes or chemical reactions. Additionally, heat itself can have positive or negative values depending on the direction of energy transfer – it is not inherently negative. Lastly, heat transfer is affected by pressure, particularly in systems involving gases, as changes in pressure can alter the temperature and thus the heat transfer properties between systems.