What aspect of a system does the variable Q represent?

The variable Q is commonly used in thermodynamics to represent heat exchange. This means it quantifies the amount of thermal energy transferred into or out of a system during a process. When heat is added to a system, Q is positive, indicating an increase in internal energy. Conversely, if heat is removed, Q is negative, indicating a loss of internal energy.

This definition is fundamental to understanding energy transfers in various thermodynamic processes. For example, during phase changes, heat transfer occurs without a change in temperature, emphasizing the importance of Q in quantifying energy transfer in both reversible and irreversible processes. In contrast, the other variables in the options refer to different types of energy interactions or system properties, but they do not specifically denote heat exchange.