4.2 Heat transfer across a boundary

The total energy stored in a system may change when energy is transferred into or out of the system. For a closed system, the energy transfer is achieved via two mechanisms: heat and work, as illustrated in Figure 1.2.3.

Heat transfer takes place when a temperature difference exists between a system and its surroundings. As heat transfer must cross the system boundary, it is a boundary phenomenon. The heat transfer between two states during a process can be written as

${}_{1}Q_{2}=\displaystyle\int_{1}^{2}{\delta Q}$

Different from internal energy, heat transfer is NOT a state function. It is a path function because the amount of heat that is absorbed or rejected by a substance in a process depends not only on the initial and final states, but also on the process path. Although heat transfer is NOT a property of a system, it has a significant effect on the changes of properties of the system in a process.

Specific heat transfer refers to the amount of heat transfer per unit mass of a substance. It is defined as

$q=\displaystyle\frac{Q}{m}$

where

$m$: mass of a system, in kg

$Q$: amount of heat transfer in a process, in kJ

$q$: amount of specific heat transfer in a process, in kJ/kg