As we learned before, energy transfer can take form in many different processes, but now we will specifically look at energy transfer by heat and work. Energy transfer due solely to a change in temperature between a system and its surroundings is also known as heat. Whereas any energy transfer not due to a change in temperature is known as work.
Although both heat and work have different requirements to occur, they have some similarities. Heat and work are both boundary phenomena, they are not properties, and they both associate with a process, not a state.
Let's imagine we have a system, and this system undergoes a change in energy. Now from our previous studies, we would write this change of energy like
[ 1 ]
$$ \Delta E_{sys} = E_{in} - E_{out} = \Delta U + \Delta KE + \Delta PE $$
where $ \Delta KE + \Delta PE = 0 $ for stationary systems. Now remember for closed systems, there is no mass flow over the boundary, and for open systems, there is. So, we could say for a closed system, the change in energy is equal to the change in heat ( $ Q $ ) plus the change in work ( $ W $ ). That means for an open system, the change in energy would be equal to $ Q + W + $ (the mass transfer of total energy).
Note: the mass transfer of total energy is the total specific energy, or $ e = u + \frac{v^2}{2} + gz $.
We can then write Eq. (1) in terms of heat, work, and mass flow energy to get
[ 2 ]
$$ E_{in} - E_{out} = (Q_{in} - Q_{out}) + (W_{in} - W_{out}) + (E_{mass, in} - E_{mass, out}) $$
Now we have one of the fundamental equations for energy balance that we will use throughout the remainder of this guide. Remember, energy is everywhere, and we are now one step closer to analyzing how much energy change takes place in a system and what the contribution is from the change in heat, work, and mass flow!