Thermodynamics Help .... Understanding Controlled Mass/Volume I don't understand the these processes. Please let me know if I am correct or not. If not, please correct me. Thanks. From what I know, when you have a controlled volume process the pressure and temperature increase when heat is added. This is in a rigid container so no mass leaves or enters. So it is controlled mass as well, right? But I do not understand how this is referred to as controlled volume. When you heat air then the volume increases and the density decreases, resulting in the mass remaining the same, correct. Also, if you know the energy of individual fluids and 'x' amount of energy is added to the above system, and then you know the resulting total energy after the addition. How do you paredo out how much energy goes to which fluid or the new temperature.