Energy Transfer Processes

In this article, we will discuss Energy Transfer Processes. So, let’s get started.

Energy Transfer Processes

The study of energy transfer processes assumes that all ”systems” possess ”internal energy”. By internal energy we mean the sum of the kinetic and potential energies of the constituents of the ”system”. In this course we will consider systems that are bulk materials like gases, liquids, and solids. One can extend these ideas to smaller systems like molecules, atoms or nuclei. Experiments verify that objects are made up of atoms and molecules and contain internal energy.

The constituents of a system can gain energy or lose energy, and consequently the total interal energy of a system can change. U is denoted as the total internal energy, and

∆U is defined as the change in the total internal energy of a system

U can change in different ways, but we divide the possibilities into two catagories: those due to mechanical work and those only due a temperature difference:

W is defined as the mechanical work done by a substance

Note the preposition by in the definition of W. If W is positive then the system loses internal energy, since the system is doing work. If W is negative then the system gains internal energy, since work is done on the system. This is the convention chosen by scientists. It will make sense when we use a P − V diagram.

Q is defined as the energy transferred to the system by processes that don’t
involve mechanical work

Note that Q is the energy transferred to the system, so if Q is positive then so is ∆U. Q is also defined as energy that is transfered as a result of a temperature difference only. Q is also called heat. Heat is a transfer of energy.

Both W and Q have units of energy. The metric unit of energy is the Joule.
However, experiments with heat processes were done before it was realized that heat processes could be related to mechanical energy. The unit of the Calorie was used.

The conversion from calorie to the Joule, the mechanical equivalent of heat, is 4.186 Joules = 1 calorie. The ”calorie” listed on food packages is equal to 1000 calories or 4186 Joules.

In general both types of processes can occur at the same time, so the change in U is the sum of these two energy transfer processes:

∆U = Q − W (1)

The minus sign on W is due to the definition of W as the work done by the system. This equation is called the first law of thermodynamics. It is a statement recognizing the existence of internal energy and its conservation.