What is First Law of Thermodynamics all about?

The first law of thermodynamics is essentially a statement of the conservation of energy, and it provides the universal framework for understanding the energy balance of a system. The type of energy bank we consider depends entirely on the system we define. For a closed system, which has a fixed mass, the total energy stored is primarily its internal energy. This internal energy represents the microscopic energy of the system, meaning the kinetic energy of molecules bouncing around in a gas, or the vibrational energy, often described as phonons, within the lattice structure of a solid. The total energy of this fixed mass can only be changed by two mechanisms that cross its boundary, namely heat transfer and work.

Fig 1: Basic Concepts of First Law of Thermodynamics

For an open system, also called a control volume, mass flows in and out, carrying energy with it, which makes the energy bank more complex. A flowing fluid possesses its own internal energy due to its molecular activity, but it also possesses flow energy, which is the product of its pressure and volume and represents the work required to push the fluid. Additionally, it has kinetic energy due to its velocity and potential energy if it is raised to a certain height. To simplify analysis, engineers combine the internal energy and the flow energy into a single property called enthalpy. The total energy of an open system can therefore be altered by three distinct means, namely heat transfer, work, and the net influx of energy carried by the mass flowing across its boundary.

Crucially, both heat and work are path functions, meaning the amount transferred depends on the specific process the system undergoes to move from one state to another. This profound realization, that a system's energy can only change through these specific interactions (heat and work) and that these interactions depend on the path taken, is what the first law of thermodynamics is all about. It is a principle that governs everything from the steam in a power plant to the air in a bicycle pump.