The first law of thermodynamics states that: "The total energy of an isolated system is neither created nor destroyed, it remains constant". It is a principle that reflects the conservation of energy.
Energy cannot be created, it is only transformed from one type to another. When one class of energy disappears, an equivalent amount of another class must be produced.
A body can have a certain speed. Having speed involves kinetic energy. If you lose speed you lose kinetic energy; which is transformed into another type of energy. The conversion can be to potential energy (if it acquires height), heat energy (if there is any type of friction that causes it to heat up), etc.
The first principle of thermodynamics allows us to define the postulate of the first law of thermodynamics:
If we supply any adiabatic system with a certain amount of mechanical energy W, this energy only causes an increase in the internal energy of system U. So:
First law of thermodynamics for non-isolated systems
If the system is not isolated, this equality is not fulfilled and the system undergoes a change in heat.
For the first law of thermodynamics, there is no trivial step of physical conception from the closed system view to an open system view.
For closed systems, the concepts of an adiabatic enclosure and an adiabatic wall are essential. Matter and internal energy cannot penetrate or penetrate such a wall. For a non-isolated system, there is a wall that allows the penetration of matter.
In general, matter in diffusive motion carries with it some internal energy. Some changes in microscopic potential energy accompany the movement. An open system is not adiabatically closed.
There are some cases where a process for a non-isolated system can, for particular purposes, be considered as an isolated system.
In an open system, by definition, hypothetically or potentially, matter can pass between the system and its environment. But when the process of interest involves only hypothetical or potential but not actual passage of matter, the process can be considered as if it were for a closed system.
If the work is done by the system, W is negative. If the work is done on the system, W is positive.
This criterion of signs related to heat and work is important since it varies depending on the author.
What is an adiabatic system?
An adiabatic process is a thermodynamic process in which the system does not exchange heat with its environment. It is in thermal equilibrium. An adiabatic process that is also reversible is an isentropic process.
The term adiabatic refers to volumes that prevent heat transfer with the environment. An isolated wall is quite close to an adiabatic limit.
An adiabatic process is performed with constant heat variation. In an isobaric process, it is carried out at constant pressure.
For all adiabatic process that takes a system from a given initial state to a given final state, the respective eventual total quantities of energy transferred as work are one and the same. It is determined just by the given initial and final states.
What is internal energy?
Joule conducted an experiment in which he concluded that the energy transferred in a heat engine became part of the internal energy of the machine.
These experiences serve to extend this observation to all thermodynamic systems and to postulate that:
If we supply any isolated system with a certain amount of mechanical energy W, it only causes an increase in the internal energy of system U:
The internal energy variation is equal to the supplied work.
This equality that applies to the isolated system, constitutes the definition of the internal energy U.
The internal energy in the international system of units is measured in joules (J).
The existence of this quantity for any system is the postulate known as: the first principle of thermodynamics.
What if the system is not isolated?
If the system is not isolated, it is observed that:
Then we can write:
In summary, we can say that the formulation of the first law of thermodynamics, the previous equation, contains three related ideas:
The existence of an internal energy function.
The principle of energy conservation,
The definition of heat as energy in transit
Summary and Conclusions
The first law of thermodynamics is the same as the law of conservation of energy. This principle states that:
In an isolated system, energy is neither created nor destroyed. It only undergoes transformations.
If mechanical work is applied to a system, its internal energy varies.
If the system is not isolated, some of the energy is converted into heat that can either enter or leave the system.
An isolated system is an adiabatic system. The heat can neither enter nor leave. No heat transfer is performed.