Laws of Thermodynamics

The Zeroth Law of Thermodynamics

The zeroth law expresses that having in existence three systems, A, B, and C, if A is in equilibrium with C and B is in equilibrium with C, then A and B will also be in equilibrium. All three systems will be in equilibrium in temperature. If any of these systems are in contact with other systems, there will be compensation in the temperature level of all the systems involved. That is, they will all have the same temperature.


The First Law of Thermodynamics

The first law of thermodynamics centralizes, generally, on the existence of the property of energy. It states: "For any process involving only the displacement of a mass between specified levels in a gravity field and no externalities to the system, the magnitude of that mass is fixed by the end states of the system and is independent of the details of the process." This law ramifies itself into many other assumptions:

1. Definition of heat.
When two objects that possess different temperatures are brought into contact, a thermodynamic process that establishes an equilibrium of temperatures takes place. Scientists in the XVIII century explained this phenomenon with the concept of "caloric" or heat. This law identified it as a form of energy that could be stored and converted into mechanical energy. It was measured in calories.

2. Uniqueness of work values.
Work is the result of a force acting on a body causing it to move. A specific quantity can be assigned to a work interaction between systems. This number of units of mass is displaced between two specified levels in a gravity field. When work is performed by a system (of a rising weight) it has a positive sign. The unit that identifies work done by energy is the joule.

3. Definition of energy.
When work is done in a system, there is always a change in state. Lets use A as the initial position and B as the final position. In A, there exists a specific amount of energy (EA) that needs work (W) in order for the object to move to B and possess another amount of energy (EB). Therefore, in mathematical terms, EA + W = EB. Its unit of measurement is erg. One calorie is equal to 4.186 x 107 ergs, or 4.186 joules.

4. Conservation of energy.
States that energy can only be modified from one form to another. It cannot be manifested or destroyed. For this reason, the sum of the amount of heat transferred in a system and the work done on the system is equal to an increase in the internal energy in the system. However, this law does not apply to nuclear energy because it is produced when atoms of matter are split or fused. The law of conservation of energy is often combined with the law of conservation of matter. This is because matter can be converted into energy.

5. Impossibility of the perpetual -motion machine of the first kind.
A perpetual- motion machine of the first kind (pmm1) is a hypothetical system in which no energy is required to perform work. In opposition, it is known that a machine needs to have some amount of energy that would be converted to work. Therefore, the ppm1 is an impossible machine.

6. The first law and relativity
According to Einstein's theory of relativity energy of a system is equal to the product of its mass and the square of the speed of light (E = mc² ). The energy and mass of the system is conserved even when there are processes occurring within the system. Further, if the energy suffers any modifications in the system, then the mass will also be altered.


The Second Law of Thermodynamics

The second law of thermodynamics focuses mainly on the equilibrium states of systems and processes that associate these states with others. The word equilibrium signifies that with time the state of a system will remain unchanged while being isolated from any other systems that may be found in an environment. It states: "Among all the allowed states of a system with specific values of energy, constraints, and numbers of particles, one and only one is a stable equilibrium state." Other hypotheses have been inferred from this law.

1. State principle.
As already known, the equilibrium state of a system corresponds to the values of energy, constraints and numbers of particles in that system. The state principle declares that the values of any property of a system in a state of equilibrium can only be expressed as a function of the values of energy, constraints and numbers of particles.

2. Reversible and irreversible processes.
If a system and its environment can change states and are capable of restoring their original states it is called a reversible process. On the other hand, if a system, for example, changes from its initial state to an equilibrium state without affecting its environment it is said to be an irreversible process.

3. Impossibility of the perpetual-motion machine of the second kind.
A system in a stable equilibrium position cannot produce any work but only receive it. If a system in a stable equilibrium state were to produce work, it would cause the system to change to a non-equilibrium state without affecting its environment. This impossible notion is the premise of the perpetual-motion machine of the second kind (pmm2). It is a device that creates work from a stable equilibrium position.

4. Work done reversibly by a system in combination of a reservoir.
If there are two systems A and B that are in a state of mutual equilibrium each system is in a stable equilibrium position. Furthermore, if the state of one of the two systems is altered, while being in contact A with B, the second system will also alter. A combination of system A and a reservoir can experience work directly through each other or indirectly using an intermediate object.

5. Definition of entropy.
Entropy is a measure of the disorder in the system or the measure of how close the system is to equilibrium. It indicates the degree to which a specific quantity of thermal energy is available for performing work. This means the less entropy, the more available the energy. The second law affirms that entropy cannot decrease for any spontaneous process. As an outcome of this law, an engine can deliver work only when heat is transferred from a hot reservoir to a cold reservoir or heat sink.


The Third Law of Thermodynamics

By virtue of the second law, an absolute zero temperature is included in an absolute temperature scale. The third law of thermodynamics remarks that absolute zero cannot be obtained easily by any procedure. It is only possible to approach absolute zero, but impossible to reach it. This law also defines the term zero entropy by stating that all bodies at absolute zero would have the same entropy.