Pressure (Thermodynamics): Definition, Formula, and Units

Thermodynamics is a field of physics that deals with energy transfers. It seeks to understand the relationships between temperature, energy, and pressure-volume work, analyse amounts of energy exchanged, and the activity performed in a physical process.

Thermodynamic science was initially developed by researchers looking for a way to improve machines during the industrial revolution to enhance efficiency. 

Today, the applications of the principle of thermodynamics are present in our daily lives. For example, thermal machines, refrigerators, car engines, transforming ores, and petroleum derivatives processes.

What is Pressure?

Pressure is a measure of the force exerted per unit area on the boundaries of a substance (or system). It is caused by the collisions of the molecules of the substance.

PV = nRT is the formula for thermodynamics pressure. Here, R is the universal gas constant. The other values are pressure (P), volume (V), temperature (T), and the number of molecules (n) under ideal thermodynamic conditions. 

Understanding Pressure and its Thermodynamics Unit

In Thermodynamics, it is assumed that all heat phenomena in bodies are characterised only by macroscopic parameters such as, 

• Pressure
• Volume
• Temperature

Pressure is a thermodynamic parameter P, which determines the elementary work performed by a certain system with a slow change in its volume V caused by the chaotic movement of external bodies. When elastic substances are deformed, the force affecting a unit surface is not perpendicular to it.

With uniform and all-round compression of the body, only normal stresses will be different from zero, which are equal to the initial pressure. In statistical physics, pressure is defined as the derivative of the mean energy ‘E’ by volume under the action of constant entropy ‘S’, or as a derivative of the free internal energy ‘F’ by volume at a constant temperature ‘T’. That is, the dependence P from T and V is determined by the equation of state. In equilibrium and stable states, metastable states are sometimes possible with P.

The internal pressure of a material body can be changed by performing certain mechanical work on it. If work is done on the matter, then pressure is automatically converted into internal energy. And if the body itself does the job, its internal pressure becomes mechanical. The more caloric flows into a substance, the warmer it will be, and vice versa. 

For example, the uneven heating of the gun barrels during drilling. The researcher suggested that the result of heating and pressure changes is the mechanical work observed during the friction of the drill on the barrel. 

Pressure thermodynamics unit

Pascal (Pa) is the SI unit of pressure in thermodynamics. 1 Pa equals 1 N/m².

Formulas of Thermodynamics

The study of pressure thermodynamics is related to the Formulas of Thermodynamics. The principles and formulas of thermodynamics explain how energy can be transferred from one system to another system in the form of heat or work.

These formulas are still responsible for postulating the existence of a quantity called entropy, capable of being determined for any, and all physical systems analysed.

• First Formula of Thermodynamics

The First formula of thermodynamics, also known as the Ideal Gas Law, is related to the principle of the relationship between temperature, pressure, and volume. It means that energy in a system cannot be created or destroyed but only transformed.

This formula is linked to pressure-volume work. It is the work resulting from the inflammation or compression of a liquid. When there is a transformation in the volume and external pressure remains constant, pressure-volume work happens. 

An example would be a person using a pump to inflate an inflatable balloon. In that sense, she’s using force to push air into the balloon, so her kinetic energy causes the bomb’s piston to go down. However, part of the energy involved in the task is transformed into heat and lost to the environment.

ΔU = Q – W (change in internal energy = Amount of Heat-Work done by the system).

Here are the other two formulas of thermodynamics that explain the principles of heat and its role in thermal processes.

• Second Formula of Thermodynamics

The Second Formula of thermodynamics states that heat transfers must occur spontaneously from the hotter body to the colder body, but not the other way around. This means the thermal energy transfer processes are irreversible.

A hot object placed in the refrigerator, for example, loses its heat to the environment. As per the second law, it is impossible for heat to ultimately convert into another form of energy, so we consider heat a degraded type of energy.

• Third Formula of Thermodynamics

The Third Formula of thermodynamics emerged as an attempt to establish absolute reference points that determine entropy. The third law is based on the Second Law.

Proposed by physicist Nernst, the third formula led to the conclusion that a substance couldn’t be considered pure at zero temperature to have an entropy of approximately zero.

Thus, the third thermodynamics formula has led to controversies, and many scientists and physicists consider it a rule and not necessarily a law.

• Zero Formula of Thermodynamics

Finally, the Zero Formula of thermodynamics cites the conditions to obtain thermal equilibrium in order to understand the influence of materials on greater or lesser thermal conductivity for the system.

According to Law Zero,

• If a body A is in thermal equilibrium, in contact with a body B, and;
• If such a body A is in thermal equilibrium, in contact with a body C, then;
• Body B is in thermal stability, in contact with body C.

Thermal equilibrium is achieved by transferring heat from the hot body to the cooler body. This formula is called the Zero Formula because understanding it is necessary for both the first and second laws to exist and make sense. 

Conclusion

Thermodynamics is governed by several formulas that apply to the system and materials. However, one of the best known and most used is the First Law of Thermodynamics formula, which is related to the concept of pressure and its effect on temperature. Understanding this principle and heat transfer relationships between bodies and physical systems is fundamental to understanding how thermodynamics works.