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Thermodynamics is a branch of physics related to the transfer of heat. The words thermo and dynamics mean heat in motion. There are three laws in thermodynamics. Temperature plays an important role in zeroth law, whereas energy is the important parameter in the first law. To understand the first law, it is essential to know about thermodynamic systems and thermodynamic processes and parameters.
Definition
“Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.”
“Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.”
First law of thermodynamics :-
The first law of thermodynamics is the application of the energy conservation principle. According to this principle, the energy of an isolated system remains constant. So before going to the discussion, we have to understand the term “energy.”
We know that energy is the capacity to do work. However, according to the conservation energy principle, energy can be converted from one form to another. So what is the equivalent form of energy in a chemical or physical system? These are “work” and “heat.” Work is the transfer of energy with a mechanical agent, and heat is the transfer of energy without any mechanical equivalent.
As an example, consider a piston and cylinder system. The cylinder contains an ideal gas. When heat is supplied to the cylinder, what happens? The gas in the cylinder will expand. When the gas expands, it does some work against the piston. So we can conclude that some of the heat energy given to the cylinder is now converted into work done by the gas on the piston.
We learn from the second law of thermodynamics that total heat given to the system cannot be converted into work completely. So what happens to the remaining energy? It is converted into “internal energy” of the gas. Let us understand “internal energy” in greater detail.
Internal energy :-
In macroscopic terms in classical thermodynamics, the internal energy is the summation of all energy except the kinetic energy and potential energy. It is the vibrational energy of the molecule of the gas. When we discuss the conservation of energy, we are not talking about the kinetic and potential energies because in the case of the gas or any thermodynamic system, we will discuss that all are neglected kinetic energy and potential energy.
Mathematically,
ΔE= ΔU + ΔKE + ΔPE
Where ΔE= change in energy
ΔU= change in internal energy
ΔKE= change in kinetic energy
ΔPE= change in potential energy
In static system, KE=PE=0
So we can say that E= U
Now we can write that
(heat given to the system)=(change in internal energy )+(work done by the system)
Symbolically, Q=U+w
Where, Q= Heat given to the system
U= change in internal energy
w=work done by the system
So this equation is called the first law of thermodynamics.
Application of the first law of thermodynamics :-
There are several applications of the first law of thermodynamics in science and engineering. Some of the examples are described below :-
Heat engine :-
The heat engine is the most common and practical example of the first law of thermodynamics. So what is a heat engine? A heat engine is a device that converts thermal energy into mechanical energy and vice versa. A heat engine consists of a cylinder-piston arrangement with working fluid. The application of heat and pressure produces work by the piston. Let us understand its working procedure.
When heat is supplied to the engine, the working fluid absorbs the heat and then expands in volume so the piston moves slowly upward and mechanical work is produced. Some amount of heat also radiates from the system, which will be discussed again in the second law. The working fluid exerts pressure due to its volume expansion, which exerts a force on the piston’s surface, causing it to travel downward. This movement can then be used to provide work equal to the total force applied to the top of the piston multiplied by the distance travelled by the piston.
Refrigerators or air conditioners :-
Another practical application of the first law of thermodynamics is in refrigerators and air conditioners. The refrigerator is a device that draws heat from a low-temperature sink to a high-temperature source. It contains a compressor that uses mechanical energy to compress the refrigerant fluid from the evaporator in the gas phase; with this compression, the pressure and temperature of the refrigerant fluid increase. When it enters the condenser, the refrigerant transfers the heat to the environment, causing its temperature to decrease and condensation to occur. This is the process of phase change from gas to liquid.
After that, the refrigerant fluid passes through the control element – capillary tube or expansion valve – which, by narrowing the passage, slows its speed on the evaporator, causing its pressure to decrease.
The refrigerant fluid arrives in the liquid state and under low pressure to the evaporator, during which it changes phase again, from liquid to gas. After a phase change, it absorbs the heat present in the conditioned items in the refrigerator case and returns to the compressor, restarting the refrigeration cycle.
Heat pump :-
A heat pump has the same application as refrigerators. The working principle is also nearly the same as the refrigerators.
Heat pumps can also transfer heat from a low to a high-temperature source. Heat pumps can be reversible or irreversible. Reversible heat pumps are commonly used as refrigerators.
Conclusion
The first law of thermodynamics is one of the fundamental laws that help understand the thermal behaviour of substances. There is a broad application of this in science and engineering. In power plant engineering and industries, there are various uses of heat engines and heat pumps. All vehicles and motors use this principle for working.
