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First Law of Thermodynamics and Second Law of Thermodynamics

First Law of Thermodynamics and Second Law of Thermodynamics

What is Thermodynamics?

Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature and energy. It mainly deals with conversion of thermal energy from and to other forms of energy and its impact on the matter. The vibrating or moving molecules possess thermal energy due to the change in temperature. 

First Law of Thermodynamics

According to the First Law of Thermodynamics heat is a form of energy and it obeys the law of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy. 

The equation of First Law of Thermodynamics can be written in terms of internal energy of a system “∆U”, heat transfer into the system “Q” and the work done on the system “W”.

∆U = Q + W

What does Internal Energy, Heat and Work mean?

The sum of all the kinetic energies of the individual molecules is called the internal energy U of the system. The speed of the molecules increases when there is an increase in temperature of the gas which further increases the internal energy of the system. Similarly, there is a decrease in internal energy when the temperature decreases which in turn decreases the speed of the molecules. So we understand internal energy and temperature increases when the speed of the gas molecules increases. That means internal energy is proportional to temperature ΔU ∝ T. If internal energy is doubled then temperature also doubles. If internal energy remains constant then temperature also remains constant.

Internal energy can be increased by transferring heat energy “Q” into the gas. The heat energy will enter the container if it is kept on a bunsen burner or boiling water. The heat “Q” will be a positive value if heat enters the system. Conversely, the value of internal energy can be reduced by transferring heat out of the gas. This is made possible by placing the container on an ice bath. If heat exits the gas, Q will be a negative number.

The internal energy can also be increased by doing work on the gas. This is done using a piston. The gas is compressed using a piston, this causes the molecules to move faster and increase the internal energy. If the work is done on the system it will be a positive number. If the gas expands and pushes the piston then the speed of the molecules will slow down, decreasing the internal energy. It the gas expands then the work done on the gas is considered negative.

Second Law of Thermodynamics

According to the second law of thermodynamics, any spontaneously occurring process will lead to an increase in the entropy of the universe. In other words, the entropy of an isolated system will never decrease over time. In some cases where the system is in thermodynamic equilibrium or going through a reversible process, the total entropy of a system and its surroundings remain constant. The second law is also known as the Law of Increased Entropy.

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