Concept of First Law of Thermodynamics

The water stored in a dam possesses potential energy by virtue of its height

. When this water is allowed to fall, its potential energy change into kinetic energy. Due to its kinetic energy, the falling water can rotate the turbines and produces electricity. Thus PE is converted into KE. In other words, the total energy remains constant when a body changes its position, though one form of energy may be converted into another. This leads to law of conservation of energy.

Chemical thermodynamics is the branch of thermodynamics which deals with study of processes in which chemical energy is involved. The study of thermodynamics is based on three generalisations derived from experimental results. These generalisations are known as first, second and third law of thermodynamics based on human experience and there is no formal proof for them.

Every substance possesses a definite quantity of energy which depends upon factors such as chemical nature of the substance, temperature and pressure. This is known as intrinsic or internal energy (E or V). The exact value of this energy is not known as it includes all types of energies of molecules constituting the given mass of matter such as transitional, vibrational, rotational, the kinetic and potential energy of the nuclei and electrons within the individual molecules and in which the molecules are linked together. Internal energy of a system is an extensive and state property.

First law of thermodynamics

It states that, energy can neither be created nor destroyed although it can be converted from one form to another. The total energy of the universe is constant. Consider a system whose internal energy is U1. If the system absorbs 'q' amount of heat, then the internal energy of the system increases and becomes U1 + q. If work 'w' is done on the system then its internal energy further increases and become U2.

U2 = U1 + q + w

U2 - U1 = q + w

DU = q + w

If a system does work (w) on the surroundings, its internal energy decreases. In this case, work is taken as negative (-w). If 'q' is the amount of heat added to system and 'w' is the work done by the system, then change in internal energy (DU) becomes.

DU = q - w

Sign Conventions, according to IUPAC

Heat absorbed by system = +q

Heat realsed by system = -q

Work done on system = +w

Work done by system = -w

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Concept of First Law of Thermodynamics

By: P. Surya Prakash

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