PHYSICS DICTIONARY

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Enthalpy

It is heat function at constant pressure. Mathematically expression is H=U+PV; ‘U’ is internal energy; ‘P’ is pressure and ‘V’ is volume.

Entropy Maximum Principle

For an isolated system in state of equilibrium, the entropy attains maximum value and remains constant.

Entropy

The thermal property which remains constant during adiabatic process is called entropy.

(or)

 Measure of randomness or disorderliness of molecules.

(or)

 Index of unavailable energy of a system.

Epicenter

The point on the surface directly above the “focus” is called Epicenter.

Epitaxial Growth

The technique of growing an oriented single crystal layer on a substrate wafer is called epitaxial growth. The substrate crystal may be a wafer of same material as the grown layer or a different material with similar lattice structure. The methods which are adopted for this growth are i) Chemical vapor deposition. ii) Liquid phase epitaxy iii) Molecular beam epitaxy

Equal Apriori Probability

A fundamental postulate of statistical mechanics is that a macroscopic system in equilibrium is equally likely to be in any of its accessible microscopic states satisfying macroscopic conditions of system.

                                               (or)

The probability of finding the phase point for given system in any region of phase space is identical with that for any other region of equal extension or volume. 

Equation

A mathematical statement or formula which shows the equality of two expressions.

Equation of Continuity

For any incompressible & non-viscous fluid flowing steadily, the product of its velocity and area of crossection at all points during its flow through a tube remain constant.  Velocity of fluid is inversely proportional to area of crossection.

Equation of Motion of Rigid Body

Relationship between torque applied to rigid body and angular acceleration of body is known as equation of motion of rigid body.

Equation of State

An equation of state is a relation between state variables. It is a thermodynamic equation which describes the state of matter under a given set of physical conditions, typically relating energy, temperature, volume, and pressure. Equations of state are most commonly used to describe properties of fluids, such as liquids, gases and plasma though equations of state may also be applied to solids.

Equilibrium of a Rigid Body

If a rigid body is subjected to number of forces acting on it and has neither translatory nor rotatory motion, then it is said to be in equilibrium. Following conditions are satisfied in such a case:

i)      The algebraic sum of forces acting on it is zero (translational equilibrium)

ii)    The algebraic sum of moments of all the forces about any point is zero.(rotational equilibrium)

Equilibrium (Phase)

The state of system where the phase characteristics remain constant over indefinite time periods is called as equilibrium. At equilibrium the free energy is least. 

Equipartition of Energy

The theorem of equipartition of energy states that molecules in thermal equilibrium have same energy associated with each independent degree of freedom of their motion.

Equipotential Surface

Surface on which every point is at same potential and the electric field is at right angles to all these points.

Erecting Lens

An eye piece sometimes used in Kepler telescopes that consists of four lenses and provides an erect image, which is more convenient for viewing terrestrial objects than the inverted image provided by simpler eye pieces. 

Erecting Prism

It is a system of prisms that converts the inverted image formed by most types of astronomical telescopes to an erect image, also known as inverting prism.

Erg

It is unit of energy in CGS system of units. It is amount of work done by a force of one dyne exerted for a distance of one centimeter. 1erg=10^-7 J.

Ergodic Hypothesis

The time average of some property of a system in equilibrium is same as instantaneous ensemble average.

Error

The difference between measured value and true value is called as error.

Escape Velocity

It is the minimum velocity with which a body should be projected from the surface of the planet so as to escape its gravitational field.

What is ENTROPY - Very important Points to be noted

1) The thermal property which remains constant during an adiabatic process is called as entropy.
     i.e. dQ/T= constant

2) It is a measure of randomness or disorderliness of molecules.

3) It is independent of the path of thermal cycle.

4) The increase in entropy implies transition from ordered state to disorder state.

5) It is an index of unavailable energy of a system.

6) Entropy could also be termed as thermal inertia since more entropy results in less amount of heat energy being converted into work.

7) The increase in Entropy of a system implies transition of thermal energy from more available energy to less available form for conversion into work.

8) The net change in entropy is zero for any reversible cycle. This statement is called as clausius theorem.

9) Clausius Theorem: - The sum of quantities of heat transfer during the isothermal change divided     by absolute temperature of the isothermal in a reversible cycle is zero. Entropy changes linearly in isothermal expansion and remains constant in adiabatic expansion or compression but decreases in isothermal compression.

The shape of Temperature (T) - Entropy (S) diagram (Tephigram) is rectangle.

10) Entropy increases in irreversible process.

11) Definition of second law of thermodynamics in terms of entropy

”Every chemical or physical or natural process in nature takes place in such a manner that total entropy increases or remains constant".

12) The principle of "degradation of energy" states that the available energy tending towards zero.

What is Chemical Potential?

For a chemical system, molar free energy is known as Chemical Potential.

A chemical substance that is free to move from one place to another place, will move spontaneously from a state of higher chemical potential to a state of lower chemical potential.

In the position of equilibrium, the chemical potential is constant through the entire system.

Let us consider a general heterogeneous system consisting of an independent components in several coexisting phases.

To start with, it is convenient to describe a given phase by its chemical composition, which is specified by the no. of mole 'Ni' of each species i, its volume V and its entropy 'S'.

If we consider internal energy (U)

U=U(S,V,N₁,N₂,.....Nᵢ,.....Nn)

μi=❴∂U/∂Nᵢ❵S,V,Nj ; j= except 'i'

'μi' is chemical potential of component 'i' in given phase.

dU=TdS-PdV+Σμᵢi.dNᵢ for i=1...n

We can also consider chemical potential 'μ' in terms of Helmoltz free energy 'F'.

F = F(T,V,N₁,N₂......Nn)

μ1=❴∂F/∂N1❵T,V,N₂,....

μ2=❴∂F/∂N2❵T,V,N₁,N₃,....

The chemical potentials are thus the rate of change of free energy per mole, at constant volume and temperature.

μ can also be expressed as

μi=❴∂G/∂Nᵢ❵T,P,Nj

A System in external field will be in equilibrium if the temperature and chemical potential of each component of the system is constant through out, i.e.

dT₁=0 and dμᵢ=0

Third Law of Thermodynamics ( The Law of zero entropy )

"Nernst" in 1906 proposed a general priniciple supported by series of experimental tests on problem of atomic heat at low temperatures. it was proposed as " The new heat theorem " and is called as third law of thermodyanmics.

Nernst statement

                       " The heat capacities of all solids tend to zero as the absolute zero of temperature is approached and   that the internal energies and entropies of all substances become equal there, approaching their common value asymptotically".

This law neither follows from first law or second law nor is totally a new law.

Other statement of Nernst:

                        " No entropy change takes place when pure crystalline solid reacts at absolute zero".

 Plank statement:

                        " The entropy of a solid or a liquid is zero at absolute zero of temperature".

Lewis and Randall statement 

                       "Every system has finite positive entropy, but at the absolute zero of temperature the entropy may become zero and does so become in the case of a pure crystalline substance".      

But this statement is confined to pure crystalline solids because theoretical argument and experimental evidence have shown that the entropy of solutions and super cooled liquids is not zero even at absolute zero.

For instance, ice always has residual entropy at absolute zero. It also doesn't apply to amorphous class of substances like glass etc.

Importance of third law of thermodynamics

• Third law is useful in explaining the nature of bodies in neighborhood of absolute zero.
• It permits the calculations of absolute values of entropy and physical interpretation of thermodynamic properties such as Helmholtz & Gibbs free energies etc.
• It can be conceived that as the temperature of system tends to absolute zero, its entropy tends to a constant value which is of pressure and state of aggregation etc. 

 "Nernst" formulated that "the entropy change in isothermal reversible process of condensed system approaches zero as temperature at which the process occurs approaches zero".

The principle of Barthelot states that "every chemical transformation which takes place with out the intervention of external energy tends towards the production of that substance or systems of substance which will give the greatest development of heat i.e that process is realized which is most exothermic.

       

All about Second Law of Thermodynamics?

Why second law of thermodynamics introduced? 

We know that some processes occur spontaneously but if we try to reverse the direction of process, the process do not occur spontaneously and further some external energy is required to move the given system away from state of equilibrium. 

The question is that "why such reversed processes do not occur spontaneously?" could not be answered by first law because the total energy of system would remain constant in the reversed process as it did in the orginal path and ther is no voilation of first law. Therfore there must be some other natural principle in addition to first law which determines the direction in which a process can take place in an isolated system. This principle is "second law of thermodynamics" .

Second law infers us that "the entropy of universe tends to maximum". 

Second law of thermodynamics in terms of entropy:

  "The entropy of an isolated system is fully conserved in every reversible process i.e. for every reversible process the sum of all changes in entropy taking place in an isolated system is zero. If the process is not a reversible one, then the sum of all changes in entropy taking place in an isolated system is greater than zero. In general we can say that in every process taking place in an isolated system the entropy of system either increases or remains constant."  

Condition for equilibrium of an isolated system

“If an isolated system is in such a state that its entropy is maximum, any change from that state would evidently lead to decrease in entropy and hence will not happen. Thus the necessary condition for equilibrium of an isolated system is that its "Entropy shall be maximum."

 Other forms of second law of thermodynamics

Kelvin-Planck statement : It is impossible to construct a device which, operating in a cycle has the sole effect of extracting heat from a single reservoir and performing equivalent amount of work.

Clausius statement : It is impossible for heat to flow from a cooler body to another hotter body  without the aid of external energy.

 "Study on heat engines is based on the above law"

What is first law of Thermodynamics?

When a definite amount of work is done a certain amount of heat is produced and vice versa.

It can be mathematically expressed as

W = JH

where 'J' is a constant called Mechanical equivalent of heat and 'H' is heat produced. 

But a true version of the law is stated as follows

 “When an amount of heat is supplied to a system a part of it is used in raising internal energy of the system and a part in doing the work.”

                                                      dQ = dU + dW 

 where

dQ - amount of heat supplied ;

  dU - change in internal energy ;

                                                            dW - change in work

 From the first law it could be inferred that it is impossible to derive any work without expenditure of an equivalent amount of energy in some other forms.

 According to first law "The energy of universe remains constant".

 For mathematical calculations it should be kept in mind that heat absorbed by the system should be taken as "positive" and rejected by the system should be taken as "negative".

 Let us now apply first law to some thermodynamic processes.

 i) In an isothermal change dU=0 and dQ = dW; thus in an isothermal change the quantiity of heat absorbed by  a perfect gas is transformed into work by the gas.

 ii) In an adiabatic process, dQ=0 and dU+dW=0 -> dU = - dW

  a) If the system is compressed, work is done on the gas and thus dW is taken as -ve.

                                           Hence  dU = -(-dW) = dW

   b) If the system expands adiabatically 'dW' is positive.

                                           Hence  dU=-(+dW) = -dW

iii) In an adiabatic compression, the decrease in volume is associated with increase in temperature and 

      increase in pressure.