Showing posts with label THERMODYNAMICS. Show all posts
Showing posts with label THERMODYNAMICS. Show all posts

Second Law of Thermodynamics

Kelvin-Planck statement: No process is possible whose sole result is the absorption of heat from a reservoir and the complete conversion of the heat into work.

Clausius statement : No process is possible whose sole result is the transfer of heat from a colder object to a hotter object.

A process is reversible if it can be reversed such that both the system and the surroundings return to their original states, with no other change anywhere else in the universe.

Spontaneous processes of nature are irreversible. The idealised reversible process is a quasi-static process with no dissipative factors such as friction, viscosity, etc.

Carnot engine is a reversible engine operating between two temperatures T1 (source) and T2 (sink). The Carnot cycle consists of two isothermal processes connected by two adiabatic processes.

No engine operating between two temperatures can have efficiency greater than that of the Carnot engine.

If Q > 0, heat is added to the system
If Q < 0, heat is removed to the system
If W > 0, Work is done by the system
If W < 0, Work is done on the system

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Heat Engines

Heat engine is a device by which a system is made to undergo a cyclic process that results in conversion of heat to work.

(1) It consists of a working substance–the system. For example, a mixture of fuel vapour and air in a gasoline or diesel engine or steam in a steam engine are the working substances.

(2) The working substance goes through a cycle consisting of several processes. In some of these processes, it absorbs a total amount of heat Q1 from an external reservoir at some high temperature T1.

(3) In some other processes of the cycle, the working substance releases a total amount of heat Q2 to an external reservoir at some lower temperature T2.

(4) The work done (W ) by the system in a cycle is transferred to the environment via some arrangement (e.g. the working substance may be in a cylinder with a moving piston that transfers mechanical energy to the wheels of a vehicle via a shaft).

The basic features of a heat engine are schematically represented in figure below.The cycle is repeated again and again to get useful work for some purpose.


If Q1 is the heat input i.e., the heat absorbed by the system in one complete cycle and W is the work done on the environment in a cycle. In a cycle, a certain amount of heat (Q2) may also be rejected to the environment. Then, according to the First Law of Thermodynamics, over one complete cycle

W = Q1 – Q2 and efficiency can be represented as

For Q2 = 0, η = 1, i.e., the engine will have 100% efficiency in converting heat into work. The First Law of Thermodynamics i.e., the energy conservation law does not rule out such an engine. But experience shows that such an ideal engine with η = 1 is never possible, even if we can eliminate various kinds of losses associated with actual heat engines.

There is a fundamental limit on the efficiency of a heat engine set by an independent principle of nature, called the Second Law of Thermodynamics.

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Heat and Internal Energy

Temperature is a marker of the ‘hotness’ of a body. It determines the direction of flow of heat when two bodies are placed in thermal contact. Heat flows from the body at a higher temperature to the one at lower temperature. The flow stops when the temperatures equalise; the two bodies are then in thermal equilibrium.

Internal energy is simply the sum of the kinetic energies and potential energies of these molecules. Internal energy is thus, the sum of molecular kinetic and potential energies in the frame of reference relative to which the centre of mass of the system is at rest. Thus, it includes only the (disordered) energy associated with the random motion of molecules of the system.and is denoted by U.

internal energy is depends only on the state of the system, not on how that state was achieved. Internal energy U of a system is an example of a thermodynamic ‘state variable’.Its value depends only on the given state of the system, not on history i.e. not on the ‘path’ taken to arrive
at that state. Thus, the internal energy of a given mass of gas depends on its state described by specific values of pressure, volume and temperature.

It does not depend on how this state of the gas came about. Pressure, volume, temperature, and internal energy are thermodynamic state variables of the system .The internal energy of a gas is just the sum of kinetic energies associated with various random motions of its molecules.It also includes rotational and vibrational motion of the molecules.

Heat and work are two different modes of altering the state of a thermodynamic system and changing its internal energy. Heat is certainly energy, but it is the energy in transit.Heat and work in thermodynamics are not state variables. They are modes of energy transfer to a system
resulting in change in its internal energy.

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Zeroth Law of Thermodynamics

This is the most fundamental law of thermodynamics and unfortunately invented after first law of thermodynamics.To say it is more basic than first law number zero is allotted to it . The number is about history of physics and community is ready to accept the truth and change accordingly.

Zeroth Law of Thermodynamics states that ‘two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other’.

The Zeroth Law clearly suggests that when two systems A and B, are in thermal equilibrium, there must be a physical quantity that has the same value for both. This thermodynamic variable whose value is equal for two systems in thermal equilibrium is called temperature (T ).

Thus, if A and B are separately in equilibrium with C, TA = TC and TB = TC. This implies that TA = TB i.e. the systems A and B are also in thermal equilibrium.

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Thermodynamics and Heat Introduction

Thermodynamics is a branch of Physics deals with transfer of heat energy into different forms of energies and its applications.

Heat was a form of energy and the experiment demonstrated conversion of energy from one form to another–from work to heat.

Thermodynamics is the branch of physics that deals with the concepts of heat and temperature and the inter conversion of heat and other forms of energy. Thermodynamics is a macroscopic science. It deals with bulk systems and does not go into the molecular constitution of matter. Thermodynamic description involves relatively few macroscopic variables of the system, which are suggested by common sense and can be usually measured directly.

Thermodynamic description of a gas, on the other hand, avoids the molecular description altogether. The state of a gas in thermodynamics is specified by macroscopic variables such as pressure, volume, temperature, mass and composition that are felt by our sense perceptions and are measurable.

In mechanics, our interest is in the motion of particles or bodies under the action of forces and torques. Thermodynamics is not concerned with the motion of the system as a whole. It is concerned with the internal macroscopic state of the body.

When a bullet is fired from a gun, what changes is the mechanical state of the bullet (its kinetic energy, in particular), not its temperature. When the bullet pierces a wood and stops, the kinetic
energy of the bullet gets converted into heat, changing the temperature of the bullet and the surrounding layers of wood. Temperature is related to the energy of the internal (disordered) motion of the bullet, not to the motion of the bullet as a whole.

The state of a system is an equilibrium state if the macroscopic variables that characterize the system do not change in time. For example, a gas inside a closed rigid container, completely insulated from its surroundings, with fixed values of pressure, volume, temperature, mass and composition that do not change with time, is in a state of thermodynamic equilibrium.

Other useful topics in physics




Cyclic Process,Reversible Process and Work Done Graphs


Degree of freedom and Law of the Equipartisien energy


Heat transfer by radiation
Heat transfer by convection
Heat transfer and conduction
Heat and Temperature
Change of State


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