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Introduction |
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Heat is a form of energy. Heat energy is also called thermal energy. When heat is given to a body, its temperature increases and when heat is removed from a body, its temperature decreases. |
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Kinetic Theory of Matter |
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This theory explains the physical properties of matter in terms of motion of its molecules. According to this theory, every substance (solid, liquid or gas) consists of a large number of minute particles called molecules. A molecule may be defined as the smallest particle of a substance that can exist in free state and has all the characteristics of the present substance. |
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Kinetic Theory of Gases |
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All the molecules of a gas are identical with respect to their shape and mass. The molecules of different gases are different. |
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Expression for Calculation |
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Consider an ideal gas enclosed in a cubical vessel of edge L. Take a corner of the vessel as the origin O and the X-,Y-, Z- axes along the edges (figure 1). Let A1 and A2 be the parallel faces, perpendicular to the X-axis. |
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Kinetic Energy and Temperature |
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Let us consider one-gram molecule (mole) of the gas. Let M and V be its mass and volume respectively. |
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Translational Kinetic Energy of Gas |
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The total translational energy of all the molecules of the gas |
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Kinetic Interpretation of Temperature |
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We know that a hotter body has greater internal energy than a similar colder body. Thus, higher temperature means higher internal energy and lower temperature means lower internal energy. |
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Deductions from Kinetic Theory |
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At a given temperature, the pressure of a given mass of a gas is inversely proportional to its volume. |
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Root Mean Square Speed in Terms of Temperature |
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We are now in a position to write the rms speed of the molecules in terms of the absolute temperature. |
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Maxwell's Speed Distribution Law |
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The rms speed of an oxygen molecule in a sample at 300 K is about 480m/s. This does not mean that the speed of each molecule is 480 m/s. Many of the molecules have speed less than 480m/s and many have speed more than 480m/s. Maxwell derived an equation giving the distribution of molecules in different speeds. |
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Degrees of Freedom and Molar Specific Heats |
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The total number of possible independent ways in which the position and configuration of a mechanical system may change, is the degrees of freedom of that system. |
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Work and Heat |
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In 1798, Count Rumford observed that the amount of heat produced is proportional to the amount of mechanical work done. Later, Dr. James Prescott Joule of Manchester, established a definite relation between the work done and the heat produced. It was shown that when a certain amount of mechanical work is done, an equivalent amount of heat is always produced. |
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Internal Energy |
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Internal energy is one of the most important concepts in thermodynamics. Energy changes in a body sliding with friction. Warming a body increases its internal energy and cooling the body decreases its internal energy. |
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The First Law of Thermodynamics |
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We have seen that heat is just a form of energy. A system can be given energy either by supplying heat to it (by placing it in contact with a hotter object) or by doing mechanical work on it. Consider an ideal gas in a cylindrical container, fitted with a piston as shown in the figure given below. |
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Heat Capacity |
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The heat capacity C of an object, is the proportionality constant between the heat Q that the object absorbs or loses and the resulting temperature change DT of the object. |
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Specific Heat |
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Two objects made of the same material, say marble, would have heat capacities proportional to their masses. It is therefore convenient to define a “heat capacity per unit mass” or specific heat that refers not to an object but to a unit mass of the material of the object. |
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Relation between Cp and Cv (Mayer's Formula) |
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Let us consider one mole of an ideal gas enclosed in a cylinder fitted with an airtight and frictionless piston. Let P, V and T be the pressure, volume and absolute temperature of the gas respectively. |
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Dulong and Petit Law (Specific Heat of Solids) |
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In 1819, two French physicists Dulong and Petit discovered that the average molar specific heat at constant pressure for all metals, except the very light ones, is approximately the same and equal to nearly 25 J mole-1 oC-1. |
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Thermodynamic Variables or Parameters |
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Thermodynamic variables are the quantities like pressure, volume and temperature, which help us to study the behavior of a thermodynamic system. There are some other thermodynamic variables such as entropy, internal energy, etc., but these thermodynamic variables can be expressed in terms of pressure, volume and temperature. |
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The Ideal Gas Equation |
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Another simple equation of state is the one for an ideal gas. The figure below shows an experimental setup to study the behavior of a gas. The cylinder has a movable piston to vary the volume, heating can vary the temperature, and we can pump the desired amount of any gas into the cylinder. |
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Kinds of Thermodynamic Processes |
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In this section, we describe four specific kinds of thermodynamic processes that often occur in practical situations. These can be summarized briefly as "no heat transfer" or adiabatic, "constant volume" or isochoric, "constant pressure" or isobaric, and "constant temperature" or isothermal. For some of these, we can use a simplified form of the first law of thermodynamics. |
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Carnot Engine |
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In 1824, a French scientist N.L.Sadi Carnot, suggested an idealized engine which is called the Carnot engine, which has an intimate relation with the second law of thermodynamics. |
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Refrigerator or Heat Pump |
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A heat engine takes heat from a hot body, converts a part of it into work and rejects the rest to a cold body. A refrigerator also known as a heat pump, does the reverse operation. |
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The Second Law of Thermodynamics |
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Experimental evidence suggests strongly that it is impossible to build a heat engine that converts heat completely to work (an engine with 100% thermal efficiency). |
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Heat Transfer Mechanisms |
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We know that there is a transfer of energy, as heat, between a system and its environment. Here we discuss how this transfer takes place. There are three mechanisms of heat transfer. They are conduction, convection and radiation. |
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Measurement of Thermal Conductivity of a Solid |
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Steam is passed into the steam chamber and a stream of water is maintained. The temperatures of all the four thermometers rise initially and ultimately become constant when the steady state is reached. The readings q1, q2, q3, and q4 are noted in steady state. |
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Kirchhoff's Law |
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A body that is a good radiator (or emitter) is also a good absorber. To understand this, suppose isotropic (i.e., equal in all directions) thermal radiation is incident on a body. |
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Stefan-Boltzmann Law |
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The energy of thermal radiation emitted per unit time by a black body of surface A. |
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Newton's Law of Cooling |
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The rate of loss of heat by a body is directly proportional to the temperature difference between the body and the surroundings, provided the difference is not very large. |
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Solar Constant (S) |
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The solar constant is defined as the amount of heat energy received per second per unit area by a perfect black body placed at the surface of the Earth with its surface being held perpendicular to the direction of the sun's rays. |
