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Basic Ideas of Thermodynamics and Statistical Physics

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1 Basic Ideas of Thermodynamics and Statistical Physics
Thermal Physics: A subject to deal with the phenomenon involving energy transfer between bodies at different temperatures. Thermodynamics: A subject concerned with the relationship between heat and work What is heat? What is temperature? What is internal energy? What is energy transfer?

2 The nature of heat Heat can be thought of as the internal kinetic energy of the atoms and molecules that make up a substance. Being a form of energy, it is measured in the standard unit of Joule, but it is also commonly measured in the following units: calorie: this is the heat energy needed to raise 1 gm of water 1 oC. 1 calorie is equal to Joules. Calorie: this is a common unit to measure the energy content of food, with 1 Calorie = 1000 calories. BTU: this is a British Thermal Unit, still used as a rating on some furnaces, and is the heat energy needed to raise 1 pound of water 1 oF. 1 BTU = 252 calories = 1,054 Joules.

3 Temperature Heat refers to the total amount of heat energy in a substance - two liters of boiling water have more heat energy than one liter. Temperature, on the other hand, is a relative term, and refers to the average kinetic energy of the atoms and molecules in a substance. There are three main temperature scales used in the world - Celsius, Fahrenheit, and Kelvin. 0 Kelvin has a very deep physical significance: It is the temperature associated with empty space that is completely devoid of all motion and/or energy. In practice it is impossible to obtain, although one can get arbitrarily close. (The empty space between distance galaxies is not at absolute zero: it contains energy and an associated temperature of about 2.7 Kelvin.)

4 Internal Energy Total translational kinetic energy of the gas due to the random thermal motion of its molecules.---Mono atomic gas where no rotaional or vibrational energy is present In general: Internal energy is the total energy associated with the internal states of atoms and molecules composing the system.

5 Internal energy-2 Internal energy: The energy a substance has because of its temperature. Value of the internal energy is determined by the state of the system. So the internal energy function is called a state function.

6 Heat Capacity The heat capacity of a substance is a measure of how well the substance stores heat. Whenever we supply heat to a material, it will necessarily cause an increase in the material's temperature. The heat capacity is defined as the amount of heat required per unit increase in temperature. Thus, materials with large heat capacities, like water, hold heat well - their temperature won't rise much for a given amount of heat - whereas materials with small heat capacities, like copper, don't hold heat well - their temperature will rise significantly when heat is added

7 Heat capacity-2 If dQ amount of heat is added to a substance to change its temperature by dT, then dQ=CdT Specific heat: heat capacity per unit mass c=C/m Therefore, Q=mcT

8 Heat Transfer Heat flow is an energy transfer that takes place as a consequence of temperature difference only. There are three ways that heat may be transferred between substances at different temperatures - conduction, convection, and radiation.

9 Thermal science is macroscopic or microscopic
Macroscopic quantities like P, V & T characterizes thermal properties of gas. So we can say that the science of ‘Thermal Physics’ is understood in a macroscopic level. It can also be understood in a microscopic level. Example: Temperature of a gas is a measure of the average kinetic energy of the gas molecules. P, V & T commonly called ‘thermodynamic variables of the system’.

10 Students should read Proper definitions of Temperature, Heat and Internal energy. Difference between a Macroscopic point of view and microscopic point of view. Derivation of the formulae of vrms vaverage and Vmostprobable.

11 Thermal contact Two objects are in thermal contact with each other if energy exchange can occur between them in absence of macroscopic work done by one on the other.

12 Thermal Equilibrium It is a situation in which two objects in thermal contact with each other cease to have any net energy exchange due to a difference in their temperatures.

13 Zeroth law If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other. It is called zeroth law because it logically proceds the statements of the first and second laws of thermodynamics, but was not recognised as an important and fundamental law of nature untill after these other laws had been stated and named.

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