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PHYS102Lec2 - Chapter 10 Heat Dr Armen Kocharian...

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Chapter 10 Heat Dr. Armen Kocharian
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Thermodynamics Historical Background Thermodynamics and mechanics were considered to be separate branches Until about 1850 Experiments by James Joule and others showed a connection between them A connection was found between the transfer of energy by heat in thermal processes and the transfer of energy by work in mechanical processes The concept of energy was generalized to include internal energy The Law of Conservation of Energy emerged as a universal law of nature
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Internal Energy Internal energy is all the energy of a system that is associated with its microscopic components These components are its atoms and molecules The system is viewed from a reference frame at rest with respect to the center of mass of the system
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Internal Energy and Other Energies The kinetic energy due to its motion through space is not included Internal energy does include kinetic energies due to: Random translational motion Rotational motion Vibrational motion Internal energy also includes potential energy between molecules
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Heat Heat is defined as the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings The term heat will also be used to represent the amount of energy transferred by this method
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Changing Internal Energy Both heat and work can change the internal energy of a system The internal energy can be changed even when no energy is transferred by heat, but just by work Example, compressing gas with a piston Energy is transferred by work
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Units of Heat Historically, the calorie was the unit used for heat One calorie is the amount of energy transfer necessary to raise the temperature of 1 g of water from 14.5 o C to 15.5 o C The “Calorie” used for food is actually 1 kilocalorie In the US Customary system, the unit is a BTU (British Thermal Unit) One BTU is the amount of energy transfer necessary to raise the temperature of 1 lb of water from 63 o F to 64 o F The standard in the text is to use Joules
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Mechanical Equivalent of Heat Joule established the equivalence between mechanical energy and internal energy His experimental setup is shown at right The loss in potential energy associated with the blocks equals the work done by the paddle wheel on the water
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Mechanical Equivalent of Heat, cont Joule found that it took approximately 4.18 J of mechanical energy to raise the water 1 o C Later, more precise, measurements determined the amount of mechanical energy needed to raise the temperature of water from 14.5 o C to 15.5 o C 1 cal = 4.186 J This is known as the mechanical equivalent of heat
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Heat Capacity The heat capacity , C, of a particular sample is defined as the amount of energy needed to raise the temperature of that sample by 1 o C If energy Q produces a change of temperature of D T , then Q = C D T
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Specific Heat Specific heat
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