chapter11 - Chapter 11 Energy in Thermal Processes Energy...

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Chapter 11 Energy in Thermal Processes
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Energy Transfer When two objects of different temperatures are placed in thermal contact, the temperature of the warmer decreases and the temperature of the cooler increases The energy exchange ceases when the objects reach thermal equilibrium The concept of energy was broadened from just mechanical to include internal Made Conservation of Energy a universal law of nature
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Heat Compared to Internal Energy Important to distinguish between them They are not interchangeable They mean very different things when used in physics
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Internal Energy Internal Energy , U, is the energy associated with the microscopic components of the system Includes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the atoms or molecules Also includes any potential energy bonding the particles together
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Heat Heat is the transfer of energy between a system and its environment because of a temperature difference between them The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment
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Units of Heat Calorie An historical unit, before the connection between thermodynamics and mechanics was recognized A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C . A Calorie (food calorie) is 1000 cal
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Units of Heat, cont. US Customary Unit BTU BTU stands for British Thermal Unit A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F 1 cal = 4.186 J This is called the Mechanical Equivalent of Heat
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James Prescott Joule 1818 1889 British physicist Conservation of Energy Relationship between heat and other forms of energy transfer
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Specific Heat Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1° C The specific heat, c, of a substance is a measure of this amount T m Q c
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Units of Specific Heat SI units J / kg °C Historical units cal / g °C
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Heat and Specific Heat Q = m c ΔT ΔT is always the final temperature minus the initial temperature When the temperature increases, ΔT and ΔQ are considered to be positive and energy flows into the system When the temperature decreases, ΔT and ΔQ are considered to be negative and energy flows out of the system
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A Consequence of Different Specific Heats Water has a high specific heat compared to land On a hot day, the air above the land warms faster The warmer air flows upward and cooler air moves toward the beach
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Calorimeter One technique for determining the specific heat of a substance A calorimeter is a vessel that is a good insulator which allows a thermal equilibrium to be achieved between substances without any energy loss to the environment
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chapter11 - Chapter 11 Energy in Thermal Processes Energy...

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