Chapt 12 - Thermodynamics Chapter 12 Thermodynamics...

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1 Thermodynamics Chapter 12 Thermodynamics c Thermodynamics is a field that describes systems with very many particles c Keeping track of this many particles with Newton’s Laws is impossible c Therefore, we use macroscopic variables such as pressure and temperature to describe them Thermodynamic Systems c Definite quantity of matter enclosed by boundaries or surfaces c We consider the exchange of energy between systems c Transfer of Heat c Performance of work c There are thermally isolated systems. i.e. well insulated c Work can still be done on them however.
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2 State Variables c State variables c Pressure c Volume c Temperature c Internal Energy c A macroscopic state of an isolated system can be specified only if the system is in equilibrium* *All parts of the system have the same temperature Equations of State c Equations of State describes conditions of thermodynamic systems. c Example: PV = nRT c For a quantity of an ideal gas, knowledge of its State Variables specifies a definite state. c Requires that the system be in thermal equilibrium and at uniform temperature Graphing States of a System A point on a p-V diagram specifies a thermodynamic state of a system c Much like a point on an x-y graph specifies the position state of a particle
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3 Processes c A process is any change in the state of a system. i.e. a change of thermodynamic coordinates, P, V, or T. c A reversible process is one in which every state along some path is an equilibrium state c It can be reversed Irreversible and Reversible Processes c An irreversible process is said to occur when a system goes rapidly and unpredictably from one thermodynamic state to another. c The intermediate states are not equilibrium states c An explosion is an example c Most natural processes are irreversible c Friction c Heat flowing spontaneously from a hot reservoir to a cold reservoir Paths of Reversible and Irreversible Processes c If it goes rapidly from (1 b 2), the process is irreversible, because we don’t know the path. c If a gas is slowly taken through many closely spaced equilibrium states, (3 b 4) the process is reversible i.e. from (4 b 3)
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4 Conductive Heat Flow on the Atomic Scale c Microscopically, energy is transferred from a hot object to a colder one by mechanical collisions of atoms. c This energy can raise the temperature of the cold object. c It can also cause the colder system to do work The First Law of Thermodynamics Q = U + W c The heat energy transferred in or out of a system is equal to the change in internal energy of the system plus the work done by that system c This is a statement of the conservation of energy given in thermodynamic terms Sign Conventions c Q is the Heat Energy transferred c Positive if energy is transferred to the system c Negative if energy is transferred out of the system c U is the Internal Energy of the system c Positive if the temperature increases c Negative if the temperature decreases c Recal : U = (3/2)k B T c W is the Work done by the system c Positive if done by the system c Negative if done on the system
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Chapt 12 - Thermodynamics Chapter 12 Thermodynamics...

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