15.1 The First Law of Thermodynamics

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• 15.1 The First Law of Thermodynamics
• A systems internal energy can be changed by
  doing work or by the addition/removal of heat
• ?U Q - W
• W is negative if work is done on the system
• Compression of the gas
• What is the state of the system?
• Described by P, V, T, m, U

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• 15.2 Thermodynamic Processes and the First Law
• Isothermal T constant ? ?U 0 ? W Q
• Adiabatic Q 0 ? ?U -W

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• 15.2 Thermodynamic Processes and the First Law
• If pressure is constant then
• W Fd PAd P ?V

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• 15.2 Thermodynamic Processes and the First Law
• The total work done during a process is equal to
  the area under the PV diagram

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• 15.4 The Second Law of Thermodynamics
• Heat can flow spontaneously only from a hot
  object to a cold object.
• A reversible process is one that is always in
  equilibrium and can return to its initial
  conditions along the same path
• Most natural processes are irreversible
• Sets an upper limit on efficiency of heat engines

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• 15.5 Heat Engines
• Heat engines convert U into other useful forms of
  energy mechanical, electrical,
• ?Ucycle 0 ? QH W QL
• Automobile engines

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• 15.5 Heat Engines
• The efficiency of a heat engine is
• Carnot (ideal) engine
• Reversible processes
• Too slow for real engines

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• 15.6 Refrigerators, Air Conditioners and Heat
  Pumps
• A heat engine in reverse.

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• 15.6 Refrigerators, Air Conditioners and Heat
  Pumps

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2. (a) The work done by a gas at constant
pressure is found from Eq. 15-3.

(b) The change in internal energy is calculated
from the first law of thermodynamics
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26. Find the exhaust temperature from the
original Carnot efficiency, and then recalculate
the intake temperature for the new Carnot
efficiency, using the same exhaust temperature.