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Thermodynamics - PowerPoint PPT Presentation

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Thermodynamics. The study of heat in motion. thermodynamic process - one in which heat is added to or taken away from a system. internal energy ( U) - defined by the system’s temperature, volume, and pressure.

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The study of heat in motion.


thermodynamic process- one in which heat is added to or taken away from a system.

  • internal energy(U)- defined by the system’s temperature, volume, and pressure.
  • first law of thermodynamics – the change in the internal energy of the system is equal to the sum of the heat added and the work done on the system.
  • second law of thermodynamics - heat will never flow from a cold object to a hot object, but only from a hot object to a cold object.
first law of thermodynamics

ΔU = Q + W

  • U is the change in internal energy of the system, Q is the heat added to the system, and W is the work added to the system (or done on the system).
  • Qis positive when it is added to the system and negative if it is taken out of the system.
  • Wis positive if it is added to the system and negative if it is done by the system. This means that the work done by an engine that uses heat is negative. Think of it as being negative because it is leaving the system.
    • output work is negative
    • input work is positive
First Law of Thermodynamics:
thermodynamic processes

Isobaric – the pressure stays the same


  • Adiabatic – there is no net heat transfer

ΔU = W

  • Isothermal – no change in temperature

Q = -W

  • Isochoric – the volume stays constant

ΔU = Q

Thermodynamic Processes
adiabatic processes

Work is done by the system at the expense of internal energy.

  • ΔQ = 0 ΔU = Q + W ΔU = W
  • Two scenarios:
    • The system is insulated so that heat can neither enter nor leave.
    • The process happens so fast that there is no time for heat to be transferred.
  • The following can happen:
    • A gas that is adiabatically expanded will lose internal energy (ΔU) and become warmer.
    • A gas that is adiabatically compressed will gain (ΔU) and become cooler.
Adiabatic Processes
isochoric processes

W = PΔV = P(0) = 0

ΔU = Q + W ΔU = Q + 0 ΔU = Q

Example: Pressure Cooker

Isochoric Processes
heat engines

Heat engines operate by converting heat into work.

  • Heat is added to the engine at a high temperature. Part of the heat is used to generate work and the rest of the heat is sent to some low temperature environment.
Heat Engines
the carnot cycle

Isothermal Expansion

  • Adiabatic Expansion
  • Isothermal Compression
  • Adiabatic Compression
The Carnot Cycle
second law of thermodynamics

Heat engines – output work will always be less than the input

  • Entropy – disorder must increase as time passes (so the amount of energy available in the universe to do useful work is constantly growing smaller)
  • Order and Disorder – a natural process takes place in a direction that increases the disorder in the universe
Second Law of Thermodynamics
perpetual motion machines

Device that is perfectly efficient.

    • Once placed in motion it would continue to operate forever with no additional energy input.
  • Prohibited by the second law of thermodynamics.
Perpetual Motion Machines