Packet 3.4 Thermodynamics

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Packet 3.4Thermodynamics

• Internal Energy
• W P?V
• 1st Law of Thermodynamics ?U Q W
• Define Adiabatic, isothermal, isobaric
  isochoric and show these on a P-V diagram
• Irreversibility disorder
• Entropy is a measure of disorder
• State 2nd Law of Thermodynamics
• Heat engine efficiency, ? W/Qh
• Carnot Engine
• Energy Degradation

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Internal Energy

• Is the total potential and kinetic energy of the
  molecules in a substance.
• Potential energy is associated with
  intermolecular forces.
• Kinetic energy includes both translational and
  rotational motion.
• When we consider an ideal gas, the intermolecular
  forces are assumed to be zero!
• Internal energy of a gas comes only from the
  random kinetic energy of the atom of the gas.

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Ideal Gas Internal Energy

• Ek 1/2mv2 3/2kT so, U 3/2 NkT
• U 3/2nRT
• ? U 3/2 nR ? T

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Ideal Gas Internal Energy

• What is the internal energy of 30 moles of oxygen
  gas at room temperature?
• U 3/2nRT

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Ideal Gas Internal Energy

• If the room were moving at a high velocity would
  that mean the internal energy of the gas would be
  greater?

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Work done on or by a gas

• Imagine compressing a gas by exerting a force on
  the piston from the outside.
• Consider heating the piston and it expands to
  perform work.

• W F x D F PA
• W P (A x D)
• W P ? V

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Example

• A gas is compressed at constant pressure 2.00 x
  105 Pa from a volume of 2.00 m3 to a volume of
  0.500 m3.
• What is the work done on the gas.
• If the temperature initially was 40 C what is
  the final temperature of the gas?

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Thermodynamic Processes

• CLICK HERE TO GO OVER EACH PROCESS!!

Isochoric Isobaric
Isothermal
Const. Volume Const. Pressure
Const. Temp

• Remember! Area under curve is Work Done!

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Adiabatic Expansion

• Rapid expansion or compression of a gas.
• No Heat (Q) can flow in or out of the system. ?Q
  0
• Any work done equals a direct change in internal
  energy. ?U ?W

• Bottle Rockets
• Diesel Engines

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Figure 18-11Adiabatic Heating
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The First Law of Thermodynamics (FLT)
-- Heat and work are forms of energy transfer and
energy is conserved.
?U Q - Wby
change in total internal energy
work done by the system
heat added to system
State Function Process Functions
or ?U Q Won
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"Process Problems"
For which process is W the largest?
smallest?For which process is Q the largest?
smallest?
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Physics Joke

• Once all the scientists die and go to heaven.
  They decide to play hide-n-seek and Einstein goes
  first. Einstein counts up to 100 and then start
  searching. Everyone starts hiding except Newton.
  Newton just draws a square of 1 meter and stands
  in it, right in front of Einstein. Einsteins
  counting ....97,98,99,100, opens his eyes and
  finds Newton standing in front. Einstein says
  "Newtons out, Newtons out." Newton denies and
  says I am not out. He claims that he is not
  Newton. All the scientists come out and he proves
  that he is not Newton. How??

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His proof

• Newton says I am standing in a square of area 1m
  square.. That means I am Newton per meter
  square.. Hence I am Pascal. Since newton per
  meter square Pascal

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Conceptual Checkpoint 18-2Which is the adiabatic
curve?
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The second law of thermodynamics

• When objects of different temperatures are
  brought into thermal contact, the spontaneous
  flow of heat that results is always from the high
  temperature object to the low temperature object.

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Heat Engines

• Energy goes in
• Useful Work taken out
• Some gets wasted

• Max Efficiency TH Tc/ Th x 100

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The 2nd Law of Thermodynamics

• The second law of thermodynamics deals with the
  limitations imposed on heat engines that is on
  devices whose aim is to covert heat (disordered
  energy) into mechanical energy (ordered energy).

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The 2nd Law of Thermodynamics

• The Entropy of an isolated system never
  decreases.
• It is impossible for heat to (spontaneously) flow
  from a cold to a hot object.
• It is impossible for a heat engine working in a
  cycle to absorb heat and perform an equal amount
  of work.
• The most efficient heat engine operating between
  two given temperatures is the Carnot Engine.

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Entropy

• Entropy like Internal energy is a State Function!
• Entropy Is a measure of the disorder of a system.
• ?S ?Q/T
• If ?Q gt 0 entropy increases
• If ?Q lt 0 entropy decreases.

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Philosophy

• Three Laws of Thermodynamics (paraphrased) First
  Law You can't get anything without working for
  it. Second Law The most you can accomplish by
  work is to break even. Third Law You can't break
  even.