Heat Transfer and the 2nd Law of Thermodynamics

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Heat Transfer and the2nd Law of Thermodynamics
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Warm Up 1

• 10 A sample liquid is cooled from 150C to 30C,
  causing the liquid to change into a solid. Which
  of the following has occurred?
• F Chemical composition has changed.
• G A physical change has occurred.
• H A new compound has formed.
• J Evaporation of a solvent has occurred.

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Heat Transfer

• Heat flows from _______________.
• To reverse the process, work must be done.

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Heat Transfer

• The amount of heat absorbed (Q) is equal to the
  amount of heat released (-Q).
• Q - Q
• mc?T mc-(?T) ( -Tf Ti)
• mc (Tf-Ti) mc (Ti Tf )
• The final temperature for both will be the same.
  (Thermal Equilibrium)

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Heat Transfer

• Positive Q value if
• -energy is absorbed, added, required, needed
• Negative Q value if
• - energy is released, removed

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Heat Transfer

• Example
• A 50 g metal bolt (cbolt 899 J/kg oC) is dropped
  into a 150 ml of water. The initial temperature
  of the water is 21 o C and they both reached an
  equilibrium temperature of 25 o C. What is the
  initial temperature of the bolt?
• Remember energy absorbed by the water is equal
  to energy released by the metal bolt.

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Solution

• Q water - Q bolt (released energy)
• mwcw (Tf - Ti) mbcb (Ti-Tf)
• (0.15 kg)(4186 J/kgoC)(25 oC 21 oC) (0.05
  kg)(899 J/kg oC)(Ti - 25 oC)
• (0.15 kg)(4186 J/kgoC)(4 oC) (0.05
  kg)(899 J/kg oC) (Ti - 25 oC)
• 
  2511.6 J (44.95 J/ oC) (Ti - 25 oC)
• 2511.6
  J (Ti - 25 oC)
• (44.95 J/ oC)
• 55.88 oC Ti - 25 oC
• 55.88 oC 25 oC Ti
• 80.88 oC Ti

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2nd Law of Thermodynamics

• The Law of Increased Entropy
• In a closed system the entropy of the system will
  either remain constant or increase.

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Law of Heat Transfer and Entropy

• Entropy disorderliness of things (gas particles
  have more entropy than solids and liquids)
• Heat flows from higher temperature to lower
  temperature.
• To reverse the process, work will be done.

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Law of Heat Transfer and Entropy

• Examples
• Ice cream easily melts but to refreeze it you
  need a heat pump (fridge) to do it.
• Coffee on the table will cool down eventually at
  room temperature. What kind of work is needed to
  warm it up?

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Entropy

• By doing work on a system you can decrease the
  systems entropy because the work you are doing
  is creating order
• It takes a lot more work to put your room in
  order than to mess it up.
• You will need an AC pump to cool down (reduce the
  entropy) of a room in Houston during summer time.

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Light bulb moment

• To reverse the process of Heat Transfer work must
  be done to do it.
• To reverse or to minimize the entropy
  (disorderliness), work must be done.

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Exit Ticket

• If energy cannot be created and heat only flows
  from high to low, what would happen to the
  universe far, far, far in the future?

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Universal Thermal Equilibrium

• Heat Death
• Eventually everything in the universe will be at
  thermal equilibrium as energy is dissipated
  (lost).
• If everything is at the same temperature, work
  will be impossible.

Work