Thermodynamics

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Thermodynamics

• The study of heat energy
• through random systems

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Laws of Thermodynamics

• 0th Law If TaTb and TbTc then TaTc
• 1st Law ?EWQ (Conservation of Energy)
• The increase in thermal energy of a system is
  determined by the heat of the system and the work
  added to it.
• The idea behind the Heat Engine

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Efficiency

• WnetQnetQhot-Qcold
• or EffWnet/Qhot1-(Qcold/Qhot)
• Ex A steam engine absorbs 1.98 x105J and expels
  1.49 x105J in each cycle. Assume that all the
  remaining energy is used to do work.
• a. What is the engines efficiency?
• b. How much work is done in each cycle?

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• a. Eff0.247
• b. W4.9 x104J

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• 2nd Law A system can be defined by its entropy.
  In a closed system entropy tends to increase.
• Entropy A measure of the disorder (randomness)
  of the system
• 3rd Law Temperature and Entropy are absolute
  scales.
• At some point no temperature or entropy exists.
• T?0 K and S?S0

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Heat Engines

• Engines allow heat energy to be transformed into
  work or mechanical energy.
• Work or Energy ?Heat increases
• no big deal. Friction does this.
• Heat ?Work or Energy
• is a big deal. Heat is easy to move around. You
  could just bring heat wherever you needed work
  done and Boom! you wouldnt have to do the
  work, a machine could.
• Work or Energy ?Heat decreases
• is also a big deal. Making food cold preserves
  it and allows it to be moved readily. Less
  spoilage means less disease.

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• Any heat engine works on the same properties.
• A hot reservoir is the source of the energy.
• Both words mean something. Hot means that there
  is plenty of heat energy and reservoir means that
  if heat is removed the temperature doesnt drop
  much.
• There is also a need for a cold reservoir.
  Again, both words mean something.
• Cold because it is at a lower temperature than
  the hot reservoir and reservoir because it must
  be large enough that you can dump heat into it
  without appreciably raising the temperature.

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What happens if we put a hot and cold reservoir
in contact? Thermal Equilibrium is not the
answer!

• Heat transfer (or flow) is the answer.
• Remember that these are reservoirs so it would
  take a long time for them to come into thermal
  equilibrium.
• This is great, but we dont get any work out of
  it.
• We need to steal some of the energy leaving the
  hot reservoir and make it do work for us.

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Heat Engine

• Stealing some energy to do work

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Types of Heat Engine

• Steam Engine

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Internal Combustion Engine
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Unfortunately we dont get an even trade!

• We lose energy to randomness/Entropy
• This is the 2nd Law of Thermodynamics
• Automobile engines are only about 15 efficient.
  That means for every 100J of heat energy, 15J
  worth of work is done on the piston and 85J of
  heat are discarded. Still, this is the source of
  energy for most of our transportation.

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• If a steam engine takes in 2.254 x 104 kJ of heat
  and gives up 1.915 x 104 kJ of heat to the
  exhaust, what is the engines efficiency?

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We can go backwards!

• This is a refrigerator or air conditioner

W
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This requires Energy/Work

• This is why your refrigerator must be plugged in.
• It is constantly dumping heat into your kitchen
• Due to the 2nd Law of Thermodynamics more heat is
  dumped than is removed
• If you left the refrigerator door open you would
  heat up the kitchen

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Perfect Heat Engine
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Efficiency

• WnetQnetQhot-Qcold
• or EffWnet/Qhot(1-Qcold/Qhot)
• Ex A steam engine absorbs 1.98 x105J and expels
  1.49 x105J in each cycle. Assume that all the
  remaining energy is used to do work.
• a. What is the engines efficiency?
• b. How much work is done in each cycle?

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• a. Eff0.247
• b. W4.9 x104J

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More 2nd Law

• Entropy
• Sentropy
• QHeat (Joules)
• TTemperature (In Kelvin)
• Entropy in a system must increase or at least
  stay the same!!!!!!!!!!!!

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• An engine has a hot reservoir at 1000 K and uses
  the atmosphere at 300 K as the cold reservoir.
  You take 2500 J from the hot reservoir to do 1900
  J of work.
• A. How much heat goes into the atmosphere?
• B. Is this engine possible? (Does the entropy
  increase?)

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• WnetQnetQhot-QcoldQcoldQhot-Wnet2500J -1900J
  600J
• EntropyEngine is not possible.
• What is the maximum amount of work we can take
  out?
• How much work is done?

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Carnot Engine

• Maximum efficiency
• In theory could run backwards
• All temperatures in Kelvin
• What is the efficiency of an ideal steam engine
  with steam at 685 K and exhaust at 298 K?
• What is Qcold ?

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Carnot Engine

• Maximum efficiency
• In theory could run backwards
• All temperatures in Kelvin
• What is the efficiency of an ideal steam engine
  with steam at 685 K and exhaust at 298 K?
• What is Qhot if Qcold is 450J?