Second Law of Thermodynamics Engines and Refrigerators

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Second Law of ThermodynamicsEngines and
Refrigerators
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Heat Engine

• Any device that transforms heat partly into work
  or mechanical energy
• working substance matter inside the engine
  which undergoes inflow and outflow of heat,
  expansion and compression, and sometimes phase
  change

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how it works

• Working substance absorbs heat from the hot
  reservoir
• Performs some mechanical work
• Discards the remaining energy in the form of heat
  into the cold reservoir
• cyclic process

Young
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1st law applied to WS

• QHgt0 WS absorbs heat from HR
• QClt0 WS discards heat into CR
• Wgt0 WS performs mechanical work
• ?U0 after one cycle

1st law Q ?U W Qnet W QH QC W QH
- QC W
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thermal efficiency

• e W/QH
• (QH-QC)/QH
• 1 (QC/QH)

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Refrigerator

• takes heat from a cold place and gives of heat to
  a warmer place
• requires a net input of mechanical work
• working substance refrigerant fluid
• cold reservoir inside of refrigerator
• hot reservoir outside of refrigerator

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how it works

• fluid absorbs heat from the cold reservoir
• work is done on the fluid
• energy from heat transfer and work done is
  discarded into the hot reservoir
• cyclic process

Young
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1. the fluid in the evaporator coil is colder than
   the inside of the ref, so it absorbs heat
2. the compressor takes in fluid and compresses it
   adiabatically (work is done on the fluid)
3. fluid is delivered to condenser at high pressure,
   fluid temperature is higher than that of
   surrounding air, fluid gives of heat and
   condenses
4. fluid expands adiabatically (expansion valve)
   into the evaporator and cools considerably

Young
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1st law applied to fluid

• QCgt0 fluid absorbs heat from CR
• QHlt0 fluid discards heat into HR
• Wlt0 work is done on fluid
• ?U0 after one cycle

1st law Q ?U W Qnet W QH QC
W -QH QC -W QC W QH
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coefficient of performance

• K QC/W
• QC/(QH-QC)

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• A heat engine takes in 1200 J of heat from the
  high-temperature heat source in each cycle and
  does 400 J of work in each cycle. How much heat
  is released into the environment in each cycle?
• 400 J
• 800 J
• 1200 J
• 1600 J

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• A heat engine takes in 1200 J of heat from the
  high-temperature heat source in each cycle and
  does 400 J of work in each cycle. What is the
  efficiency of this engine?
• ¼
• 1/3
• 3
• 4

13

• In one cycle, a heat engine takes in 900 J of
  heat from a high-temperature reservoir and
  releases 600 J of heat to a lower-temperature
  reservoir. How much work is done by the engine in
  each cycle?
• 300 J
• 600 J
• 900 J
• 1500 J

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• In one cycle, a heat engine takes in 900 J of
  heat from a high-temperature reservoir and
  releases 600 J of heat to a lower-temperature
  reservoir. What is its efficiency?
• 3/9
• 6/9
• 3/15
• 6/15

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• In one cycle a heat engine does 400 J of work and
  releases 500 J of heat to a lower-temperature
  reservoir. How much heat does it take in from the
  higher-temperature reservoir?
• 100 J
• 400 J
• 500 J
• 900 J

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• In one cycle a heat engine does 400 J of work and
  releases 500 J of heat to a lower-temperature
  reservoir. What is the efficiency of the engine?
• 4/9
• 4/5
• 5/9
• 1

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• In one cycle, a heat engine takes in 1000 J of
  heat from a high-temperature reservoir, releases
  600 J of heat to a lower-temperature reservoir,
  and does 400 J of work. What is its efficiency?
• ¼
• 1/3
• 2/5
• 3/5

18

• A heat pump takes in 300 J of heat from a
  low-temperature reservoir in each cycle and uses
  150 J of work per cycle to move the heat to a
  higher-temperature reservoir. How much heat is
  released to the higher-temperature reservoir in
  each cycle?
• 150 J
• 300 J
• 450 J
• 600 J

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• A heat pump takes in 300 J of heat from a
  low-temperature reservoir in each cycle and uses
  150 J of work per cycle to move the heat to a
  higher-temperature reservoir. What is the
  coefficient of performance (COP)?
• 1/3
• 1/2
• 2
• 3

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• In each cycle of its operation, a refrigerator
  removes 18 J of heat from the inside of the
  refrigerator and releases 30 J of heat into the
  room. How much work per cycle is required to
  operate this refrigerator?
• 12 J
• 18 J
• 30 J
• 48 J

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• In each cycle of its operation, a refrigerator
  removes 18 J of heat from the inside of the
  refrigerator and releases 30 J of heat into the
  room. What is the COP of this refrigerator?
• 18/12
• 30/12
• 12/30
• 18/30