Tag: engines and cycles

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Questions Related to engines and cycles

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

A heat engine is supplied with 250 kJ/s of heat at a constant fixed temperature of $227^0C$; the heat is rejected at $27^0C$, the cycle is reversible, then what amount of heat is rejected?

  1. 24kJ/s

  2. 223kJ/s

  3. 150kJ/s

  4. none of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The temperature in kelvin scales are $T _1=273+27=300K,T _2=273+237=500K$

$T _1$is temperature of sink And$T _2$ is temperature of source hence by efficiency we get

$\eta=1-\dfrac{T _1}{T _2}=1-\dfrac{Q _1}{Q _2}$


$Q _1=\dfrac{T _1}{T _2}*Q _2=\dfrac{300}{500}*250=150kW$
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

An engine working on Carnot cycle rejects 40% of absorbed heat from the source, while the sink temperature is maintained at $27^0C$, then what is the source temperature (in $^0C$)?

  1. 477

  2. 346

  3. 564

  4. none of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
$\eta =\dfrac { 60 }{ 100 } $

${ T } _{ 2 }={ 27 }^{ o }C$
${ T } _{ 2 }={ 300 }^{ o }K$
${ T } _{ 1 }=?$

$\eta =\dfrac { { T } _{ 1 }-{ T } _{ 2 } }{ { T } _{ 1 } } $

Here, ${ T } _{ 1 }=Source\quad temperature$
${ T } _{ 2 }=sink\quad temperature$
$\eta =Efficiency$
$\eta =100-rejecion$

$\therefore \dfrac { 60 }{ 100 } =\dfrac { { T } _{ 1 }-{ T } _{ 2 } }{ { T } _{ 1 } } $

$\therefore 0.6=\dfrac { { T } _{ 1 }-300 }{ { T } _{ 1 } } $

$\therefore { T } _{ 1 }\left( 1-0.6 \right) =300$

${ T } _{ 1 }=\dfrac { 300 }{ 0.4 } =\dfrac { 3000 }{ 4 } ={ 750 }^{ o }K$

$\therefore { T } _{ 1 }={ \left( 750-273 \right)  }^{ o }C$
$\therefore { T } _{ 1 }={ 477 }^{ o }C$
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

One reversible heat engine operates between 1600 K and $T _2$ K, and another reversible heat engine operates between $T _2$ K and 400 K. If both the engines have the same heat input and output, then the temperature $T _2$ must be equal to:

  1. 600

  2. 800

  3. 650

  4. 675
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

${ T } _{ 1 }=1600K$


${ T } _{ 2 }={ T } _{ 2 }K$


${ T } _{ 1 }^{ 1 }={ T } _{ 2 }K$

${ T } _{ 2 }^{ 1 }=400K$

Same input and same output.Then the efficiency is same.
$\eta =\dfrac { { T } _{ 1 }-{ T } _{ 2 } }{ { T } _{ 1 } } $

$\therefore \dfrac { 1600-{ T } _{ 2 } }{ 1600 } =\dfrac { { T } _{ 2 }-400 }{ { T } _{ 2 } } $

$=1600{ T } _{ 2 }-{ T } _{ 2 }^{ 2 }=1600{ T } _{ 2 }-640000$

$\therefore { T } _{ 2 }^{ 2 }=640000$
$\therefore { T } _{ 2 }=800K$

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

The freezer in a refrigerator is located at the top section so that;

  1. The entire chamber of the refrigerator is cooled quickly due to convection

  2. The motor is not heated

  3. The heat gained from the environment is high

  4. The heat gained from the environment is low
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The freezer in a refrigerator is located at the top section so that the entire chamber of the refrigerator is cooled quickly due to convection.  
The correct option is A.