Tag: physics

Questions Related to physics

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?

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

  1. 24kJ/s

  2. 223kJ/s

  3. 150kJ/s

  4. none of the above

Show answer
Correct Option: C
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

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


$Q _1=\dfrac{T _1}{T _2}*Q _2=\dfrac{300}{500}*250=150kW$

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 $^0C$)?

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

  1. 477

  2. 346

  3. 564

  4. none of the above

Show answer
Correct Option: A
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 } } $

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

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

$\therefore { T } _{ 1 }={ \left( 750-273 \right)  }^{ o }C$
$\therefore { T } _{ 1 }={ 477 }^{ o }C$

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:

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

  1. 600

  2. 800

  3. 650

  4. 675

Show answer
Correct Option: B
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 } } $

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

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

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

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

  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

Show answer
Correct Option: A
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.

If the door of a refrigerator is kept open, the room in which the refrigerator is kept 

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

  1. gets cooled

  2. gets heated

  3. neither gets cooled nor gets heated

  4. gets cooled or heated depending on the initial temperature of the room

Show answer
Correct Option: C

How can resolving power of the instrument be increased?

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. use UV light

  2. immerse in oil

  3. use IR light

  4. use one more lens.

Show answer
Correct Option: A,B
Explanation:

Resolving power for the instrument is found to be $\dfrac{\mu\sin\theta}{0.61\lambda}$ , UV light has short wavelength, hence higher resolving power. Oil is optically denser than air, that is, its $\mu$ is greater than that of air. Thus immersing in oil would increase the resolving power.

The ability of an optical instruments to show the images of two adjacent point objects as separate is called :

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. dispersive power

  2. magnifying power

  3. resolving power

  4. none of these

Show answer
Correct Option: C
Explanation:

By definition, resolving power of an optical instrument is its ability to show two closely adjacent point (closely spaced) as distinct as possible.

Two lenses of focal lengths $+ 100 cm$ and $+ 5 cm$ are used to prepare an astronomical telescope. The minimum tube length will be : (final image is at $\displaystyle \infty $)

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. $95 cm$

  2. $100 cm$

  3. $105 cm$

  4. $500 cm$

Show answer
Correct Option: C
Explanation:

the length of telescope =focal  length  of  object $(-f _0)$ +focal  length  of  eyepiece  $(f _e)$$=100+5=105cm$

In optical instruments, the lenses are used to form images by :

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. Reflection

  2. Refraction

  3. Dispersion

  4. Scattering

Show answer
Correct Option: B
Explanation:

In optical instruments, the lenses are used to form images by Refraction.

In which of the following the final image is erect? 

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. Compound microscope

  2. Astronomical telescope

  3. Simple microscope

  4. All of the above

Show answer
Correct Option: C
Explanation:

The image formed by the Compound microscope and Astronomical telescope is inverted,but in case of Simple microscope it form erect image.