Tag: heat engine: second law of thermodynamics

Questions Related to heat engine: second law of thermodynamics

In a heat engine, heat energy is converted into mechanical energy. 

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. True

  2. False

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Correct Option: A
Explanation:

The correct answer is option(A).

In thermodynamics, a heat engine is a system that converts heat or thermal energy—and chemical energy—to mechanical energy, which can then be used to do mechanical work.

How many dead centres are there in one cycle of steam engine ?

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. $4$

  2. $3$

  3. $2$

  4. $1$

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Correct Option: C
Explanation:

There are two dead centers in one cycle of the steam engine:

  • Top dead center
  • Bottom dead center
Hence option C is correct.

For a particular heat engine, 3,000 J of energy goes in at 700 K and 2000 J comes out at 200 K. The rest of the energy is used work.
What is the actual efficiency of this engine?

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. 0.71

  2. 0.33

  3. 0.67

  4. 0.29

  5. 1.5

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Correct Option: B
Explanation:

Amount of heat goes in the engine        $Q _H = 3000$ J

Amount of heat rejected by the engine      $Q _R = 2000$ J
Thus work done by the engine      $W = Q _H - Q _R =3000 - 2000 = 1000$ J

Actual efficiency of heat engine       $\eta = \dfrac{W}{Q _H} = \dfrac{1000}{3000} = 0.33$

A 60J of heat is added to the system, resulting in 15J of work being done by the system. The remaining 45J of heat is released. Find out the efficiency of the system?

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. 100%

  2. 75 %

  3. 45%

  4. 25%

  5. 15%

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Correct Option: D
Explanation:

Amount of heat absorbed       $Q _H = 60$ J

Net work done by the system     $W= 15$ J
Thus efficiency of the system          $\eta = \dfrac{W}{Q _H} = \dfrac{15}{60}  =0.25$
Thus the system is $25$%  efficient.

Find out the most efficient engine in the following

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. An engine converts 80 KJ of heat energy into 20 KJ of work

  2. An engine converts 50 KJ of heat energy into 15 KJ of work

  3. An engine converts 30 KJ of heat energy into 6 KJ of work

  4. An engine converts 60 KJ of heat energy into 24 KJ of work

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Correct Option: D
Explanation:

Efficiency of heat engine= $\frac{work done}{heat input}$ Going by the above options efficiency is maximum in option D, and is equal to 40 percent.

Which of the following engines is more efficient?

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. Heat utilised - 80 kilojoules , work done - 32 kilojoules

  2. Heat utilised - 60 kilojoules , work done - 12 kilojoules

  3. Heat utilised - 50 kilojoules , work done - 25 kilojoules

  4. Heat utilised - 90 kilojoules , work done - 27 kilojoules

Show answer
Correct Option: C
Explanation:

Efficiency of engine   $\eta = \dfrac{W}{Q _H}$  

where $W$ is work done and $Q$ is heat taken from source (Heat utilised)
(A) :  $\eta _A = \dfrac{32}{80} = 0.4$
(B) :  $\eta _B = \dfrac{12}{60} = 0.2$
(C) :  $\eta _C = \dfrac{25}{50} = 0.5$
(D) :  $\eta _D = \dfrac{27}{90} = 0.3$
Hence engine C is the most efficient.

The efficiency of a heat engine : 

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. is independent of the temperature of the source and the sink

  2. is independent of the working substance

  3. can be 100%

  4. is not affected by the thermal capacity of the source or the sink

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Correct Option: B
Explanation:

Efficiency is defined as work done per unit of heat consumed . It doesn't depend on the substance on which we work.

The earthen pots prepared by using clay only are generally

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. Porous

  2. Non-porous

  3. Conductors of electricity

  4. Transparent

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Correct Option: A
Explanation:

The earthen pots prepared by using clay only are generally porous to facilitate cooling process.This works through evaporative cooling. Capillary action causes water to seep and evaporate from the mini-pores in the pot, taking the heat from the water inside, thus making the water inside cooler than the outside temperature.

An ideal heat engine working between temperatures $T _1$ and $T _2$ has an efficiency $\eta $ . The new efficiency if the temperatures of both the source and sink are doubled, will be 

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. $\frac{\eta }{2}$

  2. $\eta $

  3. $ 2 \eta $

  4. $ 3 \eta $

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Correct Option: B
Explanation:
Efficiency of heat engine $\eta=\dfrac{output}{input}=\dfrac{{T} _{1}-{T} _{2}}{{T} _{1}}$-------(a)
Now new efficiency of heat engine when sin and source temperature is doubled${\eta} _{new}=\dfrac{2{T} _{1}-2{T} _{2}}{2{T} _{1}}$
${\eta} _{new}=2\dfrac{{T} _{1}-{T} _{2}}{2{T} _{1}}=\eta$ from a
  

The thermal efficiency of a heat engine for which the work output is $2500 J/cycle$ and the heat input is $10000 J/cycle$, is:

physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

  1. $75%$

  2. $40%$

  3. $25%$

  4. $0.25%$

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Correct Option: D