Tag: conversion of heat into work: heat engine and it's efficiency

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Questions Related to conversion of heat into work: heat engine and it's efficiency

Multiple choice 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

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

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

Multiple choice 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

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?

  1. 0.71

  2. 0.33

  3. 0.67

  4. 0.29

  5. 1.5
Reveal answer Fill a bubble to check yourself
B Correct answer
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       $\eta = \dfrac{W}{Q _H} = \dfrac{1000}{3000} = 0.33$

Multiple choice 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

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?

  1. 100%

  2. 75 %

  3. 45%

  4. 25%

  5. 15%
Reveal answer Fill a bubble to check yourself
D Correct answer
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.

Multiple choice 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

Find out the most efficient engine in the following

  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
Reveal answer Fill a bubble to check yourself
D Correct answer
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.

Multiple choice 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

Which of the following engines is more efficient?

  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
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

Multiple choice 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

The efficiency of a heat engine : 

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

Multiple choice 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

The earthen pots prepared by using clay only are generally

  1. Porous

  2. Non-porous

  3. Conductors of electricity

  4. Transparent
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

Multiple choice 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

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 

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

  2. $\eta $

  3. $ 2 \eta $

  4. $ 3 \eta $
Reveal answer Fill a bubble to check yourself
B Correct answer
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