Tag: heat engine: second law of thermodynamics

Questions Related to heat engine: second law of thermodynamics

An engine has an efficiency of 0.25 when temperature of sink is reduced by 58C,if its efficiency is doubled, then the temperature of the source 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. 150K

  2. 222K

  3. 242K

  4. 232K

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Correct Option: D
Explanation:

Let us consider $T _1=$ temperature of the source

$T _2=$ temperature of the sink
The efficiency of the engine,
$0.25=1-\dfrac{T _2}{T _1}$
$\dfrac{T _2}{T _1}=0.75$
$T _2=0.75T _1$. . . . . . . . .(1)
When the temperature of the sink is reduced by $58^0C$, then the efficiency is double,
$2\times 0.25=1-\dfrac{T _2-58}{T _1}$
$\dfrac{T _2-58}{T _1}=0.5$
$T _2-58=0.5T _1$
$0.75T _1-58=0.5T _1$ .........(from equation 1)
$0.25T _1-58=0$
$0.25T _1=58$
$T _1=\dfrac{58}{0.25}$
$T _1=232 K$
The correct option is D.

Consider a new system of units in which c (speed of light in vacuum), h(Planck's constant) and G (gravitational constant) are taken as fundamental units. Which of the following would correctly represent mass in this new system?

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. $\sqrt{\dfrac{hc}{G}}$

  2. $\sqrt{\dfrac{Gc}{h}}$

  3. $\sqrt{\dfrac{hG}{c}}$

  4. $\sqrt{hGc}$

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Correct Option: A
Explanation:

Consideration anew system of units in which speed of light in vaccum$=e$Plank's constant$=h$

Gravitational constant$=G$
Mass in the new system
$F=\cfrac{1}{2}mv^2=h\gamma$
Now,
Using the method of dimension we say that,
Here
${e}=LT^{-1}$
$[G]=M^{-1}L^3T^{-2}$
$[h]=M^1L^2T^{-2}$
Let, $mac^xG^0h^z$
By substituting the dimension of each quantity in both the sides
$M=(LT^{-1})^x(M^{-1}L^3T^{-2})^y(ML^2T^{-1})^z\M=(M^{y+z}L^{x+3y+2z}T^{x-2y-x})$
By equating the power of $M,L\quad and T$ in both the sides
$-y+z=1\x+3y+2z=0\-x-2y-z=0$
By solving the above equations
We get,
$x=\cfrac{1}{z},y=-\cfrac{1}{z},z=\cfrac{1}{z}$
So, $[m]=e^{1/2}G^{-1/2}h^{1/2}\ \quad=\sqrt{\cfrac{hc}{G}}$

For a gas $\cfrac{R}{C _{p}}=0.4$. For this gas calculate the following-

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. atomicity and degree of fredom

  2. value of $C _{v}$ and $\gamma$

  3. mean gram-molecular kinetic energy at $300\ k$ temperature

  4. none of these

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Correct Option: B

Efficiency of engine is $n _{1}$ at $T _{1}$= $200^\circ C$ and $T _{2}$ = $0^\circ C$ and $n _{2}$ at $T _{1} = 0^\circ C$ and $T _{2}=-200^\circ C$. Find the ratio of $\cfrac {n _{1}}{n _{2}}$

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. $1.00$

  2. $0.721$

  3. $0.577$

  4. $0.34$4

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Correct Option: C

An ideal heat engine has an efficiency $ \eta$ . The co-efficient of performance of the engine when driven backward is 

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. $1-\dfrac{1}{\eta}$

  2. $\dfrac{\eta}{1- \eta}$

  3. $\dfrac{1}{\eta}-1$

  4. $\dfrac{1}{1- \eta}$

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Correct Option: C

Heat flows between two bodies due to difference in their temperature.

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. True

  2. False

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Correct Option: A
Explanation:

Heat is defined as the energy which passes between two objects because of the difference in temperature. Thus energy may pass between them, but not heat. It is possible that the energy absorbed may increase the temperature, particularly if work is done, but this is not heat transfer.


Heat flows between two bodies due to difference in their temperature.

Option A is true

That 'Entropy of a system increases in all spontaneous processes' is  known as

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. 1 st law of Thermodynamics

  2. 2nd law of Thermodynamics

  3. Zeroth law of Thermodynamics

  4. 3rd law of Thermodynamics

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Correct Option: B
Explanation:

The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibriumthe state of maximum entropy

The statement - 'Heat cannot flow from colder body to hotter body' is known as

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. 1 st law of Thermodynamics

  2. Zeroth law

  3. 2nd law of Thermodynamics

  4. Law of conservation of energy

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Correct Option: C
Explanation:

 2nd law of Thermodynamics says that the heat cannot flow from colder body to hotter body.

A bullet of mass 100g moving with 20 m/s strikes a wooden plank and penetrates upto 20cm . Calculate the resistance offered by wooden plank

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. $10^2 N$

  2. $10^3N$

  3. $10^4N$

  4. $10^5 N$

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Correct Option: A

Which of the following option are related with the second law of thermodynamics (law of entropy)?

physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

  1. The heat lost by one object must be gained by another object

  2. Heat flow naturally from a hotter body to a cooler body

  3. Celsius degrees and Kelvin degrees are equivalent

  4. Heat can be transformed into work

  5. The average kinetic energy of molecules is proportional to temperature

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Correct Option: B
Explanation:

The second law of thermodynamics says that heat flows 'naturally' from an object at a higher temperature to an object at a lower temperature, and heat doesn’t flow in the opposite direction of its own accord, since the entropy in such a process increases. 

The reverse is not possible due to decrease in entropy in such a process.