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Oct 27, 2013 (3 years and 8 months ago)

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I PUC PHYSICS STUDY MATERIAL

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THERMODYNAMICS


ONE MARK QUESTIONS :


1.

What is thermodynamics
?


Thermodynamics

deals with the conversion of heat energy into mechanical


work.


2. What are thermodynamical co
-
ordinates?


Pressure, Volume and temperature are called the t
hermodynamical


Co
-
ordinates of a system.


3
.

What is cyclic process ?


If a system originates and ends at the same state after a series of changes


then the process is called
a cyclic process.


4. State the first law of thermodynamics.



Heat supplied to a system is partly used
to increase its internal energy and


the rest is used to do external work.


5. Express the first law of thermodynamics in mathematical form.


dQ = dV + dW where dQ is the heat supplied to the system, dv i
s the change


in its internal energy and dw is the work done by the system.


6
.

What is phase diagram ?


During change in state or phase, changes in pressure, volume and


temperature can be represented graphically. Such diagrams are called
p
hase


diagram.


7.
What is the basics of the first law?


First law of thermodynamics follows from the law of conservation of energy.


8.
Give the expression for the efficiency of Carnot’s heat engine.


Efficiency

=

where T1 and T2 are the temperatures of the source


and sink respectively.


9. What is a thermodynamic system?


A thermodynamic system is one which is completely described by the


thermodynamic co
-
ordinates.




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10. What is equation of state of a system?


A relation between the thermodynamic co
-
ordinates of a system is called its


equation of state.


1
1
.

What is refrigeration ?



The process of transfer of heat from a cold body to a hot body by doi
ng work



on the system is called
refrigeration.


1
2
. What is thermal equilibrium?


A thermodynamic system at constant temperature is said to be in thermal



equilibrium.


1
3
. What are the factors on which efficiency of a heat engine depends
?


Efficiency of ideal heat engine depends only on the temperature of the source



and the sink.


1
4
.

What is the effect of pressure on boiling point?



When pressure increases, the boiling point of the liquid increases.


1
5
. What is thermody
namic equilibrium?


A thermodynamic system is said to be in thermodynamic equilibrium if it is in


thermal, mechanical and chemical equilibrium


1
6
.
When is the efficiency, maximum for Carnot heat engine?


The efficiency of Carnot engine i
s maximum and equal to one when the


temperature of the sink is 0 K.


17. State Clausius’ statement for the II law of thermodynamics.


It is impossible for a self acting machine, unaided by an external agency, to


transfer heat from a bod
y at a lower temperature to one at a higher


temperature.


1
8
.

What is the effect of pressure on melting point?



When pressure increases, the melting point of wax increases. When pressure



increases, the melting point of ice decreases.


1
9
.
Name the different thermodynamic processes


The different thermodynamic process are isothermal, isochoric, isobaric and


adiabatic processes.


20
. How should the temperatures of the source and the sink must be


changed to increase the ef
ficiency of Carnot engine?


In order to increase the efficiency, the temperature of the source must be


increased and that of the sink must e decreased.



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2
1
.

Will temperature remains constant in a adiabatic change?



No.


2
2
. What is an iso
thermal process?


An isothermal process is one in which changes in pressure and volume of a


gas take place at constant temperature.


2
3
. What is the significance of the area enclosed by the indicator diagram


in Carnot’s cycle.


The
area enclosed by the indicator diagram gives the net work done by


the working substance in one cycle of changes.


2
4
. What is an isochoric process?


An isochoric process is one in which changes in pressure and temperature of


a gas take p
lace at constant volume.


2
5
. What is the physical meaning of entropy of a system?


Entropy of a system is a measure of the degree of disorder in it. If the entropy


increases, the disorder also increases


2
6
.

What is an isobaric process?


An isobaric process is one in which changes in volume and temperature of a


gas take place at constant pressure.


2
7
. What happens to the entropy of a system in an adiabatic change?


Entropy remains constant in an adiabatic change.


2
8
. What is
an adiabatic process?


An adiabatic process is one in which changes in pressure, volume and


temperature of a gas take place in thermal isolation from the surroundings.


2
9
. What happens to the entropy of a system in a reversible cycle?


In

a reversible cycle, the entropy remains constant.


30
.

What happens to the internal energy of a system in the following


cases? (‘one mark for each part’)

(1)

in isothermal expansion

(2)

in isothermal compression

(3)

in adiabatic expansion

(4)

in adiabatic co
mpression.


(1) remains constant (2) remains constant (3) decreases (4) increases.






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31
. What happens to the entropy of a system undergoin
g an irreversible


process?


Entropy of the system increases.


3
2
. What is the amount of work done

in an isochoric process?


No work is done in an isochoric process.



3
3
. State Clausius


Clapeyron equation



=

where dP is the change of pressure, dT


change in the


boiling point o
r melting point, L is l
e
ant heat, T is boiling point or melting point


and V
1

and V
2

are specific volumes


3
4
. What is a reversible process?


A reversible process is one in which the process can be retraced in the


reversed direction, so t
hat the changes in the direct process are exactly


reversed.


3
5
. How does the boiling point of water change with pressure?


Boiling point of water increases with the increase in pressure.


3
6
. What is an irreversible process


An irreversib
le process is one which cannot be run in the reverse direction


with reversed changes.


3
7
.

What is triple point?


Triple point of a substance is the temperature at which the three phases of


the substance co
-
exist.


3
8
.

Whether the follow
ing changes are reversible or irreversible?


(One Mark for each question)


(i) rusting of iron (ii) transfer of heat by conduction (iii) production of heat by


F
riction


(i) irreversible (ii) irreversible (iii) irreversible.


3
9
.

Define efficiency of a heat engine.


Efficiency of a heat engine is the ratio of the work done by the engine to the


heat absorbed from the source.


40. State Zeroth law of thermodynamics


It states that, if two systems A and B are separ
ately in thermal equilibrium


with a third system C, then A and B are in thermal equilibrium with each


other.



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41
.
Give one example of a reversible process.


Slow expansion of a gas, either isothermal or adiabatic, is a reversible



process.


42
. Give the value of triple point of water.


Triple point of water is 0.01
0
C at a pressure of 0.006 atmos.


4
3
. What is an indicator diagram?


Indicator diagram is a graph obtained by plotting pressure of a gas against


v
olume


4
4
. How can heat be made to flow from a colder body to a hotter body?


Heat can be made to flow from a colder body to a hotter body by supplying


energy to the colder body.


4
5
.
What is a heat engine?


A heat engine is a device which con
verts heat energy into mechanical


energy.


4
6
. Define change of entropy of a system.


If a system absorbs a small amount dQ of heat at temperature T, the


increase of entropy of the system is given by
.


4
7
.

Sta
te Kelvin’s statement of second law of thermodynamics.


It is impossible to derive a continuous supply of work by cooling a body below


the coldest of its surroundings.


4
8
. What happens to the total entropy of the universe?


Total entropy

of the universe increase.


4
9
. State Clausius’ statement of second law of thermodynamics


It is impossible for a self acting machine, unaided by an external agency, to


transfer heat from a body at a lower temperature to one at a higher


temperature.










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TWO MARK QUESTIONS :


1
.

State and explain the Zeroth law of thermodynamics.



It states that if two systems A and B are separately in thermal equilibrium with



a third system C, then A and B
are

in thermal equilibrium wi
th each other.




Let T
A
, T
B

and T
C

be the temperature of three systems S, B and C



respectively. If the systems A and B are separately in equilibrium with C,



then T
A

= T
C

and T
B

= T
C
. This implies that T
A

= T
B


2.

Explain what happens t
o the heat supplied to the system in the


following cases.


(i) isothermal expansion of a gas


(ii) adiabatic compression of a gas.



(i)

dQ = dV + dW




In an adiabatic change, internal energy of the system is constant.
Therefore



dV = 0 and dQ = dW. In isothermal expansion of the gas heat supplied is


used to do external work by the gas.




(ii) In an adiabatic change dQ = 0




dV + dW = 0 or dV =
-
dW




In ad
iabatic compression of the gas, work is done on the gas and as a result



its internal energy increases.


3
.


State and explain the first law of thermodynamics.



The law states that the amount of heat supplied to a system is equal to the



sum of the increase in internal energy and work done by the system.




If dQ is the amount of heat supplied to a system, dU is the increase in


internal energy and dW is the work done by the system, then according to the



first law of therm
odynamics,




DQ = dU + dW


4.

What are the limitations of the first law of thermodynamics?



First law does not explain why heat flows only from a hot body to a cold body.



It does not explain why all the heat absorbed from the source in a
heat



engine, cannot be converted into work.


5
.


What is a heat engine? Define efficiency of a heat engine.



Heat engine is a device, which continuously convert heat energy into



mechanical energy.



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Efficiency of a heat engine is

the ratio of the net work done by it to the heat



absorbed by it during each cycle.




If Q
1

is the heat absorbed by the system and Q
2
is the heat rejected by it, the



net work done is, Q
1



Q
2









6.

Distinguish between isothermal and adiabatic changes.



In an isothermal change temperature of the system remains constant



whereas in an adiabatic change it changes.




Internal energy of the system remains constant in

an isothermal change



whereas it chang
es in an adiabatic change.


7.

What is phase diagram?



During change in state or phase, changes in pressure, volume and



temperature can be represented graphically. Such diagrams are called phase




diagrams.




One type of phase diagram is P
-
V diagram, in which P is taken along the y
-


axis and V is taken along the x
-
axis. In another type of phase diagram,



called


P
-
T diagram, P is taken along y
-

axis and T is taken alon
g x
-
axis. Phase


diagrams are used to study different properties of a system.


8
.


State the 2
nd

law of thermodynamics.



1.

Kelvin’s statement::

It is impossible to construct a heat engine that works in a cyclic process to
receive heat from a s
ource continuously and produce equivalent work.



2.

Clausiu’s statement:

It is impossible for a self


acting device unaided by an external agent to
transfer heat from a cold body to a hot body.


8.

Describe Carnot’s heat engine.



Carnot’s idea
l heat engine consists of the following parts (i) A source of




infinite thermal capacity at a high temperature (ii) a sink of infinite thermal



capacity at a lower temperature (iii) a non conducting platform (iv) A cylinder



with non
-
cond
ucting sides but perfectly conducting, bottom, fitted with a



smooth piston. It contains a perfect gas as working substance.



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9
.


Discuss the application of 2
nd

law of thermodynamics to refrigerators.



In a refrigerator the working substance
absorbs Q
2

amount of heat from the



cold body (or sink) at temperature T
2
. By doing external work W on the



working substance, an amount of heat Q
1

is released to the hot body at



temperature T
1
. According to 2
nd

law of thermodynamics an
external work



must be done continuously on the working substance.


10.
What are the four thermodynamic processes forming Carnot’s cycle?


Carnot’s cycle is made of the following processes i) Isothermal expansion of


the gas at the tempera
ture of the source. ii) Adiabatic expansion of the gas


until its temperature decreases to that of the sink, iii) Isothermal compression


of the gas, iv) Adiabatic compression of the gas until the original state is


reached.


11
.

Explain
the term entropy.



Entropy of a system is defined as that physical quantity which remains



constant during reversible adiabatic process.




The change in entropy (dS) of a system is defined as the ratio of small



amount of heat supplie
d to or removed from the system (dQ) to the absolute



temperature (T).


dS =


12
.

What is reversible and irreversible process?
G
ive example for each.



Any process that can be made to go in the reve
rse direction such that all



changes taking place in the direct process are exactly reversed is called a



reversible process.




E.g., :


1.
A slow isothermal expansion or compression of a gas.


2
A slow adiabatic expansion or compre
ssion of a gas.





Any process that can not be made to go in the reverse direction is called an




irreversible process.




E.g.,:



1.
An explosion




2.
A sudden compression or expansion of a gas.


13
.

Explain the terms ‘Phase diagram’ a
nd ‘Tripple point’.



The point in the phase diagram where all the three phases of solid, liquid


and gas co


exist in equilibrium is called Tripple point.



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14. Can heat flow from a colder body to a hotter body?


In nature, heat always fl
ows from a body at a higher temperature to another


body at a lower temperature. But we can reverse the direction of heat flows


by doing work on the working substance. This is the principle of the


refrigerator.


1
5
.

The efficiency of a
carnot heat engine is 50%. When the temperature of



the sink is 300k. Calculate the temperature of the source.



n = 50% = 0.5,





T
2

= 300k then T
1

= ?



n = 1
-


i.e. 0.5 =

i.e. T
1

=

600k