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Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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BANGALORE INSTITUTE OF COACHING
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I PUC PHYSICS STUDY MATERIAL

1

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

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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I PUC PHYSICS STUDY MATERIAL

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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 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)

(4)

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

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

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
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

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