Physics
Xavier International College
Shubheksha Dhakal
Lecturer, Physics
GRADE: 11

9.1 Molecular concept of thermal energy, heat and temperature
and cause and direction of heat flow
9.2. Meaning of thermal equilibrium and zeroth law of thermodynamics
9.3 Thermal equilibrium as a working principle of mercury thermometer
Key Points
Teaching period= 3
Chapter- 9 Heat and Temperature

NO
Yes

Molecular Concept of Thermal Energy:
Internal energy : Kinetic energy + potential energy
➢ Total internal kinetic energy possessed by the molecules
present in the system is called thermal energy.
➢ It is also known as random or internal kinetic energy, due random
motion of molecules.
➢ In solid and liquid case, atoms, molecules vibrate at their mean
position.
➢ In gases, molecules move freely collide with each other and in walls
of vessels, thus posses kinetic energy.
➢ Kinetic energy is seen in three forms: Translational , Rotational
and Vibrational.
Figure: A system at certain
temperature
3
Heat of thermodynamics

Thermal Energy:
➢ Faster the molecules move higher
will be the temperature.
➢ Warmth of sun, fire

➢ The thermal energy of the system is dependent on the temperature of the system which further
depends on the motion of molecules of the system.
➢ If a cup of boiling tea is placed next to the freezing pool, the cup of tea will freeze first because it
has less thermal energy.
➢ The relatively small sample at a somewhat high temperature such as cup of tea at its boiling
condition could have less thermal energy than the large sample such as pool that is at low
temperature.
➢ Even though two materials might have the same temperature, their thermal energies might be
different due to how many particles are present in the object.
➢ For example, a glass of water can have the same temperature as a ocean , but the ocean has much
more thermal energy because the ocean contains many more water molecules.

Cause and direction of heat flow:
➢ Heat flow occur due to the more
energetic molecular activity in the
hot material transferring some of
its kinetic energy to adjacent
cooler materials.
➢ Heat transfer occur from high
temperature to low temperature.
50°C 10°C 50°C 10°C
Figure: A heat transfer by conduction method
Heat and Temperature

Thermal Equilibrium
➢ Thermal contact
➢ When two bodies of different temperature are
brought in contact with each other, then the heat
flow from hot body to cold body and finally attains
the condition where heat flow ceases and both
attain same temperature. This is the condition of
thermal equilibrium.
➢ When two thermometer read same temperature of
different objects then these objects are in condition
of thermal equilibrium.
Iron
ball T1
Cold
water
T2
Heat and Temperature

ZEROTH LAW OF THERMODYNAMICS
➢ If two system are in thermal equilibrium with the third system then they must
be in thermal equilibrium with each other.
B C
A
Conducting wall
Figure: zeroth law of
Thermodynamics
B C
A
Conducting wall
➢ An example :
Consider two glasses of water in which
One glass will have hot water and the other will contain cold water. Now if we leave them in the table
for a few hours they will attain thermal equilibrium with the temperature of the room.
Heat and Temperature

Explanation:
➢ Let us consider two systems B and C separated by an insulating wall, each being in contact
with the third system A as shown in figure in previous slide. After sometime, the system B will
be in thermal equilibrium with the system A and the system C will be in thermal equilibrium
with the system A. i.e.
➢ the temperature of system A and B will be equal, similarly the temperature of system A and C
will be equal. Now, the insulating wall between system B and C will be replaced by
conducting wall, then experiment showed that no further change in temperature takes place in
system B and C indicating thermal equilibrium in between system B and C.
➢ Therefore, when two system are in thermal equilibrium with the third system, the first two
system themselves are also in thermal equilibrium which is called zeroth law of
thermodynamics.
Heat and Temperature

Thermometer:
An instrument which is used to measure the temperature is called thermometer. The
study of temperature and its measurement is known as thermometry.
Types of thermometer:
1. Liquid thermometer
2. Gas thermometer
3. Resistance thermometer
4. Thermo-electric thermometer
5. Radiation thermometer
6. Vapour pressure thermometer
Heat and Temperature

Temperature Scales
Lower Fixed points:
➢ The temperature at which ice melts
under atmospheric pressure i.e
760mmHg is L.F.P
➢ It is also called ice point.
Upper Fixed Points:
➢ The temperature at which pure water
boils under a standard atmospheric
pressure i.e. 760mmHg is U.F.P
➢ It is also called steam point.
➢ Write about different temperature scales in detail
for making notes.

Relation Between Different Temperature Scales:
If C, F,R, and K be the temperature of a body in celsius, Fahrenheit,
Reameur, kelvin scale respectively, then we have,
Reading point –ice point =Constant
Steam point – ice point
= = =
𝐶
− 0
𝐹
− 32
𝑅
− 0
𝐾
− 273
100 − 0 212 − 32 80 − 0 373 − 273
𝐶
𝐹
− 32
100 180
= =
80
=
𝑅
𝐾
− 273
100
= = =
𝐶
𝐹
− 32
𝑅
𝐾
− 273
5 9 4 5
Heat and Temperature

°F
°C
(C,F)
0°C
32 °F
Since similar triangles i.e ABC and ADF ratio of
sides are equal.
𝐶
− 0
𝐹
− 32
100 − 0 = 212 − 32
𝐶
𝐹
− 32
100 = 180
100 °C
A
D
C
F
1B
00,212
Heat and Temperature
𝐴
𝐹
𝐴
𝐶
=
𝐷
𝐹
𝐷
𝐶

Numerical questions
1. Calculate the value of temperature at which Fahrenheit scale and centigrade scales give same reading.
Solution:
Let value of temperature at Fahrenheit scalebe F and that of centigrade scale be C. Let
us assume, F=C=
𝑥
Heat and Temperature
X = -40 degree or -40 F
𝐹
− 32
9
=
𝐶
5
𝑥
− 32
9
=
𝑥
5
5 (x-32) = 9
𝑥
5 x-160
-160 = 9x-5x
-160 =4x
X = -40
= 9x
Then, Using Relation of temperature scale

2. A faulty thermometer has its fixed point is marked 2°C and 98°C. Temperature of a body as measured
by the faulty thermometer is 64°C. Find the correct temperature of the body on correct Celsius scale.
Soln,
The lower fixed point(
𝐿
.
𝐹
.
𝑃
) :2°C
The upper fixed point (
𝑈
.
𝐹
.
𝑃
) :
98°C
Temperature of faulty thermometer: 64°C
The correct temperature of thermometer= ?
Then,
Thus, the correct temperature of the body on correct Celsius scale is 64.58°C .
Heat and Temperature
C on correct scale − (L . F . P)correct
(U . F . P)correct − (L . F . P)correct
=
Faculty on scale − (L . F . P)faculty
(U . F . P)faculty − (L . F . P)faculty
c= =64.58
𝐶
− 0
100 − 0
=
64 − 2
98 − 2
𝐶
100
=
62
96
6200

Classwork
Q. At what temperature does the kelvin scale coincide with Fahrenheit scale ?[HSEB2061]

Mercury Thermometer and its construction
➢ It is one of the most familiar used thermometer as shown in
figure aside.
Working Principle:
➢ Based on the principle of Thermal equilibrium
➢ A glass tube is filled with mercury and standard scale is marked
on the tube.
➢ We can read the temperature from the scale when the
thermometer and contact body are in thermal equilibrium with
each other.
➢ used to determine body, liquid, and vapor temperature.
➢ Householdslaboratory experiments and i n d u s t r i a l
applications.
Figure :Mercury
thermometer
Heat and Temperature

Advantages of Mercury thermometer:
➢ It is visible, because its color is silvery shining.
➢ Its expansion is regular and uniform.
➢ It does not wet the wall of the glass and can be easily obtained in pure state.
➢ It’s boiling point is 357°C and freezing point is -39°C. Therefore, it can be
used over a wide range of temperature.
➢ It has low specific heat, so it doesn’t take much heat from the body whose
temperature is to be measured.
Disadvantages of Mercury Thermometer:
➢ It cannot measure the temperature below -39°C and above 357°C.Therefore this
thermometer cannot be used in Arctic and Antarctic region.
➢ Mercury is poisonous, so it is harmful if its tube is broken.
Heat and Temperature

Construction of mercury Thermometer :
➢ A glass containing fine capillary tube having one bulb at one end is taken.
➢ The bulb is filled with pure and dry mercury by alternate
heating and cooling to remove air bubbles present in it.
➢ The glass is kept in hot water bath having temperature higher than the
temperature it has to measure.
➢ After all the air exhausted from the glass, open end of the tube is sealed.
Then, the tube of mercury is kept for cooling before marking fixed points.

Determination of fixed points:
➢ Tomark a lower fixed point, the bulb of
a thermometer is kept in melting ice as
shown in figure.
➢ Tomark upper fixed point, the bulb of
a thermometer is kept inside
hypsometer containing water inside
as shown in figure.
Note: Write about construction of mercury in detail from book

Absolute zero Temperature:
At lowest possible value of temperature, all molecular motion does not cease but it
vibrates with lowest possible energy and this energy from molecular motion cannot
be transferred to other system. Thus, this lowest possible value of temperature where
molecules are at lowest energy value is called absolute zero temperature.
Value = -273.15°C

Absolute scale of temperature:
➢ Kelvin scale is known as absolute scale.
➢ Scale of temperature whose zero corresponds to absolute zero of the
temperature(i.e. -273.15°C)
i.e 0K=-273.15°C
Adding 273.15 on both sides,
273.15 K= 0°C
Similarly adding T on both sides,
(T+273.15) K= T°C
Thus , T absolute is (T +273.15) K

Assignments
Complete exercise from principle of
physics Textbook