2. CONTENTS
Molecular Concept of Thermal Energy
Heat and Temperature
Cause and Direction of Heat Flow
Thermal Equilibrium
Zeroth law of Thermodynamics
Introduction to Mercury Thermometer
Advantages and Disadvantages of Mercury Thermometer
Construction of Mercury Thermometer
Determination of Fixed Points of Mercury Thermometer
Temperature scales
3. Molecular Concept of Thermal Energy
Thermal energy is defined as the total of all kinetic energies withing a given
system.
Thermal energy is produced in a substance when a rise in temperature causes
atoms and molecules to move at a higher speed and result they collide with
each other. The energy produced from the temperature of the heated material is
known as thermal energy.
Thermal Energy or internal Kinetic Energy is due to the random motion of
molecules in a system. The thermal energy of a given system is directly
proportional to the temperature.
The greater the temperature of the system, the greater the movement of
molecules within a given system, and the greater the thermal energy of the
system.
4. Heat and Temperature
Heat is the transfer of kinetic energy from one medium or object to another, or
from an energy source to a medium or object. Such energy transfer can occur
in three ways: radiation, conduction, and convection.
The total sum of K.E. and P.E. of the system is known as heat or thermal
energy.
Temperature refers to the hotness or coldness of a body. In specific terms, it is
the way of determining the kinetic energy of particles within an object. Faster
the movement of particles; more the temperature and vice versa.
Temperature can also be defined as the average K.E. of all the molecules of a
system.
5. Differences between Heat and Temperature
Heat
It is the sum of kinetic energy
of all the molecules of a
system.
It is measured in Joules.
It is not determined directly
by an instrument.
It is a derived quantity.
Temperature
It is the average kinetic
energy of all the molecules of
a system.
It is measured in kelvin.
It is directly measured by an
instrument.
It is a physical quantity.
6.
7. Cause and Direction of heat flow
Heat flow occurs due to more
energetic molecular activity in
the hot material transferring
some of its kinetic energy to
adjacent cooler materials.
Heat transfer occur from high
temperature body to low
temperature body.
8. Thermal Equilibrium
When two bodies of different
temperature come in contact with
each other, then the heat flow
occurs. When the flow of heat ceases
and both attain the same
temperature, then the two bodies are
said to be in thermal equilibrium.
For e.g. When thermometer reads
same temperature of two bodies then
those bodies are in condition of
thermal equilibrium.
9. Zeroth Law of Thermodynamics
If two systems are in thermal equilibrium
with a third system, then they must be
thermal equilibrium with each other.
Consider two systems A and b separated
to each other by an insulating wall but
each being in contact with third system C
through conducting wall as shown in
figure.
After sometime, the A will be in thermal
equilibrium with C and similarly B will
also be in thermal equilibrium with C.
Now if the insulating wall is replaced by
conducting wall, experiments show that
no further change takes place in A and B
indicating that A was in thermal
equilibrium with B.
10. Mercury Thermometer
It is one of the most familiar used thermometer as
shown in the figure aside.
It works on the principle of Thermal Equilibrium.
A glass tube is filled with mercury and standard scale
is marked on the tube.
It is mostly used to determine body, liquid and vapor
temperature.
The mercury in the reservoir expands with increased
temperature, climbing higher up the thermometer,
and it contracts and recedes as the temperature falls.
11. Advantages of Mercury Thermometer
• It is a good conductor of heat and hence, can measure even high
temperatures.
• It gives results quickly – has a fast response time.
• It expands linearly and less than alcohol and any other liquid.
• It is more durable than alcohol thermometer because mercury
does not evaporate easily.
• It is visible, because of its silvery shining color.
• It does not wet the wall of the glass.
12. Disadvantages of Mercury Thermometer
• If the bulb cracks and mercury spills out, it is extremely
dangerous to humans and the environment.
• Mercury is difficult to remove and can pollute the area where it
has spilt.
• It is unable to detect frigid temperatures.
• Its thermal co-efficient is low.
13. 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, the open end of the tube
is sealed. Then, the tube of mercury is kept for cooling before
marking fixed points.
14. Determination of fixed points
To mark a lower fixed point, the
bulb of a thermometer is kept in
melting ice as shown in figure b.
To mark upper fixed point, the bulb
of a thermometer is kept inside
hypsometer containing water inside
as shown in figure c.
15. Temperature Scales
Centigrade or Celsius scale: The Celsius scale, also known as the centigrade
scale, is a temperature scale based on 0o for the freezing point of water and
100o for the boiling point of water.
Fahrenheit scale: It is the scale of temperatures in which 32° represents
the freezing point of ice and 212° represents the boiling point of pure water
under standard atmospheric pressure.
Reaumur scale: In this scale the freezing point of ice is marked as zero and the
boiling point of water is marked as 80.
Absolute Temperature scale: This type of scale measures the freezing point of
ice at 273 K and the boiling point of water at 373 K.
