Thermometry
1.1 Concept of Heat and Temperature
It is not possible to speak of work in a body. Similarly, it is not possible to speak of heat in a body. Work is either done on a body or by a body. similarly, heat can flow from a body or t a body. If a body is at a constant temperature, it has both mechanical and thermal energies due to the molecular agitation and it is not possible to separate them. So, in this case, we cannot talk of heat energy. It means, if the flow of heat stops, the word heat cannot be used. It is only used when there is transfer of energy between two or more systems. Therefore, heat can be defined as energy is transit.
Temperature of a system cab be defined as the property that determines whether or not the body is in thermal equilibrium with the neighbouring systems. If a number of systems are n thermal equilibrium, this common property of the system can be represented by a single numerical value called the temperature. It means that if two systems are not thermal equilibrium, they are at different temperatures.
Example. In a mercury in glass thermometer the pressure above the mercury column is zero and volume of mercury measures the temperature. If a thermometer shows a constant reading in two systems A and B separately, it will show the same reading even when A and B are brought in contact.
Measurement of temperature of a body accurately is one of the important branches of heat in physics. It also becomes necessary to measure high temperatures and low temperatures. To make this measurement possible, it is necessary to construct a suitable scale of temperature. the scale chosen must be precise and consistent and the temperatures measures on this scale must be accurate. Assessing the temperature of a body by mere sense of touch or comparing . assessing the temperature of a body by mere sense of touch or comparing the degree of hotness of a body with respect to another body does not help in measuring the temperature quantitatively and accurately.
1.2 Thermometry
1.1 Concept of Heat and Temperature
It is not possible to speak of work in a body. Similarly, it is not possible to speak of heat in a body. Work is either done on a body or by a body. similarly, heat can flow from a body or t a body. If a body is at a constant temperature, it has both mechanical and thermal energies due to the molecular agitation and it is not possible to separate them. So, in this case, we cannot talk of heat energy. It means, if the flow of heat stops, the word heat cannot be used. It is only used when there is transfer of energy between two or more systems. Therefore, heat can be defined as energy is transit.
Temperature of a system cab be defined as the property that determines whether or not the body is in thermal equilibrium with the neighbouring systems. If a number of systems are n thermal equilibrium, this common property of the system can be represented by a single numerical value called the temperature. It means that if two systems are not thermal equilibrium, they are at different temperatures.
Example. In a mercury in glass thermometer the pressure above the mercury column is zero and volume of mercury measures the temperature. If a thermometer shows a constant reading in two systems A and B separately, it will show the same reading even when A and B are brought in contact.
Measurement of temperature of a body accurately is one of the important branches of heat in physics. It also becomes necessary to measure high temperatures and low temperatures. To make this measurement possible, it is necessary to construct a suitable scale of temperature. the scale chosen must be precise and consistent and the temperatures measures on this scale must be accurate. Assessing the temperature of a body by mere sense of touch or comparing . assessing the temperature of a body by mere sense of touch or comparing the degree of hotness of a body with respect to another body does not help in measuring the temperature quantitatively and accurately.
1.2 Thermometry
The branch of heat relating to the measurement of
temperature of a body is called thermometry. Thermometer is an instrument used
to measure the temperature of a body.
The essential requisites of a thermometer are:
(1)
Construction
(2)
Calibration, and
(3)
Sensitiveness
(1)
Construction . The physical
property of a substance plays an important role in the construction of a
thermometer. In a mercury thermometer, the principle of expansion of mercury
with rise in temperature is used. The platinum resistance thermometer is based
on the principle of the change in resistance with change in temperature. The
gas thermometer is based on the principle of change in volume or pressure with
change in temperature. Thus, for the construction of a thermometer, the proper
choice of a substance, whose physical property varies uniformly with rise in
temperature is essential.
(2)
Calibration. When a thermometer is constructed, it should
be properly calibrated. The standard fixed points are selected for calibrating
a thermometer. Melting point of ice, boiling point of water, melting point of silver
and melting point of gold are taken as fixed points. The scales are built by
dividing the interval between the two fixed points into equal parts. Centigrade
scale is built by dividing the interval between the melting point of ice and
the boiling point of water (under normal pressure) in 100 equal parts and each
part represents 1 degree C. Similarly, Fahrenheit scale is built by dividing
this interval in to 180 equal parts.
(3)
Sensitiveness. The
instrument, once constructed and calibrated, should also be sensitive. The
thermometer will be sensitive if (i) it can detect even small changes in
temperature, (ii) it shows the temperature of a body in a short time and (iii)
it does not take large quantity of heat for its own heating grow the body whose
temperature is being measured.
Fixed points
|
Degree Celsius
|
Boiling point of
oxygen
|
-182.97
|
Ice point
|
0.00
|
Steam piont
|
100.00
|
Boiling point of
sulphur
|
444.00
|
Melting point of
silver
|
990.00
|
Melting point of
gold
|
1063.00
|
Melting point of
cobalt
|
1492.00
|
Melting point of
platinum
|
1769.00
|
Melting point of
rhodium
|
1960.00
|
Melting point of
tungsten
|
3380.00
|
1.3 Types of
Thermometers
There
are different kinds of thermometer:
(1)
Liquid thermometer. These
thermometers are based on the principle of change in volume of a liquid with
change in temperature. Mercury and alcohol thermometer are based on this principle.
(2)
Gas thermometer. These are
based on the principle of change in pressure or volume with change in temperature,
e.g., Callendar’s constant pressure thermometer, constant volume hydrogen
thermometer etc.
(3)
Resistance thermometers.
These are based on the principle of change in resistance with change in
temperature, e.g., platinum resistance thermometer.
(4)
Thermo-electric
thermometer. These are based on the principle of thermo-electricity. i.e,
production of thermo-E.M.F in a thermo-couple when the two junctions are at
different temperature.
The various thermocouples commonly used
are:-
a)
Copper and constantan
b)
Iron and constantan
c)
Chromel and constantan
d)
Chromel and alumel
e)
Platinum and Rhodium.
(1)
Radiation thermometer.
These are based on the quantity of heat radiations emitted by a body e.g., furnaces.
These instruments are known as pyrometer.
(2)
Vapor pressure
thermometers. These are based on the principle of change of vapor pressure with
change in temperature. These are used to measure low temperatures, e.g., helium
vapor pressure thermometer etc.
(3)
Bimetallic thermometers.
These thermometers are based on the principle of expansion of solids. A
bimetallic strip is taken in the form of a spiral. Its one end is fixed and the other end is attached to a long
pointer. The pointer moves on a scale, calibrated in degrees. These
thermometers are used in meteorology for recording the change in temperature
during the day. They are also used to measure temperatures at high altitudes.
(4)
Magnetic thermometers.
These thermometers are based on the principle of change in the susceptibility
of a substance with temperature. These thermometers are useful for measuring
low temperatures near the absolute zero temperature.
