40 degrees Celsius is the temperature
reading of a body. 40 is the magnitude of temperature and degree Celsius
is unit of measurement of this temperature.
How to define the temperature?
Temperature is defined as the
degree of “hotness” or “coldness” of a body. We use our sense of touch
to distinguish hot bodies from cold bodies and, on the basis of this
distinguish, we arrange bodies in their order of hotness that body-1 is hotter
than the body-2 and so on.
When a hot body and a
cold body are brought into physical contact, the hot body becomes
cooler and the cold body becomes warmer. If both of these bodies maintain
contact with each other for some time, then both of these bodies will
have the same degree of hotness or coldness.
How temperature travels
through the two contact bodies? What is the basis for temperature detection?
To understand how temperature
travels through the two contact bodies, let us go through the zeroth law of
thermodynamics. This law states that when two bodies have equality of temperature
with a third body, then the two bodies also have equality of temperature with
each other. This zeroth law of thermodynamics is the basis of temperature
measurement.
Whenever a body has equilibrium
of temperature with the thermometer, then we can conclude that the body
has the temperature that we read on the thermometer whether it is a mercury
thermometer or a thermocouple or resistance thermometer.
Anybody that possesses a
measurable property such that this measurable property changes as the
temperature of the same body changes, then this measurable property can be used
as a thermometer & such property is termed as thermometric property.
Therefore, human sense of feeling
of hotness or coldness is definitely a thermometric property. The materials
that exhibits changes in the thermometric property are known as thermometric
materials.
An agreed-upon, temperature
scale is needed to assign quantitative values to temperature, with each
unit of the scale being referred to as a degree (°). A temperature scale can be
defined by specifying the constant of proportionality, as the temperature is
linearly proportional to the average kinetic energy of the atoms and molecules
in the system. By convention, the choice is made for (3/2) k, where k
represents Boltzmann’s constant. Therefore, Temperature in a specific unit
system can be expressed as:
E = 3/2 kT
Here, E denotes the average
kinetic energy per molecule or atom of the body. Since temperature is
defined as the average kinetic energy per molecule, its dependence on the size
of the system is nonexistent, making temperature always intensive. The scale
resulting from the above equation defines the absolute temperature scale, where
the temperature is zero in the absence of molecular kinetic energy. In SI
units, the temperature scale is measured in degrees Kelvin [K], and the value
of k is 1.38 x 10-23 [J/(molecule K-1)]. The temperature
scale in English units is denominated in degrees Rankine [°R].
Nevertheless, absolute zero, also
known as the temperature that is extremely cold, corresponds to a situation
where there is no molecular kinetic energy. It is often more convenient to
establish a temperature scale around temperatures commonly encountered in the
natural world. The Celsius temperature scale [°C] employs the same scale per
degree as the Kelvin scale, but with the freezing point of pure water set at
0°C and the boiling point at 100°C.
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