Thermometry

Thermometry

Three different Scales of Temperature:
Temperature is defined as the degree of hotness or Coldness of a body. The natural flow of heat is from higher temperature to lower temperature.
The Celsius Scale : In this scale LFP (ice point) is taken 0° and UFP (steam point) is taken 100°C. The temperature measured on this scale all in degree Celsius(°C)
The Fahrenheit Scale : This scale of temperature has LFP as 32°F and UFP as 212° F. The change in temperature of 1°F corresponding to a change of less than 1° on Celsius scale.
The Reaumer Scale: This scale was devised by R.A. Reaumer in the year 1730.
The interval between the lower and the upper fixed points is divided into 80 equal parts. Each division is called one degree Reaumer (1°R). On this scale, the melting point of ice at normal pressure is 0°R. This is lower fixed point. The boiling point of water at normal pressure is 80°R. This is the upper fixed point.


Conversion of temperature from one scale to another
In order to convert temperature from one scale to another, following relation is used.

=

or


Constant volume Gas Thermometer:
Suppose the pressure of the gas is p0when the bulb is placed in melting ice (ice point) and it is p100when the bulb is placed in a steam bath (steam point). We assign 0°C to the temperature of the ice point and 100°C to the steam point. The temperature t corresponding to a pressure p of the gas is defined by
t =


Constant Pressure Thermometer
Volume of the bulb = V
Volume of the mercury taken out = v'
Temperature of the ice bath = T₀
Temperature of the heat bath = T
T =


Platinum resistance Thermometer
Resistance at the temperature t = R_t
Resistance at the ice point = R₀
Resistance at the steam point = R₁₀₀.
t =
Electrical resistance of a metal wire increases gradually and uniformly over a fairly wide range of temperature has been made use of in electrical resistance thermometers. The variation of the resistance thermometers. The variation of the resistance of a metal wire with temperature may be represented by the following approximate relation.
R_t = R₀(1 + at)
Here, R_t is the resistance at t°C, R₀ is the resistance at 0°C and a is the temperature coefficient of resistance. The value of a depends upon the nature of material of the wire.

Calorimetry
Heat Calorimetry means 'measuring heat'. It is a form of energy which flows between two bodies due to difference in their temperatures.
Heat is the cause and temperature is one of the several effects.
Heat energy is transient form of energy. An isolated body at a temperature will not posses heat energy but possess internal energy.
Heat is a scalar. Units of Heat
a) S I unit is joule. b) practical unit of heat in C.G.S. is 1 calorie 1 cal = 4.186 J Dimensional formula : ML2T-2
Specific heat : The amount of heat required to raise the temperature of unit mass of a substance to raise its temperature through 10C is called its specific heat (s).
specific heat S =
Dimensional formula is M°L²T ^-2K^-1
S.I. unit is J kg^-1K^-1, CGS unit is cal gm^-1°C^-1
Latent heat : The amount of heat energy either absorbed or liberated by unit mass of substance when it undergoes change of state at constant temperature is called Latent heat (or) heat of formation (L).
L = Cal/gam , K.cal/kg , J/kg


Variation of Boiling Point with pressure
Boiling point increases with increase of pressure.
In a pressure cooker, cooking of rice is a quick process as water boils at a temperature above 1000C due to increase in pressure.
Due to addition of impurities Boiling point of liquid increases. The heating of liquid above its boiling point is called super heating and cooling of liquid below its freezing point is called super cooling. It is unstable.


Variation of melting point with Pressure :
Melting point varies with the pressure from classius - clayperon equation

The melting point of the substances which expands on melting increases with increase of pressure
Ex. Wax, Glass, Gold, Copper, Silver.
The melting point of the substances which contract on melting decreases with increase of pressure.
Ex. Ice, Cast Iron, Bismuth, Type Metal.


Triple point of Water :
The temperature and pressure at which the three states of matter coexists is known as triple point.
Triple point and phase diagram for water


OA - Ice line, OB - steam line, OC- sublimation curve