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4.1 Thermal’s Principle

The difference between Temperature And Heat



Is the degree of hotness of a body

Is a form of energy

Is a base quantity

Is a derived quantity

Depends upon the kinetic energy of the molecules

Depend upon temperature, mass and type of material ( specific heat capacity or specific latent heat) 

( Q = mcq or Q = ml) )

The  S.I. unit is K or 0C


The S.I. unit is  Joule(J)

Measured by thermometer

Measured by Joulemeter

Kelvin Scale (K)

The temperature of a substance in Kelvin , also known as absolute temperature.

 q o C =  ( q + 273 ) K


Example 1

Convert 120o C  to its equivalent temperature in Kelvin.





Thermal Contact


Two substances are said to be in thermal contact,

when heat flows from one substance to another it is in contact with. Heat flows according to temperature differences i.e.from substance  hot  to cold  substance.

The principle of thermal equilibrium


Two bodies in thermal contact are said to be in thermal equilibrium when its reach  the same temperature  and the  net rate of heat  transfer between  the two  bodies is zero.


How a liquid in glass thermometer works ?


When a thermometer is in thermal contact with a substance ( for example  hot water) , heats flows from the hot water to the thermometer(mercury)

When  thermal equilibrium is reached  the net rate of heat  transfer between  the two substances is zero. The temperature of the thermometer is same as hte temperature of the hot water.

Hence by showing its own temperature , the thermometer also reads the temperature of the hot water.


Basic principle to construct  a  thermometer.

Two important principles to construct a thermometer are.

1       Specific thermometric property  i.e. a physical quantity which varies with temperature.

          The table shows four different types of thermometers.



Thermometric property

Mercury thermometer

Volume of mercury varies with temperature

Resistance thermometer

Electrical resistance of a wire varies with temperature

Thermocouple thermometer

Electromotive force (e.m.f) varies with temperature

Gas thermometer

Gas pressure varies with temperature


2       Calibration of thermometer i.e the process of marking-up a scale on a thermometer.

To produce  a scale for a thermometer, two fixed  points of thermometer must first be selected.

a)       Lower fixed point (0oC)– is the melting temperature of pure ice at standard atmospheric pressure.

b)       Upper fixed point (100 o C) – is the temperature of steam a standard atmospheric pressure.


Calibration of a Mercury-in-glass Thermometer on the Celsius Scale


Freeze some pure water.

Crush the ice into small and fill a funnel with them.

When the ice begins to melt  inset the bulb of a thermometer so that it is covered with ice.

When the mercury stops shrinking , mark the stem of the thermometer at the mercury  level, as 0oC.

Now arrange the thermometer inside a flask so that its bulb is just above the surface of boiling water.

When the mercury stops expanding , mark  its level on the thermometer stem ,as 100oC.

Divide the distance between the marks  0oC and 100oC into 100 equal parts, marked as a scale along the stem.


The formula is used  to calibrate a thermometer



                q = temperature of a substance

                                qo= ice point                        

q100=  steam point

xo = the length of the mercury column at ice point

x100 = the length of the mercury column at steam point

x = the length of the mercury column when the thermometer is placed in a



Example 2

The lengths of the mercury column in a thermometer at the ice point and the steam point are 12 cm and 20 cm respectively. When the thermometer is placed in a  liquid , the length of the mercury column is 15 cm. What is the temperature of the liquid?




Example 3


The lengths of the mercury thread in a thermometer are 4.0 cm and   24.0 cm respectively  at 0oC and 100oC. What is the length of the thread when the thermometer is placed in a substance at 65oC.    





Mercury Thermometer



The specific thermometric property  in used in this thermometer  the changes of the volumes of mercury with the temperature i.e when the temperature increases ,the volume of  the mercury increases.


The sensitivity of the thermometer can be increased by

1           using a thinner-walled glass bulb

2           reducing the diameter of the capillary tube


Mercury is used  in the thermometer  because

                1              has a higher boiling point

                2              does not stick to the glass

                3              is opaque and therefore it is easier to read.

                4              expands and contracts uniformly    


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