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Heat and Temperature


Heat is a form of energy which produces in us the sensation of hotness or coldness.

#Heat Transfer

The process of transfer of energy from one body to another due to variation of temperature is known as heat transfer.


The property of which shows the degree hotness or coldness of a body along with identifying either they are in equilibrium or not is called temperature.


A device which is used to measure the temperature of a body is known as thermometer. It can be classified on the basis of the items used in a thermometer into following 5 categories:

a. Liquid thermometer:

liquid thermometer are based on the principle of change in volume of liquid with the change in temperature. The thermometric liquid used in thermometer must be highly sensitive to heat so, that the expansion is observed very clearly, even in a small rise in volume. The commonly used thermometric liquid are mercury, alcohol, kerosene, ethanol etc. These thermometers can measure the temperature ranging from -200°C to 600°C. For example, Mercury Thermometer, Alcohol Thermometer etc.

The types of liquid thermometer are as follows:

i) Mercury Thermometer:

Mercury Thermometer are those thermometers which are filled with mercury. Such type of thermometer are especially used to measure very high temperature as mercury is good conductor of heat, available in liquid form at NTP, whose ;

Boiling point=357°C

Freezing point=-19°C

ii) Alcohol thermometer:

Alcohol thermometer are filled with alcohol and are used to measure temperature of cold region because;

Boiling point= 78°C

Freezing point= -117°C

b. Gas Thermometer:

Gas Thermometer are based on the principle of change/ variation of pressure or volume with the change in temperature. These thermometers are more sensitive. H2, N2, He etc kinds of gases are used in this kind of thermometer. It has range of temperature form -270°C to 1600°C. Example; Constant volume H2 thermometer etc.

c. Resistance Thermometer:

Resistance thermometer depends on the principle that the resistance of a substance changes with temperature when current is flowing through it. In such thermometer material like Platinum is used whose resistance changes by current flowing through the platinum wire inside. It can measure the temperature ranging from -250°C to 1200°C. For example: Platinum resistance thermometer.

d. Thermoelectric thermometer:

Thermoelectric thermometer is based on principle of thermoelectricity i.e. production of thermo emf in a thermo couple when it's two junctions are kept at different temperature. This is very sensitive thermometer that provides accurate temperature results between -250°C to 1600°C. For example: Copper Constant thermometer.

e. Vapour Pressure Thermometer:

Vapour Pressure Thermometer is based on the principle of change in vapour pressure with the change in temperature. It is used to measure low temperature ranging from -272.5°C to -150°C. For example: Helium Vapor Pressure Thermometer etc.

On the basis of use thermometer can be classified into 2 types as follows:

i) Clinical thermometer:

Clinical thermometer is the thermometer used to measure the human body temperature. It has a constriction near its bulb. It consists of mercury. It is calibrated from 35°C to 42°C or 94°C to 108°F.

ii) Laboratory thermometer:

Laboratory thermometer is used to measure room and surrounding temperature or in scientific experiments. It has a wide range of thermometer reading from -10°C to 110°C.