Zeroth law of Thermodynamics

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The zeroth law of thermodynamics states that if two systems A and B are separately in thermal equilibrium with a third system C, then the systems A and B when brought into thermal contact will also be in thermal equilibrium. This law indicates that for systems in thermal equilibrium there must exist a common property which remains the same. This property is the temperature. Thus Zeroth law defines the temperature.

For two bodies or systems in contact, heat flows from higher temperature to lower temperature. Temperature gives a measure of the hotness or coldness of body. A hot body has more internal energy then another identical cold body. When a hot body is kept

In contact with a cold body. “There is a transport of energy from hot body to cold body. The energy transferred is called heat”.

if two bodies A and B are in thermal equilibrium, A and C are in Thermal equilibrium and B and C are also  in thermal equilibrium.

Scales of Temperature:

Name of Scale Celsius Fahrenheit Kelvin New
F.P. of H2O 0oC 32oF 273oK FPOT
B.P. of H2O 100OC 212OF 373OK BPOT


- When a person gets fever, the hotness of the body is measured by Thermometer. Thermometer is kept in contact with the sick person, so heat from person’s body transfers into the mercury in the thermometer and the volume of mercury increases. When the temperature of the body and that of the mercury in thermometer becomes one and the same, the increase in volume of mercury stops and scale on thermometer is measure which shows the value to fever on person’s body.

- If we keep two bottle of water which has two different temperature.  If we bring 1 bottle from fridge  which is too cold and second bottle filled with hot water. We kept both the bottle at normal room temperature for a while. We will see the changes in the temperature in both the bottle and it will come to equal temperature.  This is called thermal equilibrium. 

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