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Illustrate how is the zeroth law of thermodynamics used to define temperature scales?
Zeroth law of thermodynamics::
''If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other.''
EXAMPLE :
Take two separate cups of boiling water.
First place a thermometer in the first cup, it gets warmed up by the water until it reads 100°C. We now say that the thermometer is in thermal equilibrium with the first cup of water.
Next, place the thermometer into the second cup of boiling water, and it continues to show 100°C. Therefore we can say that thermometeris also in thermal equilibrium with the second cup of water.
From the zeroth law, we can conclude that the two separate cups of boiling water are in thermal equilibrium with each other.
Therefore zeroth law enables us to use thermometers to compare the temperatures of any objects we like.
Using this law, the lower point of the scale is decided by placing thermometer in freezing water and upper point of the thermometer is decided by placing thermometer in boiling water.