By the touch of our hand, we can roughly determine which is the hottest or coldest object.
The hotness or coldness of an object was called temperature.
If two objects give the same feeling of being equally hot (or cold), they are said to have the same temperature, and objects are said to be in thermal equilibrium with each other.
Zeroth Law Of Thermodynamics:
Just Like Newton’s laws of motion and Kepler’s laws of planetary motion, thermodynamics had also first, second and third, but later on development of the field, a more fundamental law was added and dubbed the zeroth law.
Statement Of Zeroth Law Of Thermodynamics:
The Zeroth law of thermodynamics states that
If two bodies A and B are separately in thermal equilibrium with third body C, they are also in thermal equilibrium with each other.
Derivation Of Zeroth Law Of Thermodynamics:
Consider Three bodies A, B and C. Let TA, TB, and TC be the temperature of bodies A, B, and C respectively.
Since A and C are in thermal equilibrium,
TA = TC
Since B and C are in thermal equilibrium,
TB = TC
From eq(1) & eq(2),
TA = TB
Since TA = TB, A and B are in thermal equilibrium.
Applications or Examples of Zeroth Law Of Thermodynamics:
- Thermometer for measuring body temperature
- The hot coffee cools down after a while
- Measurement of temperature in the food processing industries
- Coldwater and hot water
- To measure atmospheric pressure
Thermometer for measuring body temperature
The thermometer is made of a long and narrow glass tube with a bulb at one end. The narrow tube consists of mercury. Mercury(Hg) is liquid at room temperature and, it expands or contracts at a small temperature change.
When a thermometer is in contact with a person having fever, the temperature of mercury increases, and it expands, so it’s level rise and reading correspond to the level of mercury will be the numerical value of body temperature in degree Fahrenheit or degree Celsius.
In this example of a thermometer, mercury and the human body are at the same temperature, so we can say that they are in thermal equilibrium with each other.
The hot coffee cools down after a while:
Hot coffee cools down after some time if we did not drink it.
Hot coffee continuously loses heat until its temperature equals to room temperature.
When the room temperature and temperature of the coffee are equal, coffee is in thermal equilibrium with the surrounding.
Measurement of temperature in the food processing industries:
In food processing industries pyrometer or infrared thermometer is used.
The working of the pyrometer is based on the zeroth law of thermodynamics.
Everybody having mass emits some amount of energy, and this energy emits in the form of heat.
A pyrometer measures the food temperature by sensing this heat emitted from the object without making contact with it.
Coldwater and hot water:
Consider two glasses of cold water(with ice in it) and hot water.
After some time, ice in the first glass will melt, and water in that glass will come at room temperature.
Similarly, hot water in the second glass will be cool, and it will also be at room temperature.
In this case of cold and hot water in a glass, water in both glasses are in thermal equilibrium with the surrounding, and they have the same temperature.
This follows the zeroth law of thermodynamics.
To measure atmospheric pressure:
Atmospheric temperature means how cold or hot the atmosphere is. Atmospheric temperature can be measured using a traditional thermometer consisting of mercury.
Atmospheric temperature can be measured as discussed in the example of measurement of body temperature.