18.2:
Zeroth Law of Thermodynamics
The zeroth law of thermodynamics states that if any two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.
To understand this experimentally, consider three beakers filled with water at three different temperatures.
After connecting beaker A with B and beaker B with C, heat transfer will take place.
After a period of time, the water in beaker A arrives at thermal equilibrium with the water in beaker B, and the water in beaker B arrives at equilibrium with the water in beaker C.
On recording the temperature of the beakers, a uniform temperature is observed in all three beakers, which corresponds to the zeroth law of thermodynamics.
The most significant application of the zeroth law of thermodynamics is the different types of thermometers.
When a thermometer is used to measure the temperature of a body, it attains thermal equilibrium by changing its own temperature.
Thus, the measured temperature of the body is actually the temperature of the thermometer.
18.2:
Zeroth Law of Thermodynamics
Experimentally, if object A is in equilibrium with object B, and object B is in equilibrium with object C, then object A is in equilibrium with object C. That statement of transitivity is called the "zeroth law of thermodynamics." For example, a cold metal block and a hot metal block are both placed on a metal plate at room temperature. Eventually, the cold block and the plate will be in thermal equilibrium. In addition, the hot block and the plate will be in thermal equilibrium. By the zeroth law, we can conclude that the cold block and the hot block are also in thermal equilibrium.
British physicist Ralph Fowler suggested the number "zeroth" in the 1930s. At that time, the first, second, and third laws of thermodynamics were already named and numbered. The zeroth law had seldom been stated, but it needed to be discussed before the others, so Fowler gave it a smaller number.
Now consider the case where A is a thermometer. The zeroth law tells us that if A reads a certain temperature when in equilibrium with B, and it is then placed in contact with C, it will not exchange energy with C; therefore, its temperature reading will remain the same. In other words, if two objects are in thermal equilibrium, they have the same temperature.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/books/university-physics-volume-2/pages/1-1-temperature-and-thermal-equilibrium