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…
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.
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Q1: What does the zeroth law of thermodynamics state?
The zeroth law states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This principle establishes transitivity: if object A equilibrates with object B, and object B equilibrates with object C, then A and C are also in equilibrium. This foundational concept enables temperature measurement and comparison across multiple systems.
Q2: How does the zeroth law apply to thermometer measurements?
When a thermometer measures an object's temperature, it reaches thermal equilibrium by changing its own temperature. The measured reading reflects the thermometer's temperature, not the object's initial state. By the zeroth law, if the thermometer and object are in equilibrium, they share the same temperature, making thermometers reliable measurement tools.
Q3: Why is the zeroth law called 'zeroth' rather than first?
British physicist Ralph Fowler suggested the name 'zeroth' in the 1930s because the first, second, and third laws of thermodynamics were already established and numbered. The zeroth law needed to be discussed before the others logically, so Fowler assigned it a smaller number to reflect its foundational importance in thermodynamic theory.
Q4: What happens when three beakers at different temperatures are connected?
When beakers A and B are connected, heat transfers until they reach thermal equilibrium. When B and C are then connected, they also equilibrate. Eventually, all three beakers reach the same uniform temperature, demonstrating the zeroth law experimentally. This shows that thermal equilibrium is transitive across multiple bodies.
Q5: How do cold and hot blocks demonstrate the zeroth law on a metal plate?
A cold metal block and hot metal block placed on a room-temperature metal plate will each reach equilibrium with the plate separately. By the zeroth law, since both blocks equilibrate with the plate, they must also be in thermal equilibrium with each other, even without direct contact. This illustrates that equilibrium is transitive through an intermediary.
Q6: What does thermal equilibrium tell us about temperature equality?
If two objects are in thermal equilibrium, they have the same temperature. This relationship is central to the zeroth law: it defines temperature as the property that is equal when objects are in equilibrium. Without this principle, temperature measurement and comparison between different systems would be impossible.
Q7: Why doesn't a thermometer exchange energy with objects already in equilibrium?
When a thermometer is placed in contact with an object already in thermal equilibrium, no temperature difference exists between them. Since energy transfer requires a temperature gradient, no heat flows. The thermometer's reading remains unchanged, confirming that both objects share the same temperature and are in equilibrium.