1. Introduction to Zeroth Law of Thermodynamics
The Zeroth Law of Thermodynamics is one of the four fundamental laws of thermodynamics. It establishes the concept of temperature and explains how thermal equilibrium is achieved between bodies. This law forms the foundation for temperature measurement and is the reason why thermometers work.
Although it was formulated after the First and Second Laws of Thermodynamics, scientists realized that it was more fundamental than the other laws. Therefore, it was named the Zeroth Law instead of the Fourth Law.
Thermodynamics deals with the study of heat, temperature, work, and energy transfer. Before understanding heat transfer or energy conversion, it is essential to know whether two objects are at the same temperature.
The Zeroth Law provides the basis for comparing temperatures. It states that if two systems are separately in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.
This principle allows engineers and scientists to define temperature as a measurable physical property.
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2. History of the Zeroth Law
The concept of thermal equilibrium existed long before the formal development of thermodynamics.
In 1931, British physicist Ralph H. Fowler recognized that this principle was more fundamental than the First and Second Laws. Since those laws had already been numbered, he named this principle the Zeroth Law of Thermodynamics.
3. Zeroth Law of Thermodynamics
The zeroth law of thermodynamics states that, ” If two systems are each in thermal equilibrium with a third system, then the three system will be in equilibrium with each other “.
When System, ‘A’, is in thermal equilibrium with another System, ‘B’, and also separately in thermal equilibrium with a system, ‘C’, then system, ‘B’ and ‘C’, will also be in thermal equilibrium with each other refere Fig. 1.1. This statement defines the Zeroth law of thermodynamics. The law is based on temperature measurement.
In simple term, it can be said, “Systems that are in thermal equilibrium exist at the same temperature”.
Example:
We can observe the Zeroth law in action by taking a very common thermometer having mercury in a tube. As the temperature increases, this mercury expands since the area of the tube is constant. Due to this expansion, the height is increased. Now, the increase in the height of the mercury label shows the changes in temperature and basically helps us to measure it.
4. What is Thermal Equilibrium?
Thermal equilibrium is the condition in which two or more bodies have the same temperature, so there is no net heat transfer between them.
Conditions for Thermal Equilibrium
- Same temperature
- No temperature gradient
- No heat flow
- Stable thermodynamic condition
5. Why is it Called the Zeroth Law?
The First and Second Laws had already been established before scientists realized that thermal equilibrium was even more fundamental.
Since changing the numbering of the existing laws would have caused confusion, this law was inserted before them and named the Zeroth Law.
6. Real life Examples
1. Measuring Body Temperature
A thermometer is placed under the tongue.
After a few seconds,
- Body temperature equals thermometer temperature.
- No further heat transfer occurs.
- The thermometer displays the correct reading.
This directly demonstrates the Zeroth Law.
2. Cup of Tea
A thermometer dipped into hot tea eventually reaches the same temperature as the tea.
Both become thermally balanced.
3. Room Air Conditioner
After sufficient time, the room air reaches the thermostat’s sensing temperature.
The thermostat then controls the AC based on thermal equilibrium.
4. Refrigerator
Food, air, and refrigerator shelves gradually reach nearly the same temperature.
5. Laboratory Thermometers
Mercury or digital thermometers measure temperature only after thermal equilibrium is achieved.
7. Practical Engineering Applications
1. Temperature Measurement
- Mercury thermometers
- Alcohol thermometers
- Digital thermometers
- Thermocouples
- RTDs (Resistance Temperature Detectors)
- Infrared thermometers (after calibration)
2. Power Plants
Used for monitoring:
- Steam temperature
- Boiler temperature
- Condenser temperature
- Feedwater temperature
3. Internal Combustion Engines
Helps measure:
- Coolant temperature
- Lubricating oil temperature
- Exhaust gas temperature
- Cylinder wall temperature
4. Refrigeration and Air Conditioning
Essential for:
- Thermostats
- Temperature controllers
- Refrigerant monitoring
5. Manufacturing Industries
Temperature control is critical in:
- Heat treatment
- Welding
- Casting
- Forging
- Plastic molding
6. Chemical Industries
Maintains process temperatures in reactors, distillation columns, and heat exchangers.
8. Importance in Mechanical Engineering
The Zeroth Law is essential because it:
- Defines temperature.
- Forms the basis of thermometry.
- Enables calibration of temperature sensors.
- Supports heat transfer analysis.
- Is fundamental to HVAC systems.
- Is used in boiler and turbine instrumentation.
- Ensures process quality in manufacturing.
9. Advantages
- Simple and universally applicable.
- Basis for all temperature measurements.
- Essential for thermodynamic analysis.
- Supports instrument calibration.
- Widely used in engineering and science.
10. Limitations
- Applies only to thermal equilibrium.
- Does not explain heat transfer rates.
- Does not describe energy conversion.
- Does not predict system efficiency.
- Not applicable during transient (non-equilibrium) conditions.
11. Comparison with Other Laws
| Law | Main Concept |
|---|---|
| Zeroth Law | Thermal equilibrium and temperature |
| First Law | Conservation of energy |
| Second Law | Direction of heat flow and entropy |
| Third Law | Entropy approaches zero at absolute zero temperature |