1. What Is Temperature?
Temperature is the physical quantity that tells us how hot or cold a body is. It is a measure of the average kinetic energy of the particles inside a substance.
A higher temperature means the particles are moving more energetically, and a lower temperature means they are moving more slowly.
2. How Temperature Determines Heat Flow
Heat always flows from a body at higher temperature to a body at lower temperature. Temperature difference is the driving force for heat transfer.
Once temperatures become equal, heat flow stops and the bodies reach equilibrium.
2.1. Everyday Examples
- Ice melts when placed in warm water because heat flows from the warm water to the cold ice.
- A hot metal spoon in cold water cools down as heat flows from spoon to water.
3. What Is Thermal Equilibrium?
Thermal equilibrium is a condition where two bodies in contact have the same temperature and no net heat flows between them.
Once equilibrium is reached, their temperatures remain constant unless an external influence acts.
3.1. Reaching Thermal Equilibrium
When two systems at different temperatures are brought into contact, heat begins to flow from the hotter to the colder system. This continues until both reach the same temperature.
3.2. Examples of Thermal Equilibrium
- A thermometer left in your mouth eventually shows a steady reading—that is thermal equilibrium.
- Tea left on a table cools and reaches room temperature after some time.
4. Temperature as a State Variable
Temperature is a state variable, meaning it depends only on the current condition of the system and not on how the system reached that state. It uniquely helps define the state of a system along with pressure, volume, and internal energy.
4.1. Why Temperature Is Reliable
Temperature allows comparison between systems even if they are not in direct contact. This makes it one of the most important variables in thermodynamics.
5. Role of Temperature in Thermodynamics
Temperature plays a central role in almost every thermodynamic process. It determines heat flow, helps define equilibrium, influences work done by gases, and appears in all major thermodynamic laws.
5.1. Key Connections
- Heat flows due to temperature difference.
- Zeroth Law uses temperature to define equilibrium.
- First Law relates heat, temperature, and internal energy.
- Second Law explains the direction of natural processes using temperature.
6. How Thermometers Use Thermal Equilibrium
Any thermometer works by reaching thermal equilibrium with the object whose temperature is being measured. Once the thermometer and the object have the same temperature, the reading becomes stable.
6.1. Examples of Thermometer Use
- A clinical thermometer measures body temperature after coming in contact with skin or mouth.
- Lab thermometers measure the temperature of liquids by reaching equilibrium with them.