Thermal equilibrium is the condition in which objects in contact no longer exchange net heat because they have the same temperature. It matters because temperature is only meaningful when it can be compared reliably between objects. The Zeroth Law of Thermodynamics gives the logical foundation for measuring temperature.
It explains why a thermometer can tell us the temperature of a cup of water, a metal block, or the air in a room.
Key Facts
- Thermal equilibrium means no net heat flows between objects in thermal contact.
- Zeroth Law: If A is in thermal equilibrium with C, and B is in thermal equilibrium with C, then A is in thermal equilibrium with B.
- Heat flows spontaneously from higher temperature to lower temperature: hot to cold.
- At equilibrium, temperatures are equal: T_A = T_B.
- A thermometer measures temperature by reaching thermal equilibrium with the object it touches.
- Temperature is related to average particle kinetic energy, but heat is energy transferred because of a temperature difference.
Vocabulary
- Thermal equilibrium
- A state in which two or more objects in thermal contact have the same temperature and no net heat flows between them.
- Zeroth Law of Thermodynamics
- The rule that if two objects are each in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.
- Temperature
- A measure related to the average kinetic energy of particles in a substance.
- Heat
- Energy transferred between objects because of a temperature difference.
- Thermometer
- A device that measures temperature by coming into thermal equilibrium with the object or environment being measured.
Common Mistakes to Avoid
- Confusing heat with temperature. Temperature describes the thermal state of an object, while heat is energy transferred because temperatures are different.
- Thinking heat always means something contains a lot of energy. Heat is not stored in an object, it is energy in transit from warmer matter to cooler matter.
- Assuming equilibrium means two objects have the same amount of thermal energy. Objects at the same temperature can contain different total thermal energies if their masses or materials differ.
- Reading a thermometer before it reaches equilibrium. The thermometer must exchange energy with the object until its reading stops changing.
Practice Questions
- 1 Object A is at 80°C and object B is at 20°C. They are placed in thermal contact in an insulated container. In which direction does heat initially flow, and when will net heat flow stop?
- 2 A thermometer initially at 22°C is placed into water. After a short time its reading becomes 60°C and then remains constant. What is the temperature of the water, assuming the thermometer has reached thermal equilibrium?
- 3 Objects A and B are not touching. A is in thermal equilibrium with thermometer C, and B is also in thermal equilibrium with thermometer C. Explain what the Zeroth Law allows you to conclude about A and B.