CIE AS/A Level Physics 14.1 Thermal equilibrium Study Notes- 2025-2027 Syllabus
CIE AS/A Level Physics 14.1 Thermal equilibrium Study Notes – New Syllabus
CIE AS/A Level Physics 14.1 Thermal equilibrium Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Physics latest syllabus with Candidates should be able to:
- understand that (thermal) energy is transferred from a region of higher temperature to a region of lower temperature
- understand that regions of equal temperature are in thermal equilibrium
Understanding Thermal Energy Transfer
Thermal (heat) energy is the energy transferred due to a temperature difference between two regions or bodies. It always flows from the hotter region (higher temperature) to the colder region (lower temperature).
Key Concept:
\( \mathrm{Energy \ flows \ spontaneously \ from \ higher \ temperature \ to \ lower \ temperature.} \)
Explanation:
- Temperature is a measure of the average kinetic energy of particles in a substance.
- When two bodies at different temperatures come into contact, faster-moving particles (in the hotter region) collide with slower-moving particles (in the cooler region).
- As a result, energy is transferred from the particles with higher kinetic energy to those with lower kinetic energy until both reach the same temperature.
- The transfer continues until there is no net flow of thermal energy — the bodies are then in thermal equilibrium.
Modes of Thermal Energy Transfer:
- Conduction: Transfer through direct particle collisions (mainly in solids).
- Convection: Transfer through bulk movement of fluid (liquids and gases).
- Radiation: Transfer through electromagnetic waves (no medium required).
Thermal energy always moves from hotter to colder regions until temperatures equalise — this is a fundamental law of thermodynamics.
Example
Explain what happens when a metal spoon is placed in a hot cup of tea.
▶️ Answer / Explanation
Initially, the tea has a higher temperature than the spoon. Thermal energy is transferred from the hot tea to the cooler spoon by conduction through particle collisions. The particles in the spoon gain kinetic energy, causing the spoon to heat up until it reaches the same temperature as the tea.
Example
Why does a cold drink become warmer when left in a warm room?
▶️ Answer / Explanation
The air in the room is at a higher temperature than the drink. Thermal energy is transferred from the warmer air molecules to the cooler liquid molecules by convection and conduction through the container walls. The drink absorbs energy until it reaches the same temperature as its surroundings.
Understanding Thermal Equilibrium
Two or more bodies are said to be in thermal equilibrium when they are at the same temperature and there is no net transfer of thermal energy between them.
Condition for Thermal Equilibrium:
\( \mathrm{T_1 = T_2 \ \Rightarrow \ No \ net \ heat \ flow.} \)
Explanation:
- When two bodies of different temperatures come into contact, heat flows from the hotter to the colder one.
- As heat transfers, the hotter body cools down and the colder body warms up.
- Eventually, both reach the same temperature — at this point, the system is in thermal equilibrium.
- In thermal equilibrium, microscopic energy exchanges still occur, but there is no net gain or loss of energy between the bodies.
Key Principle (Zeroth Law of Thermodynamics):
If body A is in thermal equilibrium with body B, and body B is in thermal equilibrium with body C, then A and C are also in thermal equilibrium. Thermal equilibrium occurs when all parts of a system are at the same temperature and no further heat transfer takes place.
Thermal Energy and Equilibrium
| Concept | Definition | Direction of Energy Flow | Condition |
|---|---|---|---|
| Thermal Energy Transfer | Movement of energy due to a temperature difference. | From higher temperature to lower temperature. | Occurs until both regions reach same temperature. |
| Thermal Equilibrium | State where bodies in contact have equal temperature. | No net energy transfer. | \( \mathrm{T_1 = T_2.} \) |
Example
A cup of tea is left in a room at \( \mathrm{25^\circ C.} \) Explain what happens to its temperature over time.
▶️ Answer / Explanation
The hot tea (at higher temperature) transfers thermal energy to the surrounding air (at lower temperature). The tea cools down while the surrounding air gains a small amount of energy. Eventually, both reach the same temperature — the tea is then in thermal equilibrium with the room.
Example
Two blocks of metal, one at \( \mathrm{90^\circ C} \) and the other at \( \mathrm{20^\circ C,} \) are brought into contact. Describe what happens.
▶️ Answer / Explanation
Heat flows from the \( \mathrm{90^\circ C} \) block to the \( \mathrm{20^\circ C} \) block. The hotter block loses energy (its particles slow down), while the cooler block gains energy (its particles speed up). After some time, both blocks reach the same intermediate temperature — they are now in thermal equilibrium, and no further net heat transfer occurs.