CIE AS/A Level Physics 16.1 Internal energy Study Notes- 2025-2027 Syllabus
CIE AS/A Level Physics 16.1 Internal energy Study Notes – New Syllabus
CIE AS/A Level Physics 16.1 Internal energy 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 internal energy is determined by the state of the system and that it can be expressed as the sum of a random distribution of kinetic and potential energies associated with the molecules of a system
- relate a rise in temperature of an object to an increase in its internal energy
Internal Energy of a System
The internal energy of a system is the total energy contained within it due to the motion and interactions of its molecules. It depends only on the state of the system (its temperature, volume, and pressure), not on how the system arrived at that state.
Internal energy consists of two components:
- Kinetic energy of the molecules (due to their random motion)
- Potential energy of the molecules (due to intermolecular forces)
Internal energy = random molecular kinetic energy + intermolecular potential energy
Key Ideas:
- A system’s internal energy increases if the molecules move faster (higher temperature).
- Potential energy changes when distances between molecules change (e.g., during phase changes).
- Internal energy is a state function: it depends only on the current conditions, not the history.
For an ideal gas, internal energy depends only on kinetic energy because intermolecular forces are assumed negligible (so potential energy is zero).
Example
Describe how the internal energy of a gas changes when it is heated at constant volume.
▶️ Answer / Explanation
Heating at constant volume increases the random kinetic energy of the molecules (they move faster). Since the container’s volume does not change, their potential energy remains nearly constant. Thus, the internal energy increases mainly due to increased molecular kinetic energy.
Example
Explain why the internal energy of ice increases during melting even though the temperature stays constant at 0°C.
▶️ Answer / Explanation
During melting, the temperature remains constant, so the average kinetic energy of the molecules does not increase. However, thermal energy is still absorbed to break intermolecular bonds, increasing molecular potential energy. Therefore, internal energy increases due to an increase in potential energy, not kinetic energy.
Example
A gas is compressed slowly so that its temperature increases. Explain in terms of kinetic and potential energy how the internal energy changes.
▶️ Answer / Explanation
During slow compression, work is done on the gas, reducing the volume. This forces molecules closer together, which:
- increases their average kinetic energy (temperature rises),
- increases their potential energy because intermolecular forces become more significant at shorter separations.
Since both kinetic and potential energy increase, the total internal energy of the gas increases.
Temperature Rise and Internal Energy
When the temperature of an object increases, its internal energy also increases.
This is because temperature is directly related to the average kinetic energy of the molecules within the object.
Key Idea:
A rise in temperature means the molecules move faster → their kinetic energy increases → the internal energy increases.
Important Points:
- Internal energy = molecular kinetic energy + molecular potential energy.
- For most heating processes (without phase change), the temperature rise is due almost entirely to an increase in kinetic energy.
- If no change of state occurs, potential energy changes very little.
- The increase in internal energy is proportional to the heat supplied (for constant mass).
When temperature rises:
- Molecules vibrate faster (solids)
- Molecules move faster (liquids and gases)
- Average kinetic energy increases
- Internal energy increases
Example
A metal rod is heated from 20°C to 40°C. Explain what happens to the internal energy of the rod.
▶️ Answer / Explanation
As the rod’s temperature rises, the atoms vibrate more vigorously. Their average kinetic energy increases, causing the internal energy of the rod to increase.
Example
Two blocks (A and B) receive identical amounts of heat. Block A increases in temperature by 15°C, while block B increases by only 5°C. Which block has the higher increase in internal energy?
▶️ Answer / Explanation
Both blocks receive the same heat energy, so both experience the same increase in internal energy. The difference in temperature rise is due to their different specific heat capacities, not different internal energy changes.
Example
A gas in a sealed container is heated, causing its temperature to rise. Explain how both kinetic and potential energy components of internal energy may change.
▶️ Answer / Explanation
The rise in temperature means gas molecules move faster, so their average kinetic energy increases. If heating also causes the gas to expand slightly, the average distance between molecules increases, affecting intermolecular forces and increasing potential energy slightly. Thus, internal energy increases through increases in both kinetic and potential energies.