Edexcel A Level (IAL) Physics-5.4 Internal Energy- Study Notes- New Syllabus

Internal Energy as the Random Distribution of Energy Among Molecules

Internal energy is the total energy stored within a substance due to the random motion and interactions of its molecules. It is a microscopic form of energy and cannot be observed directly.

What Is Internal Energy?

Internal energy is the sum of:

• Kinetic energy of molecules due to random motion
• Potential energy of molecules due to intermolecular forces

\( \text{Internal energy} = \text{random kinetic energy} + \text{random potential energy} \)

The word random emphasises that molecular motion is disordered, not directional.

 Random Kinetic Energy of Molecules

• Associated with the random motion of molecules
• Depends on temperature
• Higher temperature → greater average kinetic energy
• Includes translational, rotational, and vibrational motion

Important: Temperature is a measure of the average kinetic energy of molecules.

Random Potential Energy of Molecules

• Due to intermolecular forces between molecules
• Depends on separation between molecules
• Significant during phase changes
• Changes without changing temperature

Example: During melting, energy increases potential energy by weakening intermolecular bonds.

How Internal Energy Changes

Internal energy can change by:

• Heating: increases molecular kinetic energy
• Doing work on the system: increases internal energy
• Phase changes: increase potential energy

For example:

• Heating ice → increases kinetic energy (temperature rises)
• Melting ice → increases potential energy (temperature constant)

Key Distinction

• Internal energy ≠ temperature
• Temperature depends only on kinetic energy
• Internal energy depends on both kinetic and potential energy

Two objects at the same temperature can have different internal energies if their masses differ.

Importance in Thermodynamics

• Explains heating and cooling processes
• Explains phase changes
• Forms the basis of the first law of thermodynamics