Internal energy

Summary and Exam Tips for Internal Energy

Internal energy is a subtopic of Thermodynamics, which falls under the subject Physics in the Cambridge International A Levels curriculum. Internal energy ($U$) is the total of kinetic and potential energies within a system's molecules, influenced by the phase of the substance—solid, liquid, or gas. It is measured in Joules (J) and is determined by factors such as temperature, molecular motion, and the phase of matter. Generally, gases possess higher internal energy compared to solids. Internal energy can increase through work done on the system or by adding heat, and it decreases when heat is lost to the surroundings.

The relationship between internal energy and temperature is crucial. In gases, heating increases kinetic energy, causing molecules to move faster, while in solids, it enhances vibrational motion. Both liquids and solids contain kinetic and potential energy. For ideal gases, which are assumed to have no intermolecular forces, internal energy is purely kinetic and directly proportional to temperature change ($\Delta U \propto \Delta T$). This means that heating an ideal gas results in increased kinetic energy and a corresponding rise in temperature.

Exam Tips

• Understand Key Concepts: Grasp the distinction between kinetic and potential energy and how they contribute to internal energy.
• Memorize Relationships: Remember that for ideal gases, $\Delta U \propto \Delta T$, meaning internal energy changes are directly linked to temperature changes.
• Phase Matters: Be aware of how the phase of a substance (solid, liquid, gas) affects its internal energy.
• Units and Symbols: Ensure you know that internal energy is symbolized by $U$ and measured in Joules (J).
• Practical Examples: Practice with examples involving changes in internal energy due to heating or cooling to solidify your understanding.