Internal energy
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Explainer Video
Summary
Internal energy
In a nutshell
The internal energy of a system is defined as the sum of the kinetic and potential energies of all its particles. The temperature of a substance does not change while its phase is changing.
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Internal energy
Internal energy is defined as the sum of the random distribution of the kinetic and potential energies associated with the molecules of a system. It can be expressed as an equation:
The higher the temperature, the higher the internal energy of a system, as kinetic energy increases. As temperature decreases, the particles move less and they become completely immobile at absolute zero.
Absolute zero or is the lowest possible temperature and it is the point at which the internal energy is at a minimum. Kinetic energy is zero but potential energy is still stored between the particles.
Changing phases
When adding energy to a substance its kinetic energy increases, however when a phase change occurs the potential energy increases instead.
When reaching its melting or boiling point, the temperature of a substance does not increase as all the energy is transferred to the electrostatic potential energy which causes changes in the electrostatic forces holding the particles together.
This is better shown through state change graphs:
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As the substance reaches its melting or boiling point, the temperature stops increasing until the phase change is completed.
Electrostatic potential energy in different phases
Particles have different electrostatic potential energies depending on their phase:
- Solids- the electrostatic potential energy between molecules has a large negative value, due to large electrostatic forces.
- Liquids- the electrostatic potential energy between molecules has a moderate negative value, as energy is still required to break bonds.
- Gases- the electrostatic potential energy is zero due to the large distances between molecules.
Therefore, the electrostatic forces decrease when going from solids liquid gases, but the electrostatic potential energy increases.
FAQs - Frequently Asked Questions
What is internal energy?
Internal energy is defined as the random distribution of the kinetic and potential energies associated with the molecules of a system.
What is absolute zero?
Absolute zero or 0 K is the lowest temperature possible and is the point at which there is no kinetic energy in a system.
Why does temperature stay the same during phase changes?
Temperature stays the same during phase changes as heat increases the electrostatic potential energy of the system instead of the kinetic energy.