Internal Energy (Copy)

What Is Internal Energy?

• Internal energy is the total microscopic energy possessed by the molecules of a system due to their motion and positions.
• It is a state function — it depends only on the current condition (temperature, pressure, volume) of the system, not on how the system reached that state.

Internal Energy = Kinetic Energy + Potential Energy

The internal energy of a substance is made up of:

1. Random Kinetic Energy

• Due to the random translational, rotational, and vibrational motion of molecules.
• Directly related to temperature.
• In an ideal gas, where no intermolecular forces exist, internal energy consists entirely of kinetic energy.

2. Potential Energy

• Due to intermolecular forces (attractive or repulsive) between molecules.
• Becomes significant in liquids and solids, or when gases are compressed or cooled.
• Changes in potential energy occur during phase changes (melting, boiling, condensing).

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

Internal Energy is a Function of State

• Internal energy depends only on the current state of the system:
  • Temperature
  • Volume
  • Pressure
  • Phase (solid, liquid, gas)
• It is not affected by the path or process (e.g. whether energy was added through heating or compression).

Changes in Internal Energy

Internal energy can change in two main ways:

1. Heating

• When a substance is heated, energy is transferred to it.
• This usually increases the average kinetic energy of particles → temperature rises.
• If the substance undergoes a phase change, energy is used to increase potential energy without a temperature rise.

2. Doing Work

• A system can gain internal energy if work is done on it (e.g. compression of a gas).
• A system loses internal energy if it does work on the surroundings (e.g. expansion).

Temperature and Internal Energy

• Temperature is a measure of average kinetic energy of the molecules.
• Therefore:
  • An increase in temperature → an increase in internal energy.
  • The change is only in the kinetic energy part, unless there’s a phase change.

Internal Energy in Different Phases

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

Important Points

• Internal energy is microscopic, unlike mechanical energy (which is macroscopic).
• It is not directly measurable, but changes in internal energy can be observed by:
  • Measuring temperature changes
  • Observing phase changes
• Internal energy increases when:
  • Heat is supplied
  • Work is done on the system (compression)
• Internal energy decreases when:
  • Heat is removed
  • Work is done by the system (expansion)

Heating vs Doing Work

Real-World Examples

• Friction: Increases internal energy by converting mechanical energy into random motion (heat).
• Compression pumps: Doing work on air increases its temperature by increasing internal energy.
• Boiling water: Temperature remains constant, but internal energy increases due to rising potential energy (breaking bonds).

Summary Table

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course