What is internal energy change?

Short Answer

Internal energy change refers to the increase or decrease in the total energy stored inside a system due to heat transfer or work done. When heat is added or work is done on a system, its internal energy increases. When heat is removed or the system does work, its internal energy decreases.

This change in internal energy is represented by ΔU in thermodynamics. It depends on how much heat enters or leaves the system and how much work is done by or on the system. Internal energy change helps explain temperature rise, phase changes, and energy flow inside a system.

Detailed Explanation :

Internal Energy Change

Internal energy change refers to the variation in the total internal energy of a system when it absorbs or releases heat or performs work. Internal energy is the sum of all microscopic forms of energy inside a system. This includes:

  • Kinetic energy of molecules
  • Potential energy due to molecular forces
  • Vibrational and rotational energy
  • Energy stored in chemical bonds

The First Law of Thermodynamics expresses how internal energy changes. This helps us understand how systems behave when heated, cooled, compressed, or expanded.

Definition of Internal Energy Change

Internal energy change is defined as:

“The difference between the final internal energy and the initial internal energy of a system.”

Mathematically:

ΔU = U₂ − U₁

Where:

  • ΔU = Internal energy change
  • U₂ = Final internal energy
  • U₁ = Initial internal energy

If ΔU is positive → internal energy increases
If ΔU is negative → internal energy decreases

Relation with the First Law of Thermodynamics

The First Law states:

ΔU = Q − W

Where:

  • Q = Heat added to the system
  • W = Work done by the system

This means:

  • If heat is added (Q > 0), internal energy increases
  • If heat is removed (Q < 0), internal energy decreases
  • If work is done by the system (W > 0), internal energy decreases
  • If work is done on the system (W < 0), internal energy increases

Thus, internal energy change depends on both heat transfer and work.

How Internal Energy Changes

Internal energy changes in two major ways:

  1. Due to Heat Transfer

When a system absorbs heat:

  • Molecular motion increases
  • Temperature rises
  • Internal energy increases

Examples:

  • Heating water raises its internal energy
  • Sunlight increasing temperature of soil

When a system loses heat:

  • Molecular motion decreases
  • Temperature falls
  • Internal energy decreases

Examples:

  • Cooling a hot drink
  • Ice cube losing heat and remaining cold
  1. Due to Work Done

If work is done on the system:

  • Molecules get compressed
  • Potential and kinetic energy increase
  • Internal energy increases

Example: Compressing gas in a piston.

If work is done by the system:

  • Gas expands
  • Energy is used to push surroundings
  • Internal energy decreases

Example: Gas pushing a piston outward.

Internal Energy Change and Temperature

Internal energy is directly linked with temperature for ideal gases:

  • Higher temperature → higher internal energy
  • Lower temperature → lower internal energy

However, internal energy may change even without temperature change during phase transitions.

Internal Energy Change in Phase Changes

During melting, boiling, freezing, or condensation:

  • Heat is absorbed or released
  • Temperature remains constant
  • Internal energy changes due to change in molecular arrangement

Examples:

  • Ice melting → internal energy increases
  • Water freezing → internal energy decreases
  • Water boiling → internal energy increases
  • Steam condensing → internal energy decreases

Even though temperature does not change, internal energy changes because molecular structure changes.

Examples of Internal Energy Change in Daily Life

  1. Pumping Air Into a Tyre

Air is compressed → internal energy increases → tyre becomes warm.

  1. Cooling Food in a Refrigerator

Refrigerator removes heat → internal energy decreases → food cools.

  1. Steam Engines

Steam expands → internal energy decreases → work is done.

  1. Pressure Cooker

Heat increases internal energy → pressure rises → food cooks faster.

  1. Hot Water Bag

Water retains internal energy → slowly releases heat → provides warmth.

Importance of Internal Energy Change

Understanding internal energy change is important for:

  • Designing engines and refrigerators
  • Studying chemical reactions
  • Predicting temperature changes
  • Calculating work done in thermodynamic processes
  • Understanding heating and cooling systems
  • Analyzing natural processes like cloud formation and wind movement

Internal energy change helps engineers, scientists, and researchers control energy flow efficiently.

Internal Energy is a State Function

Internal energy depends only on the state of a system (temperature, pressure, volume), not on how it reached that state.
Therefore:

  • Internal energy change depends only on initial and final states
  • Path taken does not matter

This makes calculations easier and reliable.

Conclusion

Internal energy change refers to the increase or decrease in the internal energy of a system due to heat transfer or work. It is an important concept in thermodynamics and is expressed through the First Law of Thermodynamics. Internal energy change explains how systems heat up, cool down, expand, compress, and undergo phase changes. Understanding this concept helps in analyzing energy flow in engines, refrigerators, natural systems, and various physical processes.