What is internal energy (U), and why is it important?

Here’s a explanation:

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Internal Energy (U)

• Definition: Internal energy is the total energy contained within a system.
• It includes:
  • Kinetic energy of molecules (movement of particles)
  • Potential energy of molecules (energy due to interactions between particles)
• It does not include energy from the system moving as a whole—only energy inside the system counts.

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Why is Internal Energy Important?

1. Measures the energy of a system:
   • It tells us how much energy the molecules inside the system have.
2. Relates to heat and work:
   • When a system absorbs heat or does work, its internal energy changes.
   • ΔU = Heat added to the system – Work done by the system
3. Helps predict chemical and physical changes:
   • Reactions, phase changes, and other processes involve changes in internal energy.
4. State function:
   • Only the initial and final energy matter, not the path taken.

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Example:

• Boiling water: Heat increases the water molecules’ movement → internal energy rises.
• Compressed gas: Work done on the gas → internal energy increases.

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