4.2 What is the internal energy of a substance

(KE shorthand for kinetic energy)

• The particles of solids, liquids and gases all have kinetic energy (KE).

• In solids the particles vibrate with kinetic energy but can't move around to another position, but in gases and liquids the particles freely move from place to place with kinetic energy.

• Particles also have energy in their potential energy stores due to their positions - the motion from their kinetic energy keeps them separated as it opposes the forces attracting the particles together.

  • The particles in gases have the most potential energy because they are the farthest apart.

  • In potential energy order: gases >> liquids > solids

  • Remember there is a little space on average between liquid particles, but virtually non between the particles of a solid.

• Therefore the internal energy of a system is stored by the particles (atoms, ions, molecules) because of their kinetic energy and spacing-position.

  • Total internal energy of particle system = kinetic energy store plus potential energy store

  • The thermal energy store of the particles = kinetic energy stores of the particles (see Part 4.1)

• When you heat a system energy is transferred to the particles eg they move faster in gases and liquids (increase in KE of movement from one place to another) or the particles vibrate more strongly in a solid (increase in vibrational KE), so the internal energy is increased when you heat a material.

  • Here, due to increase n temperature, the increase in the thermal energy store is effectively the increase in kinetic energy store of the particles.

• This absorption of heat, ie increase in internal energy can cause an increase in temperature OR a change of state e.g. melting or boiling if the particles are given sufficient thermal energy.

• Removing heat decreases the internal energy, so the material cools to a lower temperature OR undergoes a change of state e.g. condensing or freezing.

• The size of the change depends on the energy input, the mass of substance involved and the specific heat capacity (which depends on the nature of the material).

Know the internal energy of a substance is the sum of the kinetic energy of the particles plus the potential energy.

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INDEX of notes on Particle model theory state changes and latent heat