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Internal Energy Q

The internal energy (sometimes called the random thermal energy) of a substance is the sum of its kinetic energy and potential energy.

Potential energy is due to the interaction of neighbouring particles, this is therefore very significant in solids and liquids but less so in gases.

Kinetic energy is due to the movement of the particles in the substance - it is therefore very important in gases, less so in liquids and negligible in solids.

In the realm of thermodynamics we are interested in the internal workings of matter. That is the sum total of all of the energies of the particles in it.

The energy that a particle possesses depends on two factors:

how tightly it is held onto by other particles around it - its potential energy - the higher this value is the more tightly it is held

how much movement or kinetic energy it has - the higher the temperature, the faster it moves and the bigger this factor is

The restraining influence of the other particles surrounding a particle, works against the particle whizzing off - its kinetic energy.

So, if a particle has net 'negative' energy it means that the 'potential' enegy is greater than the kinetic energy and the particle is not 'free' - it will be constrained in a solid or liquid form. If it has no potential energy - only kinetic energy - then it is free! (an ideal gas particle).

Water particles at 0oC have the same average kinetic energy as ice at 0oC but ice has less internal energy because the negative potential energy it has is greater than in water. Energy has to be 'put in' from the outside to melt the ice. When ice melts, the energy that is supplied to it has to break down the rigid structure of the ice crystal and 'free' the water particles.