UNIT 27 Internal Energy

To see how the various components in the internal energy arise, let us begin by considering a system of just two atoms. When they are at their equilibrium separation, and stationary, there is neither potential nor kinetic energy, so the internal energy is simply zero. If we now disturb the system by increasing the interatomic separation, by a few per cent say, the system will acquire potential energy. If the atoms are then released, they are free to move and they will approach each other with increasing velocity, ultimately passing the equilibrium separation and then overshooting this, so that the separation is now less than the equilibrium distance. As the equilibrium point is passed, the potential energy will be momentarily zero, but the kinetic energy will be finite (and at its maximum value for the given starting conditions). The initial potential energy of the displacement will thus have been fully converted into kinetic energy. Ultimately, when the overshoot is such that the potential energy has again reached the value it had when the atoms were released, the kinetic energy will once more have become zero. The direction of motion will then be reversed, and the interatomic separation will again start to increase. As it passes the equilibrium separation, there will again be zero potential energy but finite (and maximum) kinetic energy. If the system is not further disturbed, these oscillations will continue indefinitely, the mutually staggered situations of ξ_pot = 0 and ξ_kin= 0 regularly alternating. In between these extremes, both potential energy and kinetic energy will of course be finite.

As can readily be imagined, the situation becomes far more complicated if the number of atoms is increased, and it is then best handled statistically. In an arbitrarily disturbed system, it will be highly unlikely that all atoms simultaneously have zero kinetic energy, and in any event the zero potential energy situation cannot prevail unless the number of participating atoms is very small. This is not to say that equilibrium cannot be attained, however, because a balance between repulsive and attractive forces can still be achieved even though some of the interatomic spacings differ from the equilibrium two-body value.

Define the following words using a dictionary

To release, velocity, to prevail, finite, an overshoot, to alternate, interatomic spacing

Continue the sentences

1. When two atoms are stationary,…
2. If we now disturb the system by increasing the interatomic separation…
3. If the atoms are released,...
4. As the equilibrium point is passed,…
5. The direction of motion will then be reversed,…
6. If the system is not further disturbed,…
7. the situation becomes far more complicated if…
8. In an arbitrarily disturbed system, it will be highly unlikely that
9. This is not to say that…