Currents

When you consider a current of electricity, electrons are flowing around the circuit. However for this to happen the electrons must detach themselves from their parent atom where they were orbiting around a nucleus. To do this the electrons must overcome the attractive force of the nucleus and so must increase in energy. There are many excited states that the electron can exist in, and although the difference in energy between the ground state (original energy level) and the excited state are precisely prescribed, the change in energy level is responsible for many effects.

For instance when metals are heated they glow specific colours. The colours are due to electrons moving from the heated excited state to a lower energy level and emitting a photon of light with a quanta of energy corresponding to that of the level change.

If the excited state exceeds the energy of the nucleic attraction the electron is no longer bound to that atom and is said to have moved from the valence band of energies to the conduction band ¹. As with all energy level transitions the gap between these two bands is specific to the material but in insulators is very large so it is unlikely that the electrons will reach the excited state. In conductors the two bands are very close and enough energy will be present in the surroundings for the electrons to delocalise. For instance: solid metals exist as a regularly packed configuration of cations in a sea of electrons that all had enough energy to delocalise because there is no 'forbidden gap' between valence and conduction bands.

Sometimes there are circumstances in which there are intermediate levels located in the 'forbidden gap'. This leads to phonomenon like phosphorescence where electrons excited by nuclear decay in rock formation drop down to their ground state. These levels can be caused by impurities in crystal lattices.

Laura White: 24/05/04