The secret to electric currents is metallic bonding, a property found at the microstructure level of a conductor. Metals take their properties from the way metallic atoms are joined to their neighbors. Metal atoms have extra electrons that they need to shed to satisfy the octet rule. If there were non-mental atoms around, they would give the electrons to the nonmetal atoms so that those atoms would satisfy the octet rule. This is called ionic bonding. But what if all the atoms in an object were metallic? In this case, the atoms repel the excess electrons until they all wind up on the surface of the metal object. This shell of electrons forms a nonlocal bonding of the metal atoms very different from the local bonding characterized by ionic or covalent bonds. The nonlocal nature of the bonding gives the metals properties such as malleability, ductility, luster, and high thermal conductivity. Most importantly for our discussion, it gives the metal high electrical conductivity. This is due to the fact that the electrons on the surface of the metal are free to move on the surface.
What is often glossed over in discussions of current is the statistical nature of the phenomenon. The surface electrons normally move at random on the surface due to thermal effects yielding a statistical average of zero for the velocity. However, given an electromotive force that causes the random motion of the electrons to favor a direction, the statistical average of the electron velocities is no longer zero. The current is then defined by the number of electrons that, on average, pass a point on the surface per second multiplied by the charge of an electron.
The electromotive force is caused by an electric field generated by some source such as a battery or power supply. The electric field generates the “drift” in the statistical motion of the electrons by exerting an electrical force on the electrons.
It is often enlightening to think of electron flow in analogy to the flow of water in a pipe. The current corresponds to water flow, electromotive force to pump pressure, and resistance to pipe surface viscosity and flow constrictions due to changes in the pipe diameter. However, the microstructure description of current, along with the implications of this description for effects such as superconductivity, can be lost in this analogy. Care must be taken to emphasize macro and micro effects in the understanding of the flow of electrons in conductors.
