Big Power From Tiny Particles: Current and the Periodic Table
We know the drill—a voltage source produces current if there is a continuous path, and resistance limits the flow of current to convert power. But at this sub-atomic level, how does that actually work?
Have you heard any good jokes today? If not, here’s one for you: Why can’t you trust atoms? They make up everything!
Maybe funny, but the truth is that the interactions of atoms and sub-atomic particles are responsible for the operation of all electrical devices. Most of us know that electricity bears the name of the electrons that flow through wires, giving us light, heat, motion, and any other form of electrically-generated work.
Figure 1. Although we cannot see electricity, even under the best microscope, its properties are well-researched.
The problem with the sub-atomic level of physics is that it’s too small to be seen, so we can’t visualize the motion of the particles, and this leads to (sometimes false) assumptions that lead to errors in system design and troubleshooting. In this article, we’ll look at conductors, insulators, and semiconductors from an atomic and molecular level, explaining some of the phenomena that can happen when energy is applied to circuits.
Electrical Conductors
Anything can technically conduct electricity, but we reserve the name conductor for those elements which allow the flow of electricity relatively well.
Most conductors are metals, the section of the periodic table near the center. The inside hub (nucleus) of the atom contains the protons and neutrons, while the electrons orbit freely around at a distance. The electrons orbit at statistical distances away from the nucleus, and the number of electrons likely to be at the greatest distance are called the ‘valence’ electrons.
Metals have fewer valence electrons than many other elements, but this causes them to not be held so tightly by the molecular forces. When many atoms of a metal are touching, the electrons are shared by the whole surrounding group of atoms in a constantly moving mass. This is called ‘metallic bonding’.
If you hold up a piece of metal, like a plate or a wire, billions of electrons are in motion, even if no voltage source is present. The flow of electrons is not limited just to circuits. Directional current will only begin to flow if a voltage source forces the electrons, en masse, to move slowly from one direction to the other in a mostly organized bundle. This can only happen if there is a source of electrons and a return path, like in a battery or generator.
Figure 2. Metals used for conductors must balance ideal conduction properties with realistic material supply and cost.
How Fast Does Electricity Flow?
When electrons move in a wire, the energy input from the voltage source will cause one electron to jump to an adjacent atom, knocking off an electron on the opposite side to maintain a stable valence orbit. This, in turn, knocks off another electron, and this energy propagates through the wire.
That first electron didn’t make it very far in this amount of time, yet the energy applied to one end of the wire made it to the other side nearly instantly.
So, the answer to the question is: Individual electrons move very slowly through the wire. Electrical energy, however, propagates through a conductor very quickly. It does not travel at the speed of light, though, regardless of what some people might believe.
Electrical Insulators
Several materials are well known for being insulative, particularly most plastics, ceramics, and organic materials. Unlike metals, these insulative materials are molecules that contain stable mixtures of atoms, most commonly oxygen, hydrogen, carbon, and nitrogen. When these are arranged into certain structures—many of them very, very large as far as molecules go—two adjacent molecules will bond by sharing a pair of electrons (two valence electrons, or ‘covalent’ bonds) or by the hydrogen atoms sharing a force called hydrogen bonding. The mass of moving electrons is simply not present in insulators.
Without the mass of electrons, a voltage applied to one end of an insulative molecule will not propagate electron motion to the other side, and the flow of electricity is reduced nearly to zero.
Figure 3. Electrical insulators are used to isolate and protect people and nearby equipment from unwanted voltage. Image used courtesy of Canva
A large enough voltage, however, can be enough to force a breaking of strong bonds between these molecules, which is why insulators have a voltage rating. Exceed this energy input, and electricity can flow. If that happens, the result may be catastrophic, and the energy dissipated by the material comes off as heat, melting the material in an instant and reducing the path resistance.
Semiconductor Materials
Semiconductors, or solid-state materials, are usually made of elements near the center of the periodic table. These materials hold on tighter to their valence electrons, which makes them somewhat poor conductors on their own. Usually, the substrate (like silicon) will have other elements added to it in order to either increase or decrease the availability of more electrons in the general moving mass.
These other elements will add an overall negative (N) charge with more electrons or an overall positive (P) charge with fewer electrons. This is why transistors will have labels such as NPN and PNP. The name refers to the structure of the material, like a little electron sandwich.
Figure 4. Semiconductor devices are found near the top center of the table, with valence orbits moderately filled.
A diode, like an LED, consists of P material on one side and N material on the other. When a sufficient voltage is applied to the N side, enough surplus electrons will be supplied by the N material, and the source that the P side will become saturated enough for current to keep on flowing through. If the material is just right, the energy supplied to the electrons for motion must be lost when it reaches the other side of the material junction, and it comes off as light: the LED.
If the voltage source is applied to the P side of the material with fewer electrons, there simply are not enough to fill the voids, and current is blocked. A large enough voltage will still allow the current to flow in reverse, but this is how the diode functions as a one-way valve for electricity.
Electricity and Chemistry
Electrons are far too small for anyone to see, even with the most powerful microscope. Even if we could see one, we could never keep track of billions of them in motion. This is why it can be so important to be able to visualize the process and understand how and why certain materials are chosen for key tasks.
