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Hello again. In the last session, I talked a bit about electrons and charge, and tried to hook them up to concepts you're probably familiar with, like voltage and current. Now, I'm going to talk about conduction, and resistance, and semiconduction (this last being pretty vital for transistors, and the integrated circuits that are made out of them). I'm going to skim over them to some extent, because of all the material in this "course", the physics of conduction is the area in which I have least knowledge.
An atom, as I said in the last advent science session, can be considered as a nucleus (containing protons and neutrons) and a series of orbiting electrons. But those electrons don't all orbit at the same "altitude". It turns out that there are set "altitudes" (energy levels) at which electrons can orbit around the nucleus, and a certain number of electrons can fit in each. The energy levels aren't called energy levels for nothing, as an electron can move between them by gaining or losing energy - the more energy an electron has, the higher altitude it orbits around the nucleus, the higher its energy level. The terminology all kind of makes sense (although this is a massive simplification on the actual physics).
Different elements have different energy levels, and the spacing between the levels varies. The energy levels tend to be occupied from the nucleus out, as with time electrons will lose energy and fall inward to the lowest unoccupied level. An electron that has enough energy to transition to a higher level is said to be "excited". The highest energy level in which unexcited electrons are normally found for a particular element is called the Valence Band. The energy level immediately above it (which is "normally" empty of electrons) is called the Conduction Band. Electrons in this level can be exchanged between atoms moderately freely, as they are highly energised and loosely bound to their nucleus, but because electrons are drawn to positive ions (such as those created when an electron leaves an atom), it tends to be more a case of atoms swapping electrons with their neighbours than of electrons migrating freely around the material. As a rule, electrons in the Valence Band are too strongly fixed in place to move around like this.
As I said a while back, an electron must take in energy (and become excited) in order to step up to a higher energy level. This is ... mostly true. The trouble is, an energy level isn't just a single, clearly defined circle around a nucleus, it's a fuzzy band within which electrons can have one of several excitation levels, all very close together. So the bands can be different widths, and different elements have different distances between their bands... can you see where this is going?
CONDUCTORS are materials, such as metals, in which the valence band and the conduction band actually partially overlap. Electrons can swap in and out between the two bands freely. As a result, the conductor can be considered as a virtual sea of electrons, coexisting with a virtual sea of positive ions, despite the fact that the material as a whole is approximately neutral. Electrons within the conductor have a drift velocity (on the order of millimetres per second), which is the speed at which they move about, trading between atoms. Note, however, that electricity moves much more quickly than this (about 0.75 light-speed in vacuum). This is because electrons are traded, not moved - the whole thing behaves a bit like a Newton's Cradle. Push an electron in one end, and all the electrons jump forward one place in line, and an electron comes out the other end almost immediately. It's not the same electron, but it may as well be.
INSULATORS are materials, such as glass or rubber, in which the valence band and the conduction band are widely separated. Electrons need to gain a great deal of energy to move between bands, and they rarely manage it. As a result, attempting to push electrons in one end of a insulative material (or pull them out) has minimal effects, BUT it can cause charge buildup at the end. This is why insulative materials can hold a static charge - electrons are added to or removed from the material, but because it isn't conductive they can't go anywhere. The "not-electrons" that are left behind when electrons are removed are called Holes. They'll be useful later.
Finally, SEMICONDUCTORS are materials (like silicon) in which the valence band and the conduction band are close together. Electrons don't need a lot of energy to hop the gap, but they can't do it spontaneously. Semiconductors, it turns out, are really useful if you're smart in how you prepare them and lay them out - all integrated circuits are made from semiconductors. If a semiconductor is "doped" with other materials, it can be made to have a slight excess of electrons or holes, and when the two types of semiconductor are laid out in certain patterns, you can use a flow of electrons in one area to control the flow of electrons in a nearby area very precisely. But that's transistors, which is another session, yet to come.
I can't promise to post every day, as you've noticed, but I'm trying. The next session is on other fun particle physics trickery that's used in the field of electronics, then we'll move on to the actual machinery involved, starting with the transistor and moving on from there.