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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.
Hello, and welcome to Advent Science 2011, the pewterfish edition. This is a little experiment being performed by myself, pufferfish and duckbunny: we rather suspect we can teach our readership something interesting in the month of December, and we've each chosen a subject we know fairly well. It's kind of like an advent calendar, but each door conceals awesome knowledge instead of chocolate or paintings or Lego. Not that such things are bad, of course.
Pufferfish is talking about genetics, and duckbunny about the history of the atmosphere. In the next 24 days, I hope to lead you on a journey from science to engineering, and beyond, in one very narrow field.
Computers are everywhere these days, on our desks, in our pockets, in the back rooms behind companies like Google, and Facebook, and Oracle. They cook our food, wash our clothes, and guide our aircraft. But to many people, the computer is a magic box, a Thing within which humankind is not meant to peer.
That's nonsense. They're machines, just like everything else, the parts are just smaller and less obvious in function. I happen to know my way around them quite well, so I'm going to impart some of that knowledge to you, if you keep reading. I'll necessarily have to gloss over details from time to time, but I mean to touch everything important, and leave a trail of links for you to follow if you want to learn more.
Join me, then, as we wander from Electrons, to Email.
If we're going to go from Electrons to Email, I guess we'd better start at the beginning. Time for some particle physics.
The electron is a particle with a unit negative charge. That is, it is a particle with the smallest negative electrical charge known to exist - there's no such thing as "half an electron's worth of charge". An atom consists of protons and neutrons in the nucleus, and orbiting electrons: a proton has a unit positive charge, an uncharged atom has an equal number of protons in the nucleus as it has electrons orbiting it.
Opposites attract, as well we all know, and electrons are no different. Electrons, being negative, are drawn towards positively charged objects. But what's charge? It's the presence or absence of "the right number" of electrons. When I said that an uncharged atom has an equal number of electrons and protons a paragraph ago, I didn't mention what happened if the numbers weren't equal. If an atom has less electrons than protons, it is positively charged. If it has more, it is negatively charged. A charged atom is called an ion. Positive ions "want to" gain electrons, and achieve neutrality. Negative ions "want to" lose electrons. I put "want to" in quotes because there is, of course, no desire involved. Equally, it's not something I can easily explain without going deeper into partical physics than I really want to, so you kind of have to accept it.
Electrons are drawn to positively charged objects, and repelled by negatively charged objects. This is as true at the macroscopic level as it is at the atomic: a positive electrode will attract electrons, and negatively charged objects, to itself.
It follows that given a difference in charge, electrons will tend to flow from a negatively-charged object to a positively charged object, until the charges are equalised. This flow is called electricity. The chemical reactions within an AA battery cause one end to become more positive and the other more negative as they proceed, and it is possible to extract energy to light lamps, sound buzzers and so on by placing the item to be powered between the positive and negative ends: the electrons flow and, on their way, they power the item. Given enough time, the chemical reactions within the battery will cease as their fuel is consumed and the electrons will slowly drift back to equal out across the battery - this is what happens when a battery is discharged, or "flat".
The difference in charge between two objects is called the Voltage, or potential difference, that exists between them. The number of electrons per unit of time that flow between them is called the Current. Current is measured in Amperes, or "amps", and a single ampere is 6.241x10^18 electrons per second.
Given that that's an awful lot of electrons (roughly six quintillion), it should come as no surprise that in electronics, it is normal to deal with thousandths or millionths of an amp, or even smaller amounts.
So, that's the electron, more or less. I've glossed over some bits, because down at the tiny tiny scale of particle physics, it can be a bit too easy to get sucked down the rabbithole of detail, but that's enough that I can explain the next bit with. See you tomorrow.
For more in-depth information...
Well.
Well well well.
I live in Cambridge. And I work at (but not yet for) ARM. Life is more or less good. And I'm back on the net, as this entry shows. Hello again.
I have a (rented) house, which is still a mess, because I haven't finished unpacking (it's traditional, you know).
I have a job, and I'm responsible for a manual the company will be releasing soon. This is ... well, not going perfectly smoothly, but so it goes. I'm doing the best I can under tricky conditions, and I don't think it'll be too late (but when one's co-author, manager and mentor all independently go on holiday at the same time, there's only so much a new hire can manage).
Life is, for the most part, good.
Watch this space, I guess :)
So, I've been moved in for a bit over a week now, in theory, and I've unpacked .. some things. By the gods, do I have a lot of stuff. I've gone from a room and a half to a house and I'm still having trouble finding spaces in which to put everything. But, so it goes. If I unpack a box every so often, it'll get sorted out eventually.
I'm due to start work on Monday, and all the details for that are sorted out now. I still need to finish servicing my bicycle for the commute, but that's a couple of hours' work, and will probably be done on Saturday morning. Then I just have to find the route...
Not a lot to say here, really, but thought I should just put up a "things still happening" post. Money is tight, as expected after just moving house, but all the major bills are taken care of and I'm (barely) solvent this month. Once I'm actually being paid again, and Brighton Uni get that last pay-cheque to me, everything should be just fine.
For now, well, I guess it's cheese sandwiches for me :)
Having the river and Stourbridge Common thirty seconds' walk from my door is a lovely thing, though. I'll try and get some pictures at some point, and post them here. Realisation of the day, though: rowing skiffs have head- and tail-lights, and they appear to be exactly the same models as used on bicycles - horizontal lines of LEDs, powered by two AAA batteries per unit. It's odd, and interesting, to see the same design reused in a way I'd never have expected.