Electromagnetism+1.


 * Electromagnetism 1.**

During the day we will look at permanent magnets, starting with the basic rules for magnetic interaction and hopefully ending up with a little insight into what makes a magnet... magnetic.

We will lok at the motor effect, refreshing our memories on the obscure hand signals of Maxwell and Fleming and look at applications of the motor effect - including motors. The 'opposite' of the motor effect is the generator effect, we'll look at how we can use the same devices to generate electricity and extend the models to explain how a transformer works. By the end of the session you should be able to explain why the North pole is a south pole, why you need to know a bit about the Theory of Relativity to explain magnets and why it's a good idea to transmit electricity at such dangerously high voltages.

As we've chattted about the poles of the Earth, it's become apparent that we don't really know whether or not the world will end in 2012. As this could affect our holiday plans, it's probably worth looking into. Place any links relevant to the magnetic reversal of the Earth, the shielding effect of the Earth's magnetic field, coronal ejections (matron) and similar subjects below.

Motors - this is quite a good start from the schoolscience website - There's also some good material there on the motor effect and other related topics. Worth a browse. Also quite comprehensive is this stuff from physics animations: electric motors and generators There's always hyperphysics - not the most effusive explanations but concise and clear.

I've been hunting high and low for an easy to understand explanation of why a transformer gives an input:output voltage ratio equal to its input:output turns ratio. If you want to get deeply involved in Faraday's Law of Induction it can be explained mathematically but I suspect that, at this stage, you'd rather not. In terms of a simple physical model, I've always understood it like this (and I'm not saying that this is correct, it's just how I get my head around it...) Condiser a transformer with one turn on each coil. Probably not wildly efficient, but let's pretend that it produces enough magnetic flux to get the domains in the iron core swivelling about and the transformer works. The energy that we've put into jiggling electrons about in the input (or primary) coil is all transferred into reversing magnetic domains in the core, and then transferred into the jiggling of electrons in the output (or secondary) coil. We'd expect that, given the same number of electrons (pretty much) and the same amount of energy transferred, that each electron would have about the same urge to move in the output coil as was imposed on them in the input coil. Now suppose you put two coils on the output coil. Each electron is in the same field and thus pushed just as hard by the field. The two coils are joined, though, so I've always asumed that the voltages produced are added, like cells in series. Thus twice the output coils, twice the output voltage. OK, so now why does the voltage doubling halve the current? Easy answer is that the law of conservation of energy is broken otherwise but a transformer doesn't know and thus obey the laws of physics; there has to be a reason. The current in the secondary coil will depend on the voltage produced and the resistance to current flow (usually called //impedance// in ac circuits, the combined effects of //resistance// and //reactance//). The current in the secondary coil also generates a magnetic change in the core and in turn produces a voltage that opposes the change that produced it. This is where it gets a bit hard to follow without studying inductance but as far as I can follow, this has the effect of decreasing the overall resistance of the primary coil so that the current rises to supply sufficient flux in the core to allow the secondary coil to draw its current. Confused? Don't worry, so are people with far more physics background than you. Take a look at this forum discussion. It's basically an argument between engineers and physicists. I learned everything I know about transformers from Megan Fox.
 * Transformers - a few notes.**

Sober reassurances from the British Geological Survey are echoed by NASA - so there's a bit of academic cred. In the name of balance (much in the way that the BBC often invites a homeopath or other witch doctor onto TV to talk to a scientist) there are some crackingly hysterical websites about Mayans, the end of the world and so on - this one made me laugh until milk came out of my nose* and this one, rather cheekily, actually steals the diagrams from the NASA site! (another update: looks like this second site is where the nutters have migrated. Check out the links!). We should, however, note that some reasonably sober looking journals take some of this guff quite seriously - as in here. Suite 101 presents itself as a non-loony forum but there are some articles leding credibility to Doomsday theories. There are some nice debunking sites, too, if you search and some good science blogging on the subject. I think that an interesting difference between science blogs and alt-science conspiracy-style blogs are that the former quite often feature a photo of the writer with a little bio... the authors of the latter then to remain shrouded in bullshit... sorry, I mean mystery. All in all there's plenty of material there for a HSW activity - getting students to critically review the quality of evidence presented on the various sites.
 * 2012 - a disaster? Links below.**


 * To my great disappointment, this previously hilarious website has been radically updated with rational and sensible articles and is now debunking what it once seemed to take quite seriously. Still some good stuff on the lizard people, though.