Quick Intro and Magnetism/Energy Question

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Quick Intro and Magnetism/Energy Question

Postby jfarquhar » Tue Jul 10, 2007 11:13 am UTC

First of all, I'd like to introduce myself to the xkcd forums. I do not have very much experience in physics but I am very interested in it, and I try my best to get my head around some of the threads in here! I study Computer Science at Uni, and I am considering taking some basic physics or astronomy classes purely out of interest. Now, onto my question!

I have had an idea in my head for some time, and I thought here would be a great place to discuss it. First, I began thinking about the laws of conservation of energy. This is basic stuff, so I understand it to an extent, and I know that energy cannot be created, it is simply converted from one form to another. Building upon this, I became interested when playing around with magnets one day as to where they draw their energy from. They are able to exert force upon an object, by pulling it closer or pushing it away, with no apparent energy source! I know this might be really simple for you guys and girls, but I'd be happy to be enlightened.

This also leads on to another line of thought I had, can we not convert magnetism into a form of usable energy? Seeing as magnetic energy seems to be constant (again, I am probably wrong), could it not be converted somehow into [effectively] infinite energy? Or would the device that accomplishes this require more energy for conversion than the conversion itself would output?

Thanks in advance :)

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Postby aoanla » Tue Jul 10, 2007 11:55 am UTC

In answer to the second part of your question, a further question for you to ponder: Since the mass of a body seems to be constant, why can't we use gravity to produce an infinite source of energy?

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Postby skeptical scientist » Tue Jul 10, 2007 12:15 pm UTC

You can work this out from aoanla's hint, but for a solution, highlight this:
Just as there is potential energy inherent in certain configurations of massive objects, there is potential energy inherent in certain configuration of magnets and ferromagnetic objects. Consider the following situation, where you have two magnets separated by a gap and oriented so they attract. They feel a force towards each other, so if they are released, they pull each other together. This can be converted into energy (in fact, the equation is energy=force * displacement, or rather an integral since the force is non-constant, but this is an unimportant detail for the moment) if you extract the energy produced when the magnets slam together. However, in order to keep extracting energy you would need to return them to their starting configuration. Can this be done without exerting energy? The answer is no, because you have to spend at least as much work pulling the two magnets apart as you were able to get out of them in the first place.
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Postby hyperion » Tue Jul 10, 2007 12:18 pm UTC

First off: some people (most) don't like new people introducing themselves outside the intro thread in general. Just a heads up.

I don't have much experience in electromagnetism either, but I can tell you why it can't be harnessed for free energy (yet): Once the body has been pulled to the magnet, there is no way to remove it without adding energy.
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Postby jfarquhar » Tue Jul 10, 2007 1:36 pm UTC

My apologies. I had read over the forum rules, but after reading through some of the science threads and becoming engrossed in them I completely forgot about the intro thread and I let my enthusiasm take over :S I'll head over there right now and properly introduce myself.

As for my question, I had not even thought of the problem of removing the magnets from one another! Thanks for the replies :)

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Postby 22/7 » Tue Jul 10, 2007 3:14 pm UTC

This has become the thread for aussies I guess...
Totally not a hypothetical...

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Postby BlochWave » Tue Jul 10, 2007 4:34 pm UTC

Well first it's important to understand what you mean when you say magnetism. Do you know what really causes it and how it's related to electricity?

Magnetism is caused by moving charges. If you have a straight wire with current running down it the magnetic field lines circle the wire at any particular distance. If you take two parallel wires(the ends have to loop around back to your battery or whatever so it has to be long enough we can ignore the ends and just look at the parallel part)put 'em beside each other and crank current in the same direction, they'll attract each other! Opposite directions and they'll repel. This is the magnetic force

Also of importance, instead of circular magnetic field lines around a straight current, a circular current gets you...well, a magnetic dipole. If you have a long cylinder with circular currents going all up and down its length, the magnetic field lines will be straight inside the cylinder, and when they reach the ends they kinda loop around just like a bar magnet's(which is a big magnetic dipole, you'll have to google pictures I can't explain it well)

So if magnetic fields(and hence magnetic forces)are generated by currents, how does your block of solid iron in your bar magnet do it? Same way! We'll look at the classical picture(if you see a textbook entitled quantum theory of magnetic materials DO NOT TOUCH or your brain will melt)the electron orbits(remember you can think of a circling electron as basically a circular current, all current is is moving charge)in a ferromagnetic material like iron all magically(don't touch that textbook!)get aligned in domains, and the domains more or less align in one direction and you have lots of little magnetic dipoles giving the material the property of one big magnetic dipole(incidentaly the borders of these domains are called bloch walls cough my name cough)

Anyways, I say alllll this to explain why magnetism of this nature isn't eternal, as you said it appears. You can expect by various physical laws(entropy leaps to mind)that unless you're at absolute zero thermal energy is gonna disrupt this alignment until everything's dipole moment is randomly orientated, and then there's no net magnetism. So a magnet that you have will eventually become demagnetized, unless you regularly stick it in a strong magnetic field to realign its domains(a strong magnetic field that takes energy to make since you have to pump current to create it)

Sooo that's that, as for using stored magnetic energy for something useful, we do that all the time. Perchance do you know how we actually generate electricity? Google Faraday's Law, if I get started I won't stop. An obvious example though of what you described is using a Faraday flashlight, it's one of those that you crank and crank, then press the button(well if it has a capacitor, otherwise crank and crank and it shines)and it gets the current to light the bulb from turning that magnet(or the wire around it)again, google Faraday's Law to learn. Ignore the math, unless you're not scared of surface integrals and vector calculus

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Postby MFHodge » Tue Jul 10, 2007 5:11 pm UTC

Thanks, Bloch. Good post.
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Postby evilbeanfiend » Tue Jul 10, 2007 5:18 pm UTC

BlochWave wrote:if you see a textbook entitled quantum theory of magnetic materials DO NOT TOUCH or your brain will melt


he is not joking, quantum theory melts brains
in ur beanz makin u eveel

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Postby BlochWave » Tue Jul 10, 2007 7:51 pm UTC

he is not joking, quantum theory melts brains


Some of my research I did as an undergrad involved calculating dielectric constants from quantum principles O_O I didn't do well at that particular job

Anyhoo, another point worth mentioning, skeptical scientist compared magnetic fields to gravitational fields, but I'm not sure that's fair(one might say I'm skeptical oh-hohoho!!)since gravitational fields can do work, no questions asked, but magnetic fields cannot, in fact, do work, period. If it appears like a magnetic field is doing something it's because of electric fields that are induced when changing the magnetic field, that's why when you calculuate the energy stored in a magnetic field you need to know the field producing current and the inductance of the material.

I don't think that actually changes the poster's POINT though, but I do like my long-winded explaining and expanding >_>

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Postby QuantumTroll » Tue Jul 10, 2007 7:59 pm UTC

Good timing on this post!

A company called Steorn has developed a machine (called Orbo) that uses magnets to achieve > 100% efficiency. They get more power out than they put in! :shock:

In fact, they were putting up a demo at a museum in London this week. Let's see how it went.

http://blog.wired.com/gadgets/2007/07/s ... updat.html

Ah. Too much heat. And the "robust" bearings broke. Not willing to commit to a date for the demo...

Oh hey, a thread on this forum: http://forums.xkcd.com/viewtopic.php?t=7327

Btw, nice post Blochwave. I have nothing to add to the science in this thread. A bittersweet moment...

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Postby BlochWave » Tue Jul 10, 2007 8:02 pm UTC

I have nothing to add to the science in this thread. A bittersweet moment...


I know the feeling, I practically haunt this board looking for new topics where I can make constructive comments that haven't already been said. Damned degree has to be good for something, right?!

Edit: And now to go above and beyond, http://www.radioelectronicschool.net/fi ... dyanim.gif

I'll leave it to the topic creator to google the details but a changing magnetic flux induces an emf(a voltage, basically). That gif practically shows you how a faraday flashlight works. There's a voltage while the magnet is MOVING(or you can move the wires, it's all relative)and none when it's stopped. In the case of, say, a hydroelectric plant, we use the energy from flowing water to do things like turn a coil of conducting wire while in a strong magnetic field, hence changing the flux through it, producing an emf, and hence current! Notice in the .gif that you get both positive and negative voltage as it goes in and out, similarly in a generator like I'm describing, as the wire coil rotates you'll get it swinging from + to -, which is why most all of our electricity is produced in the form of AC, it's like a prerequisite of nature

Edit AGAIN sorry but I keep having thoughts:

Nuts I confused myself. Or maybe not. In a Faraday flashlight if you induce the emf by rotating the magnet, you're gonna need some type of wave rectifier circuit too, right? Since light bulbs don't like AC. Either that or....a diode and a capacitor? Then pressing the button discharges the capacitor and turning the crank charges it? Well I guess that's a half-wave rectifier

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Postby Solt » Wed Jul 11, 2007 3:42 am UTC

HYPERiON wrote:I don't have much experience in electromagnetism either, but I can tell you why it can't be harnessed for free energy (yet): Once the body has been pulled to the magnet, there is no way to remove it without adding energy.


This isn't an entirely correct explanation. To see why, just pretend that the magnets involved are electromagnets. Let them attract and come together, extract the energy, then just turn the electromagnets off and separate them at almost 0 cost in energy. Wallah, infinite energy right?

BlochWave gives the better explanation, which basically says that every time you pull energy out of a magnet, you reduce the strength of its field (proportionally?). This translates, in electromagnets, to a drop in voltage thus if you keep the field at a constant strength throughout, you will lose resistance + extracted energy through an increase in current.

I'm not sure why you can't treat a magnetic field like a gravitational field. If I remember my calculus correctly, both are conservative force fields meaning that the energy that you put in, moving a certain distance, is the same as the energy you will get out, moving in the opposite direction. Though I do seem to recall that doing work in a magnetic field will cause heat to be generated in the object creating the field and I can think of no parallel effect in gravity. Although, when you extract energy from a gravitational field it does get weaker but in a different way. For example, the energy that you can extract from the tides comes from the moon- every Joule that you pull out from the tides pushes the moon further away.



By the way BlochWave, light bulbs tend to run fine on AC ;)
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Postby hyperion » Wed Jul 11, 2007 5:05 am UTC

Solt wrote:
HYPERiON wrote:I don't have much experience in electromagnetism either, but I can tell you why it can't be harnessed for free energy (yet): Once the body has been pulled to the magnet, there is no way to remove it without adding energy.


This isn't an entirely correct explanation. To see why, just pretend that the magnets involved are electromagnets. Let them attract and come together, extract the energy, then just turn the electromagnets off and separate them at almost 0 cost in energy. Wallah, infinite energy right?

Except for the fact that you're adding energy to power the electromagnet.
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Postby Solt » Wed Jul 11, 2007 5:18 am UTC

HYPERiON wrote:
Solt wrote:
HYPERiON wrote:I don't have much experience in electromagnetism either, but I can tell you why it can't be harnessed for free energy (yet): Once the body has been pulled to the magnet, there is no way to remove it without adding energy.


This isn't an entirely correct explanation. To see why, just pretend that the magnets involved are electromagnets. Let them attract and come together, extract the energy, then just turn the electromagnets off and separate them at almost 0 cost in energy. Wallah, infinite energy right?

Except for the fact that you're adding energy to power the electromagnet.


Assuming nothing is moving, the only energy you put into an electromagnet is that which is wasted by the resistance of the wire. Essentially the same thing as a ferromagnet.

Hell, just assume you're using superconducting electromagnets.
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Postby yy2bggggs » Wed Jul 11, 2007 5:21 am UTC

To address the "conservation of energy" side of the question, there's another way to look at things. The following explanation is rough and qualitative--it's only meant to be sufficient to get what's going on.

Suppose we suddenly discover a new force between some special types of objects. Furthermore, suppose the force between these objects is some function of the distance between the objects. It sort of makes sense that this would happen--if you hold the objects apart at some distance, they will tug towards each other (or repel if you so fancy, but for now let's say they just attract), and you'll have to resist this by applying an opposite force. If you let go, the same force would pull the objects together; if you want the objects to be further apart, you need to struggle to overcome this force.

The actual function doesn't matter that much, so let's just abstract it. Given some d, some function f(d) determines how much force there is between the objects.

Now, to side track...

Imagine we have the strange capability of being able to reverse the flow of time--so that time plays from future to past at normal "speed". When time is played forward, these two objects, separated by a distance of d, will seem to attract each other with a force of f(d). Now if we flip the switch on time, and make it flow backwards, what do you suppose this force will look like?

Easy. With a backwards time flow, the force will seem to be an attractive force of f(d) between the objects--in other words, it's the same force in the same direction. Okay, maybe that's not intuitive, but it's true. Think of gravity. Imagine you throw a ball up into the air as hard as you can. The ball will start going up pretty fast, and as it gets higher, it will slow down, then eventually stop. At this point, it turns around and goes the opposite direction, slowly at first, and then progressively faster. Once it reaches the level where you threw it, it will be falling down as fast as you threw it up. Gravity pulls the ball down. Play this sequence backwards now, and the ball starts going up fast, slows down, stops, reverses, then goes back down, slowly at first, until it reaches its starting speed and lands in your hand. Sound familiar?

Now, back to our mystery force....

Let's hold two of the objects out at a distance of d now, perfectly still, and just let go of one of the objects. It will fall towards the other object until it hits. This creates kinetic energy from the application of the force--let's say the amount of kinetic energy generated is E just as it hits. So what we did was utilize the force to produce energy, and E is the amount we got.

If we're to make a perpetual motion machine, we need to reset the mechanism to the original distance, and we have a budget of E. But it's going to cost us exactly E units of energy to get back to a distance of d; you can imagine one possible way to reach this distance as just backtracking the object's speed, at the same speeds it fell, until you have the objects separated by a distance of d. The physics involved in doing this will wind up looking exactly as if we time reversed the process (since forces are time symmetric), so once we hit the distance of d, we would literally be back where we started. (If you pull back at different speeds, you're still going to wind up with the same energy investment, though this treatment doesn't illustrate this fact).

The point of all of this is to illustrate that forces are not energy--discovering a new force doesn't really help you in your quest to produce a perpetual motion machine. Energy is just conserved.

Now, it's remotely possible to imagine an energy producing force... the only thing is, that force has to behave differently than prescribed above (it can't just be some force between objects that is a function of the distance between them), and it would wind up being a strange force indeed.

Now, hopefully I'm just barely sane enough to have made the above a coherent and true explanation, but if not, feel free to correct me.

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Postby jfarquhar » Wed Jul 11, 2007 8:22 am UTC

Wow, you guys sure know your stuff! Thanks so much for all the replies, I am currently looking through some things on Faraday's Law. It is very interesting. I knew of course, that magnets would have to draw their energy from somewhere, I just wasn't quite sure where, and to the untrained eye they would seem as if they produced the energy.

I have been following that Orbo project for a while now, the announcements of delays that is :) I don't have much hope for it really, but I am still curious to see what they have done.

On a quick side question, what are your views on using Wikipedia as an information source? I have a fear that I will visit a page after a recent vandalism and learn something completely outrageous >< Normally though, I check the sources at the end of the page.

Thanks again, I'll keep reading up on it :)

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Postby skeptical scientist » Wed Jul 11, 2007 9:05 am UTC

When consulting wikipedia on math and science issues, I find it fairly reliable as long as the issue in question isn't controversial (e.g. global warming). Usually vandalism on these types of articles is pretty blatant and obvious (most if it is the occasional obscene word at the end of a paragraph), so you wouldn't get confused. Usually being aware of the possibility of inaccuracies is sufficient - if something makes you suspicious, you'll know to check it out, but otherwise you can usually tell the good information from the bad information.
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Postby aoanla » Wed Jul 11, 2007 9:21 am UTC

I should note some minor extensions to what people have been saying.

While you can look at magnetic fields being produced by moving charges, it is actually more accurate to say that all there is, is an electromagnetic field, which looks different to you (that is, more or less magnetic or electric), depending on your velocity.

As BlochWave noted, magnetic fields are incapable of doing work, but this merely means that only the "electric component" of the electromagnetic field in your frame of reference can do work.

In addition, it should be noted that subatomic particles can and do have magnetic moments (and thus magnetic fields) in the rest-frame of an observer. These magnetic properties cannot be removed by entropic considerations, and are due to symmetry properties of the particles, rather than their motion. (Of course, as BlochWave notes, the large scale effect of those particles can be cancelled by arranging them at random orientations, which is what happens in most materials.)


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