Magnetic Electricity Generation

there is no known substance that can block magnetic feilds.

the difference between your setup and a coal fired setup is that the energy required is supplied from outside the system namely coal

in your system as soon as you start to draw current from generator a back emf will be produce. additional energy must be supplied to countract the back emf.

 

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Ok, well could you put them far enough apart that they don't affect one another? I know that magnetic fields don't range forever, otherwise all metal would be attracted to the closest magnet.

 

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Ok, it's clear that there is something you don't know about generators. Unloaded, they spin relatively freely, ignoring bearing friction and the like. But as soon as you start to draw current from the output terminals, a drag is induced in the rotor. It gets harder to turn and starts to slow down. You have to supply energy (mechnical work) to the input shaft to prevent it from slowing to a stop. And of course the best you can do is for the electrical output to equal the mechanical input.

 

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What the man is trying to say, DJ, is that yes, presumably, you could spin a wheel with magnets (perpetually, assuming a perfect system - no friction or other drag forces).

However, as soon as you draw a current from that system, you introduce a drag (back emf). Your wheel will start to slow down, unless you either help it by turning a crank, or introduce additional magnets.

It is irrelevant how far apart the magnets and the generator are placed, physically. They must be connected (by means of a drive shaft), and that shaft will transfer the drag forces between the two points (just as it transfers the productive forces).

 

Actually, that's not back emf. Back emf is the voltage that is induced in a motor's windings opposite to the drive voltage. This limits input current and is an aspect of a motor's efficiency. It also occurs in transformers and other coils and is why a well designed transformer draws little current from the line untill a load is drawn.

 

Iron can. For example, there is a 7 Tesla MRI machine at NYU that I have used. It has to have something like 600 tons of iron plates around it. The plates are there for one purpose only, to keep the magnetic field from pulling on or magnetizing things outside the room. Iron is not terribly effective at it, which is why 600 tons are needed. Basically it works by the little magnetic subdomains in the iron becoming aligned opposite to the machine's field and cancelling it out.

This doesn't negate your comments about how generators work and require external energy. The OP could use his idea as a magnetic bearing, to passively reduce wasted energy, but not as a generator to actively produce energy.

-Dale

 

Just as a point of interest, I have worked with mu-metal many times to shield sensitive military magnetometers in various test setups. It has an extremely high permeability and can effectively shield against static or reasonably low frequency magnetic fields.

 

You are correct here and in other post about back EMF (which rather than be created is reduced to let the current flow, as you basically said) I just want to mention an old (more than 50 years) illustration of the above, never to be used again, now that we have transistors etc.

There existed just after the war, a military motor / generator (WWII's "PE 103" - God only knows why the number still sticks in my head when so much has fallen out.) quite cheap surplus unit with at least a few hundred watts capacity. Back then "tubes" were the only way to make a radio, and they required a few 100 volts of DC (the B+ or "plate voltage")* to work which was not available in a jeep (or car) but 6V was from the vehicle battery (12V batteries came later) The PE-103 had one shaft, one end of which was a 6V DC motor and the other was a 500 Volt DC generator so that unit served as a DC to DC voltage converter. I had one in the truck of my car (actually my father's car) and a moble amateur radio transmitter (single "807 tube" as final RF stage was driven by a "6L6" tube - why is my memory space retaining this crap?).

One day we nearly wrecked the car as we got so excited when our "CQ" (= calling anyone or phonetically approximately "seek you") just transmitted on 10 meters was answered by a "ham" in Cuba.
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It was the "B+" as the "A+" was the first battery you had to turn on to heat up the filaments.

 

I can't see how a ferromagnetic material would develop magnetic domains opposite the direction of the applied magnetic field. I would think that the mechanism would be "completing the circuit." The field follows the shielding around from one end of the magnet to the other as the most attractive path, a path of negative resistance.

 

then what makes a generator harder to turn the more current that is drawn from it?

 

leo,

I think kevin is being a bit picky with the words. The back EMF (a voltage) technically causes a current to flow that generates a magnetic field that opposes the driving field. Yes? My Motor-Gen theory is pretty old...

 

yes, that's right.

 

Don't you remember the old addage "opposites attract"? This is exactly what inspired that saying. The North poles in the iron domains line up to the South poles in the MRI magnet, thereby canceling out some of the field.

Don't forget that a magnetic field contains a certain amount of energy. As they are installing these iron plates the field does some work on the plates. The plates are pulled towards the magnet as you might expect based on your own experience with iron and magnets. This work also aligns the subdomains so as to reduce the magnetic field. The work done by the field on the subdomains is equal to the energy lost by cancelling the field. Fundamentally, that is why opposites attract, because they cancel out the field and reduce the energy associated with the magnetic field.

-Dale

 

http://www.physlink.com/Education/AskExperts/ae512.cfm

 

Exactly. mu-metal.

http://metals.about.com/library/bldef-Mu-Metal.htm

http://en.wikipedia.org/wiki/Mu-metal

http://www.mumetal.com/

 

What you describe are opposite poles, but in order to attract, the magnets have to be oriented roughly the same direction. When you magnetize an object the magnetic field of the magnetized object is the same direction as the applied field. You and I may be describing the same thing while talking past each other. What happens to the "direction" of the field when it circles around to meet itself again? This is why if you set two magnets side by side facing the same direction they repel each other.

I think that the major mechanism here is that the magnetic field lines are coerced to follow the iron surrounding the magnet so that they do not do what they naturally do, and that's spread out. We can call that creating domains that repel the field or something like that, but that is what I think is happening.

I have a pair of cheap supermagnets that I got from Radio Shack. They seem almost able to pull a wood screw through wood from the other side. An interesting thing here is that I have not been able to find a piece of stainless steel or brass in the entire house that won't stick to them. I don't have any metallic bismuth or I could try something else.

 

I may have been a bit picky. Sorry. My point is that back emf usually refers to an inductively induced voltage that limits current. This doesn't happen significantly in a generator. Back emf and generator drag do arrise in a similar fashion. The induction law that states that induced currents appear in such a way as to oppose the magnetic field change that produced the currents.

 

No prob kevin. I knew what you meant.

 

I thought superconductors expelled magnetic fieldsd from their interiros, and thus a cage of it couldbe used as a magnetic shield?

 

http://www.sciforums.com/showthread.php?t=47314