How does a simple battery-magnet-screw motor work?

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Refering to the simple diagram attached, could someone please describe to me exactly what you would expect to happen in this scenario (batter with screw and magnet hanging loosely below the battery from magnetic force, with a wire held to the negative end of the battery, and lightly brushed against the edge of the circular magnet).

What would happen, and can you explain why precisely?

Thank you
 

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I can't tell from the diagram if the magnet is in contact with the screw. If it is, you have a direct short across the battery and things will heat up quickly. If not, nothing will happen.
 
The magnet is in contact with the screw, and it is what holds the screw to the battery.

I would like to know if your response Danger, is the standard expectation or not. Because it isn't what happens.

What actually happens is the screw and magnet spins, quite fast too. I'm not sure whether it heats up much or not because it is hard to keep a solid contact with the wire and the magnet (since you are lightly brushing it), and instead you just lots of brief brushing contacts. But after sitting there holding the wire to the magnet for over 10 minutes watching it spin, I didn't notice and remarkable amounts of heat.

So, if anyone doubts what happens, this is simple enough to do in your own home. Get a D size battery, a screw, a disc magnet and a wire. Make sure the screw is hanging loosely but aalso not swinging, then lightly brush the edge of the magnet with the wire.
 
Well, I certainly hope that someone else gets involved in this soon, because I have no idea what's going on. I'm not denying your observations; I just can't figure out why anything happens at all. I would expect the result to be the same as if you just hooked the wire straight from one end of the battery to the other.
On the other hand, I didn't realize that it was an intermittent contact between the wire and the magnet. Maybe you're switching the polarity from it being north on top as a regular magnet to being north on one side as an electromagnet, and then back. I still don't see how, but if so perhaps it's interacting with the Earth's magnetic field like a fluctuating compass.
 
That's the problem of being one of the smartest on this forum :P You won't receive any help.
 
You made a unipolar motor. Did you do this on your own, or did someone suggest it to you? Kind of weird if you did it on your own. You could get a stronger magnet to hold the screw to the battery better, though you would want to decrease the potential (voltage) if you did this.

It's a pretty basic concept in E&M that makes this work - magnetic induction. You have a current, which is a relative magnetic field, and a magnetic field acting on a loop of conductor carrying a current can produce a torque on the loop.
 
Thanks for the clarification, Mindscrape, but I'm still messed up. Wouldn't the magnet have to be on edge for that to happen? :confused:
 
You mean if you kept the polar orientation as shown in the diagram, and just put the magnet against the side of the screw? That should work too, but would be really tricky.

Oh, you must mean to put the magnet against the side so that the north touches the side. That would make torque upward and have the screw dig against if I am visualizing this right, though maybe it is downward, I'm a little too lazy to draw it out and do any formal analysis right now. Maybe Another God can try it.

Anyway it's just another approach to the standard motor where you have the loop of current that acts like a magnet, and instead you actually do have a magnet. Can you see the resemblance now? I'll post a link to the standard.

http://home.hiwaay.net/~palmer/motor.html

First link I found, probably better ones out there.

Edit: Reading that again it is a little unclear. Surely you remember that general E&M tutorial where the experiment above was done, which was also the first electric motor Faraday made if I remember right. Because the ring of wire is essentially a magnet when current passes through it, you can just replace it with a magnet. The screw does nothing but serve as a conductive material.
 
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Mindscrape said:
You made a unipolar motor. Did you do this on your own, or did someone suggest it to you? Kind of weird if you did it on your own.
Someone did demonstrate it to me first. I am posting it because I have no idea about it really. I have basic High School physics and High School electronics behind me and some Uni chemistry, but my understanding of how a magnetic field interacting with an electron flow creates torque is extremely limited. After about 3 days of thinking about it and the reminder of that hand signal you use to determine the direction of force exerted on an ion moving through a field, it seemed to make sense that an electron flow from the center of the magnet to the edge of the magnet would move through the magnetic field in a way which would produce a force that would push one edge of the magnet around, hence creating torque.

but as the person who showed me pointed out, what is the force pushing against? Every actions equal and opposite and all that...? So I can't answer that, and I was hoping someone here might have a better idea than I do.

The person who showed me (who i will be emailing this thread to) is very cynical about modern science (because he seems to do a lot of things which physicists and electrical engineers say is impossible), but he also has a tendency to paint strawman claims of science, so I wanted to hear from others who have faith in the current scientific paradigm what their take on this phenomenon was.

Mindscrape said:
It's a pretty basic concept in E&M that makes this work - magnetic induction. You have a current, which is a relative magnetic field, and a magnetic field acting on a loop of conductor carrying a current can produce a torque on the loop.
Unfortunately I don't understand this at all :(

Maybe a diagram of the 'relative magnetic field' would help? Are you saying that ...no... I've got no idea. Is 'the loop', the magnet? or the field...or what? LOL. :confused:
 
Mindscrape said:
. Maybe Another God can try it.

I don't have a magnet on me anymore :( I considered asking if I could take it home with me, but didn't bother. But anyone with a circular magnet at home can do it. Batteries, screws and wires are pretty easy to find.
 
Here is a good video that explains everything pertaining to this topic:

http://mfile.akamai.com/7870/rm/mit...870/8/8.02/videolectures/wl-802-lec11-220k.rm

Essentially a current has moving charges, and moving charges create a magnetic field around them. If you point your thumb in the direction of the moving charges, the current then the magnetic field circles around in the direction that your fingers curl.

In response to your friend, perhaps he just doesn't explain the theory of operation of the devices all that well. I don't know about EEs, but physicists are infallible. ;)
 
Are you sure the screw and magnet aren't turning in opposite directions?
 
cesiumfrog said:
Are you sure the screw and magnet aren't turning in opposite directions?
quite. The two turning in opposite directions would create far more problems than both turning in the same direction. Friction and the like specifically. I didn't check, but it seems incredibly unlikely.

PS mindscrape, that video was rm...any chance you can find a youtube or video.google version, or mpeg or anything other than rm?
 
Mindscrape said:
Essentially a current has moving charges, and moving charges create a magnetic field around them. If you point your thumb in the direction of the moving charges, the current then the magnetic field circles around in the direction that your fingers curl.
OK, I remember that, but I don't understand how that makes torque. Why does a magnetic field around the flow of electrons turn the magnet?
 
Look at it this way:

[tex]F = q(\vec{v} \times \vec{B})[/tex]

You have the current going left, and cross it with a magnetic field going up, and that makes a force inward, which because the magnet is free will make it spin counter-clockwise.
 
(the equation doesn't mean anything to me. I'm a molecular biologist, not a physicist :p)

But the field, current force description is pretty mcuh what I said above I think...the question that was posed to me was what was the force pushing against?

Actually, to make this easier to understand, as the magnet spins, does the magnetic field spin with the magnet, or does it stay stationary?
 
Magnetic field is stationary; up.

Electric forces are easy because they are F=qE, where q is the charge and E is the magnitude of the electric field. Magnetic forces would be F=qB, where q would be the magnetic charge and B would be the magnitude of the magnetic field, however that would be a for a magnetic monopole that doesn't exist. So the magnetic force is associated with a moving charge, a relative magnetic field, which is where the other equation comes from.

I don't really understand the question posed to you. Is it asking where the force creating the torque is coming from?
 
Another God said:
quite [sure that the screw and magnet aren't turning in opposite directions...] I didn't check, but it seems incredibly unlikely.

But you've realized the obvious problem with a statorless motor: total angular momentum is sure to be conserved somehow, or somebody is being fooled (it certainly seems unlikely that the missing angular momentum is just being radiated).

Mindscrape said:
You have the current going [radially], and cross it with a magnetic field going up, and that makes a force [...] will make it spin

Isn't that too naive? Consider the simpler rail gun: you pump electricity across a conductive "puck", which is sitting in some magnetic field. This produces a force on the puck, and can be used to fire it at great velocity. But there must be an equal and opposite force (recoil), presumably acting directly on the coils that produce the magnetic field.

Now, if the puck happens to be a permanent magnet (so as to provide the magnetic field itself), is the puck fired forward by the force on the current? Or is it fired backwards by the reaction force on the magnetic domains?

Has anyone verified the existence and direction of this spin yet?
 
Huh? A rail gun will have recoil because it needs to conserve momentum, just like any mechanical/non-magnetic gun. What does a motor have to do with conserving momentum? I don't understand your argument for angular momentum either. There is a force applied to the magnet.

Here is a video I found that has this same set up.

 
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Mindscrape said:
Huh? A rail gun will have recoil because it needs to conserve momentum, just like any mechanical/non-magnetic gun. What does a motor have to do with conserving momentum? I don't understand your argument for angular momentum either. There is a force applied to the magnet.

Here is a video I found that has this same set up.



Bingo. That film, or more accurately the one it is in reply to: is exactly the phenomenon I was describing in my diagram. You can see the direction of rotation in both of the films too. One has the screw hanging from the positive end (the nobbly end anyway) and it turns clockwise. The version hanging from the negative end (the flat end of the battery), turns counter-clockwise.
 
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Mindscrape said:
...What does a motor have to do with conserving momentum? ... There is a force applied to the magnet.
Newton's third law still applies, so I was trying to see what experiences the opposite reaction force. This relevant http://en.wikipedia.org/w/index.php?title=Homopolar_generator&oldid=91495513" even depicts a sculpture on my campus, so I feel obliged to study the references until I understand.
 
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It seems obvious (this morning, knowing rotation will not affect the magnetic field in any way) that the current inside the magnet (or rotor) will cross just as many field lines as that through the battery and wire does. So MindScrape's explanation is not only correct but explains an equal (opposite) torque being exerted on the wire (or "stator" :biggrin:).

Apparently my description of a rail gun was also incorrect: The recoil force should be exerted on the conductors that complete the puck's circuit, not on the magnet/coil itself.
 
OK, still trying to come to terms with this, and these paragraphs in the wikipedia entry stuck out to me:

When a magnet with a symmetrical field is rotated about its axis of symmetry (as it may or may not do in a homopolar machine), people often ask whether the field lines rotate with the magnet. Field lines can be a useful visualization-aid in predicting the behaviour of some electro-mechanical machines, but are misleading in the case of homopolar machines. There are no field lines mentioned in special relativity or Maxwell's equations or the Lorentz force equation.

A magnetic field merely has a magnitude and direction at every point in space, and is only defined relative to an inertial frame of reference (i.e. a non-accelerating, non-rotating frame of reference). No one has ever succeeded in making a device that can tell whether or not a symmetrical non-conducting magnet, hidden inside a black box, is rotating about its axis of symmetry.

However the Lorentz force law predicts that a rotating conductive magnet should be detectable, at least in principle, by the electric field produced when its free charges separate radially due to the (absolute) rotation of the conductor within its own magnetic field. This is the basis of one construction of a homopolar generator. The failure to appreciate this difference between conducting and non-conducting magnets is yet another source of confusion.

So, it seems the question I am about to ask may be hard or impossible to answer, but I am going to ask it anyway: If the passage of electrons through the field creates a radial force on the magnet, is there not an equal and opposite force created on the magnetic field? Would that field be pushed away, and thus rotate in the opposite direction to the magnet, or do we expect the field to be 'locked' in place (like the Earth is 'locked' in place when we jump up).
 
*bump*
I'd appreciate any attempts to answer my question :) Or even someone telling me that its a stupid question if need be... :)
 
I thought my previous post answered that question: no, there is no opposite force "on the magnetic field" nor on the magnet itself, and in fact it is impossible (or physically meaningless according to relativity) for the field to rotate or be pushed in that way. However, since you need a loop to make current flow, in this kind of design there will always be an equal and opposite force created on the "other" part of the loop.
 
cesiumfrog said:
there is no opposite force "on the magnetic field" nor on the magnet itself, and in fact it is impossible (or physically meaningless according to relativity) for the field to rotate or be pushed in that way. However, since you need a loop to make current flow, in this kind of design there will always be an equal and opposite force created on the "other" part of the loop.
I find it hard to believe that there is no force on the magnet itself. The magnet is stationary, then it is spinning, Newtonian physics says there must be a force acting. To say that no force is acting on the magnet just doesn't make sense to me, can you explain it further??

Also, by 'loop' I assume you refer to the circuit itself? The top of the batter, the wire, the magnet, the screw connecting back to the battery = the loop right? If so, then can you please explain what the "other" part of the loop is. I don't follow that at all.
 
The magnet spins because of the force on the first part of the circuit/loop, ie. on the current through the magnet, as I think Mindscrape expained already. Although their isn't an opposite force on "the magnetic field", there will be an opposite force on the wire connecting the battery to the magnet.
 
So there should be a detectable repulsion of the wire from the magnet right?
 

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