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Author Topic: Physics/Electromagnitism question
mr_porteiro_head
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My son was learning a little bit about electricity, so he and I were fiddling around with simple projects like making a simple switch, making an electromagnet, etc..

I wanted to make a DC motor like I remember Mr. Wizard doing when I was a kid, and I found these instructions. (2/3 down the page is where you'll find the instructions.)

I looked at the instructions, and thought that there was no way it could work -- it's just a free-spinning coil in a magnetic field. Without anything functioning as a commutator, applying current to it would just cause it to properly orient itself in the magnetic field, and then stay there.

I figured I'd try it anyway, and when it didn't work, I'd apply fingernail polish on one half of the the wire where it connects to the bracket to act as a crude commutator, turning off the motor during half of the rotation.

But when I made it according to the instructions, it worked! Sometimes it would get stuck, and just flip over like I predicted. But if I started it spinning, it would usually just keep going.

It still doesn't make any sense to me this thing should work.

Help!

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mr_porteiro_head
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quote:
When the coil flips over the axis of rotation is reversed,
Um, what? When the coil flips over, its North and South poles flip over as well, just like would happen if it were a permanent magnet.
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Dobbie
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After the coil flips, momentun keeps it going long enough for it to move close enough to its initial position to start the process over again.
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Dobbie
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I think.
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mr_porteiro_head
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As it rotates 360 degrees, it's working against the magnetic force just as much as it's moving with it. There's just as much force slowing it down as there is speeding it up. All else being equal, it should slow down and stop.
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dem
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EMag, ReMag, DMag, CMag
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mr_porteiro_head
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It's like an unbalanced wheel -- it wants to rest in one place. If you give it a spin, it will spin for a while, half the time with gravity pulling it down, half the time with it fighting against gravity.

Except, it's obviously not like that.

But how?

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mr_porteiro_head
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quote:
Originally posted by dem:
EMag, ReMag, DMag, CMag

????
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Dobbie
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By the time the force switches to "slowing down" the coil has enough momentum to keep going until it gets close enough to its original position for the force to switch back to "speeding up".
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mr_porteiro_head
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Why would that happen here when it doesn't happen with a freely spinning unablanced bicycle wheel?
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mr_porteiro_head
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OK, here's an explanation that makes sense to me:

The coil is pretty unbalanced, and as it spins and bounces around, it loses contact with the uprights, which shuts off the current. This bouncing and turning on and off of the current happens in time with the rotation, so it acts like a commutator.

This also explains why sometimes it just wouldn't work, but fiddling with it, trying to balance it out better, would fix it.

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MattP
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You can get a faster/more efficient version by carefully sanding off only half of the insulation on each side so that it spends half of the rotation with current going through, half without.
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mr_porteiro_head
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Right! Applying the fingernail polish after it didn't work was intended to give me the same effect as only stripping off half of the insulation in hte first place.
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Glenn Arnold
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First of all, this is just a guess, and I'm trying to think this through as I type. I'm not claiming this is right, but it doesn't seem to me that "the contact is lost and it coasts through" would make a reliable motor. There must be something more reliable here.

Looking at the coil, I find it difficult to imagine the axis of the magnetic field. I think that may be the key. In a traditional electromagnet, the wire is wrapped sequentially around an iron core, which fixes the polarity of the axis along the length of the core. Here there is only a bundled loop of wire, with no core. What causes the north end of the field to project from one particular end of the loop?

Also, the circular magnet is lying on its side. Which direction do the magnetic lines project? My best guess is that both of these magnets have unstable fields, which can be moved by the presence of the other magnet. As the loop turns toward the fixed magnet, it pushes the fixed magnet's field, but when one edge of the loop is perpendicular to the fixed magnet, the magnetic field of the electromagnet is parallel to the plane of the circular magnet (the field projects through the center of the coil). At this point, the polarity of the electromagnet is unstable, and the field of the fixed magnet rebounds and flips the polarity of the electromagnet. Etc.

What bothers me about this is that it isn't a good demonstration for kids, because it's too dynamic for a kid to visualize. A motor with a nice visible commutator is easier for a kid to see the event cycle, and this just doesn't do it.

I built a version of my own, using a spool with nails sticking into it, and brass commutator brushes. I'm heading off to bed, but if I still see this thread tomorrow I'll try to take some photos of it and post them somewhere.

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mr_porteiro_head
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quote:
it doesn't seem to me that "the contact is lost and it coasts through" would make a reliable motor
Well, it wasn't a reliable motor. [Smile] We were constantly fiddling with it to get it working again.

quote:
Looking at the coil, I find it difficult to imagine the axis of the magnetic field. I think that may be the key. In a traditional electromagnet, the wire is wrapped sequentially around an iron core, which fixes the polarity of the axis along the length of the core. Here there is only a bundled loop of wire, with no core. What causes the north end of the field to project from one particular end of the loop?
Whether there's a ferrous core or not, the direction of the magnetic north pole is determined by the direction that current flows through the loops, according to the right-hand rule. The ferrous core mostly just makes it easier to add together the magnetic fields from a bunch of loops

quote:
Also, the circular magnet is lying on its side. Which direction do the magnetic lines project?
The North-South axis of that magnet is coaxial with the circle, so when it's laying down, as in the motor, either the North or South pole is sticking straight up.
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King of Men
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Suppose we reduce the coil to being half a circle, like so:

code:
--<-|          |-<--
| |
|----<-----|

^ ^ ^
MAGNET

When the half-circle is in the down position, the force is into the screen, taking the current as running right to left and the magnetic field as going straight up. When the half-circle is in the up position, the force is likewise into the screen. But, because it is further away from the magnet, the force is weaker; therefore, neglecting friction, it is not braked as hard as it was pushed, and continues through to reach the down position and get another kick. The full coil can be considered as consisting of many half-circles.
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mr_porteiro_head
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I thought of that. While the half loop does get closer and further away, thus increasing and decreasing the force, it seems to me that half of the time the increased force is pushing it in the correct direction, while the other half the increased force is slowing it down, resulting in a net zero increased force.
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MEC
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quote:
Originally posted by King of Men:
When the half-circle is in the down position, the force is into the screen, taking the current as running right to left and the magnetic field as going straight up. When the half-circle is in the up position, the force is likewise into the screen. But, because it is further away from the magnet, the force is weaker; therefore, neglecting friction, it is not braked as hard as it was pushed, and continues through to reach the down position and get another kick. The full coil can be considered as consisting of many half-circles.

Almost right, You're failing to take into account the direction of current on the other half of the loop.

code:
    |---->-----|
| |
--<-| |-<--
| |
|----<-----|

^ ^ ^
MAGNET


|----<-----|
| |
--<-| |-<--
| |
|---->-----|

^ ^ ^
MAGNET

In both the above time frames of the loop the magnitude of the Torque generated is equal, but opposite in direction, averaging out to zero over time.

It would work if it was supplied by an AC current, then the motor would rotate at the angular velocity of the current. However the AA Battery acts as a DC supply in the absence of any other components.

Is this motor sustainable? (i.e. if you start spinning the coil does it continue to spin indefinitely?)

Edit to add: If you want a cheap and easy demonstration of electromagnetic force, you could make a simple homopolar motor.

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MattP
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quote:
Is this motor sustainable?
Yes. I've built a few of these. You can get one in kit form too: http://www.scientificsonline.com/worlds-simplest-motor-kit.html
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DSH
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Try a homoploar motor (which is really the worlds simplest motor [Big Grin] )
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dem
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MPH...very inside joke
EMag was a flush out course at Georgia Tech (back in the day..80's-early 90%) that everyone in engineering (90% of the school) had to get through. I had a friend that worked in the physics department and even professors that were not currently involved in teaching EMag (many of them with international reputations) could not solve the problems on the test from this freshman level course. It was a rite of passage.
Thus
EMag - take the course
RMag - drop mid quarter and sign up next time
DMag - stay in the course on 2nd try and make a D, not good enough
CMag - A C isn't pretty, but you can graduate with it.

Some new president came in and put a lot of pressure on the Physics department to ease up and they finally relented. Having to endure things like this made Georgia Tech #1 on recruiter's lists during this period. When I went there the hill was steeper, the classes were harder and there were 10 guys for every girl!

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MEC
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quote:
Originally posted by DSH:
Try a homoploar motor (which is really the worlds simplest motor [Big Grin] )

I'd recommend going with something a little safer.

DangerouslyFun has a good model. And yes, I do see the irony of saying you should do something safer and then posting a link to DangerouslyFun.

Video of said motor at work.

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mr_porteiro_head
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quote:
Is this motor sustainable? (i.e. if you start spinning the coil does it continue to spin indefinitely?)
When it's working? Yes.

Although, we once got a really strange behavior -- it would spin in one direction for a few seconds, slow down, and reverse direction, spin fore a few seconds, slow down, and reverse direction. It kept doing that indefinitely.

Yeah, we did a homopolar motor as well, the simple version with the screw. It was fun. Honestly, it didn't strike me as unsafe. Sure, it's rotates very quickly, but because of the low mass and size, it wasn't worrisome. The angular momentum was quite small, and it wasn't going to go flying out into somebody's eye.

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DSH
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Holy cow!! [Eek!] DangerouslyFun isn't just a clever name is it?
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King of Men
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quote:
Almost right, You're failing to take into account the direction of current on the other half of the loop.
Yes, I realised this after going to bed. I observe that one of the two half-circles has to have one more strand of wire than the other, so there is an asymmetry. But I don't know whether that's sufficient to overcome the friction; it depends on the strength of the magnet.
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mr_porteiro_head
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KoM -- could you reply to my response to your half-loop idea?
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King of Men
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If we are sticking with a simple half-loop, and not filling out the coil, then I don't understand where your proposed symmetry comes from; can you expand? If you do fill out the coil, then your objection is basically the same as MEC's, and MEC is correct except for the minor asymmetry that one half of the coil must have one more wire in it than the other half. If this is the source of the force, then basically the windings of the coil are only there for structural strength.

Edit: A possible test would be to try coils with different numbers of windings; the half-loop theory predicts that the torque will be constant and therefore the speed will be inversely proportional to the number of windings. If, on the other hand, the windings are providing the torque in some way that none of us have been able to see, then presumably the speed will be constant-ish, since each winding adds the same amount of both current and mass, and a very small resistance. So, this may be a bad demonstration of electromagnetic force but perhaps you can use it to teach scientific method instead! And, by the way, if you do this please let us know the result. [Smile]

Edit the second: Perhaps a better way would be to measure the torque on the coil directly, rather than trying to infer it from the rotational speed, which you've said is a bit unreliable.

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MEC
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quote:
Originally posted by King of Men:
quote:
Almost right, You're failing to take into account the direction of current on the other half of the loop.
Yes, I realised this after going to bed. I observe that one of the two half-circles has to have one more strand of wire than the other, so there is an asymmetry. But I don't know whether that's sufficient to overcome the friction; it depends on the strength of the magnet.
Ah yes, I see that now.

quote:
Originally posted by mr_porteiro_head:
KoM -- could you reply to my response to your half-loop idea?

I know I'm not KoM but I think I can address your post. As per KoM's diagram, at any point in the rotation the electromagnetic force pushes into the screen. However, since the Torque is derived from the portion of the force that is normal to the plane formed by the wire and the axis of rotation, any point at which the wire is below the axis in elevation causes one direction of torque, while the opposite direction is caused when the wire is above the axis. The magnitude of the electromagnetic force is dependent on the intensity of the magnetic flux, thus the torque created when the wire is below the axis (closer to the magnet) will be greater than the torque created when it is above the axis.
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Dobbie
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Could gravity have anything to do with it? When the coil is properly aligned magnetically, it would be imbalanced gravitationally.
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Jenny Gardener
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My 5 year old is interested in electricity...any ideas on activities/projects/basic education I can use to help him better understand how it works? Electricity is pretty mysterious to me still. He does understand that fuel from squished and heated dinosaur-era living things makes stuff go. But how? And he is also fascinated with the whole earth around the sun causing seasons thing. I'm actually pretty delighted. But I'd like to keep the momentum going, and these concepts usually prove difficult to middle schoolers - and I have a preschooler asking me these questions!
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The Rabbit
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I'm trying to get a clear picture of how the magnetic field lines would go in this motor and its not intuitively obvious.

The current wouldn't flow around the wire coil. It's going to flow the same direction (+ to -) along each half loop. For it to work, each half of the ring has to be attracted as it approaches the magnet and repelled once it crosses the center of the magnet. Now I just need to get a clearer grasp of how the magnetic field lines go on a ring magnet. Isn't one pole in the center of the ring and the other pole on the outside of the ring? If that's the case, once a half loop passes the center of the ring magnet, the direction of the magnetic field flips, so it will be repelled rather than attracted?

Electromagnetism is one of my weaker areas so I could be completely wrong here.

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Dobbie
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The current does flow around the coil. If it were a solid metal ring the current would flow straight from positive to negative, but with a wire the current flows along the path of least resistance, which is along the wire.
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The Rabbit
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quote:
Originally posted by Dobbie:
The current does flow around the coil. If it were a solid metal ring the current would flow straight from positive to negative, but with a wire the current flows along the path of least resistance, which is along the wire.

But the net current has to be from + to -, regardless of the path of least resistance. If both ends of the wire are on the same terminal which must be the case for the coil to be symmetric (as indicated in the instructions), There is no driving force for flow around the coil. There is only a driving force toward the opposite pole.
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Dobbie
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The electromotive force drives the current through the wire from the positive end of the battery to the negative end, no matter what shape the wire is wound into.
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Dobbie
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Actually that should be "from the negative end to the [/i]positive[/i] end".
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The Rabbit
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quote:
Originally posted by Dobbie:
Actually that should be "from the negative end to the [/i]positive[/i] end".

Electrons flow from negative to positive. Current moves in the opposite direction of the electrons. Damn Ben Franklin.
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The Rabbit
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quote:
Originally posted by Dobbie:
The electromotive force drives the current through the wire from the positive end of the battery to the negative end, no matter what shape the wire is wound into.

Yes, that's what I'm saying. The current flowing in the copper wire will be from the positive terminal to the negative terminal along both sides of the copper coil because that's direction of the EM force. If you attached one end of the wire to one terminus and the other end to the other terminus, the current would flow around the coil. But that isn't what is being done in this experiment. One side of the coil is attached to the the + terminus, the other side of the coil is attached to - terminus. As long as the coil is symmetric, there is no driving force for electrons to flow around the coil.
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King of Men
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I think you are visualising something wrong. (Or else I am.) There is a single wire forming both the coil and the connections; this is not obvious in the picture because the connections have had the insulation stripped off, so it looks different, but it's all one wire. So there is a wire with one end at each terminus.

That said, your proposed setup is interesting, because I think I see how it would work. Consider it as a circuit diagram:

code:
      |------<-------|
| |
--<---| |---<--
| |
|------<-------|

If the coil didn't consist of a single wire, but rather of a bunch of wires all of which were connected to the terminals separately, then in effect you've got two resistors connected in parallel. Consequently the current flows through the top and bottom loops in the same direction, right to left. Then the force due to the magnetic field is into the screen on both parts of the coil, so as far as rotation goes they oppose each other; but because of the aforementioned asymmetry due to distance from the magnet, there is a net torque.

So this looks more sensible to me, in terms of the motor working, than relying on the asymmetry due to the extra half-loop; but on the other hand it also contradicts my understanding of the instructions. Perhaps Mr Squicky could clarify how he built his motor? Is the coil a single spiral, one end connected to each terminal; or can we consider it as a bit wire going in a circle, with one end of the circle connected to the positive and and to the negative terminal? (I'm having a hard time making myself clear without a blackboard to sketch on; I hope the distinction I'm making is getting through.)

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MattP
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The coil is a spiral. Imagine taking a long wire and wrapping it many times around a cylinder. The current will be flowing through one end of the wire, around the coil several times in the same direction, then exit through the other end of the wire.
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MattP
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The instructions here demonstrate the construction of the coil: http://www.stmary.ws/highschool/physics/home/lab/lab3/Electromagnetism_Lab_latest_files/image003.jpg
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Dobbie
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quote:
Originally posted by The Rabbit:
The current flowing in the copper wire will be from the positive terminal to the negative terminal along both sides of the copper coil..

That would be true of a solid ring of copper, not a coil of copper wire. The copper atoms are bonded to each other along the length of the wire. The current flows more easily along these bonds, so the current tends to follow the path of the wire, even when the wire comes into contact with itself.
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The Rabbit
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quote:
Originally posted by King of Men:
I think you are visualising something wrong. (Or else I am.) There is a single wire forming both the coil and the connections; this is not obvious in the picture because the connections have had the insulation stripped off, so it looks different, but it's all one wire. So there is a wire with one end at each terminus.

I just read the directions more closely and it looks like your initial interpretation was correct. The wire doesn't look to me like its insulated. What are the chances that the contact between the loops of the coil is good enough that current actually flows the way I'd initially assumed.

I happen to have a stack of ring magnets in my drawer. The top and bottom of the ring magnet act as opposite poles, but so do the inside and outside of the ring. If I hold one ring magnet perpendicular to the other, they attract on one side of the ring and repel on the other side of the ring. If a bar magnet were swinging past the ring, it would accelerate toward the first side, momentum would carry it past the whole and then it would be repelled past the second side. The motor couldn't work with two ring magnets, since the second side would be repelled rather than attracted to the first side.

I think this means that current can't be flowing around the loop, it has to acting like two resistors in parallel.

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King of Men
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quote:
The wire doesn't look to me like it's insulated.
The instructions explicitly call for stripping off the insulation at the ends; it's common for insulation on lab wire to be a very thin coat of varnish, not obvious if you don't know what you're looking for. But at this point I think we had better wait for Squicky to give us more information about the construction.
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The Rabbit
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quote:
Originally posted by MattP:
The coil is a spiral. Imagine taking a long wire and wrapping it many times around a cylinder. The current will be flowing through one end of the wire, around the coil several times in the same direction, then exit through the other end of the wire.

Yes, but when you make a regular solenoid, you don't tie the opposite sides together with the wire. That will reduce the contact resistance between the wires. It appears that the combined resistance of a bunch of strips of wire short wire in parallel + contact resistance is less than the resistance of one long thin wire. If it weren't, this motor couldn't work.

We could design some experiments to test my theory. Try loosening the coil to decrease the contact between the wire loops. If I'm right, the motor will stop working.

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MEC
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quote:
Originally posted by The Rabbit:
quote:
Originally posted by MattP:
The coil is a spiral. Imagine taking a long wire and wrapping it many times around a cylinder. The current will be flowing through one end of the wire, around the coil several times in the same direction, then exit through the other end of the wire.

Yes, but when you make a regular solenoid, you don't tie the opposite sides together with the wire. That will reduce the contact resistance between the wires. It appears that the combined resistance of a bunch of strips of wire short wire in parallel + contact resistance is less than the resistance of one long thin wire. If it weren't, this motor couldn't work.

We could design some experiments to test my theory. Try loosening the coil to decrease the contact between the wire loops. If I'm right, the motor will stop working.

You could just use an Ohmmeter to test this.
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MEC
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quote:
Originally posted by MattP:
The instructions here demonstrate the construction of the coil: http://www.stmary.ws/highschool/physics/home/lab/lab3/Electromagnetism_Lab_latest_files/image003.jpg

From these instructions I see that the insulation is only removed on one half of the wire that is touching the conducting supports. This would cause the wire loop to conduct only half of the time in the spin, thus the opposite half of the spin would get no current and thus no forces to oppose that of the conducting half.

quote:
Originally posted by The Rabbit:
I'm trying to get a clear picture of how the magnetic field lines would go in this motor and its not intuitively obvious.

The current wouldn't flow around the wire coil. It's going to flow the same direction (+ to -) along each half loop. For it to work, each half of the ring has to be attracted as it approaches the magnet and repelled once it crosses the center of the magnet. Now I just need to get a clearer grasp of how the magnetic field lines go on a ring magnet. Isn't one pole in the center of the ring and the other pole on the outside of the ring? If that's the case, once a half loop passes the center of the ring magnet, the direction of the magnetic field flips, so it will be repelled rather than attracted?

Electromagnetism is one of my weaker areas so I could be completely wrong here.

for the magnetic field generated by the coil, point your right thumb in the direction of the current and imagine your hand tangential to the loop. Coil your fingers through the loop, the side of the hole in the loop your fingers are coming out of is the North face, the side of the hole your fingers are entering the hole is the south face.
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The Rabbit
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quote:
Originally posted by MEC:
quote:
Originally posted by The Rabbit:
quote:
Originally posted by MattP:
The coil is a spiral. Imagine taking a long wire and wrapping it many times around a cylinder. The current will be flowing through one end of the wire, around the coil several times in the same direction, then exit through the other end of the wire.

Yes, but when you make a regular solenoid, you don't tie the opposite sides together with the wire. That will reduce the contact resistance between the wires. It appears that the combined resistance of a bunch of strips of wire short wire in parallel + contact resistance is less than the resistance of one long thin wire. If it weren't, this motor couldn't work.

We could design some experiments to test my theory. Try loosening the coil to decrease the contact between the wire loops. If I'm right, the motor will stop working.

You could just use an Ohmmeter to test this.
Yes that occurred to me after I made the post. [Big Grin] Sadly, I don't have a spool of magnet wire and an Ohm meter in my desk drawer to test it out.

Back of the envelope calculation: Based on the specs, it looks like you should get 8 and 1/2 windings around the battery, so we are talking about 17 resistors in parallel + contact resistance vs. 17 resistors in series. The resistors in series will have 306 times the resistance of the resistors in parallel so if the contact resistance is less than ~300 times the resistance of ~0.75 cm of magnet wire, current would flow through the parallel rather than the in series configuration. I'm calculating a restance of ~4 micro Ohms for a cm of magnet wire. A contact resistance of less than 1.2 milli-Ohms seem possible but not highly likely.

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The Rabbit
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quote:
for the magnetic field generated by the coil, point your right thumb in the direction of the current and imagine your hand tangential to the loop. Coil your fingers through the loop, the side of the hole in the loop your fingers are coming out of is the North face, the side of the hole your fingers are entering the hole is the south face.
I'm fine with that in two dimensions. My difficulty is visualizing what happens in the third dimension. The field strength drops off by distance squared, so because the distance between the permanent magnet and the coil is much smaller (at closest approach) than the diameter of the coil or the permanent magnet, what the magnet field lines are doing around the edges of the coil and center of the permanent magnet matters -- doesn't it?
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The Rabbit
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quote:
Originally posted by MEC:
quote:
Originally posted by MattP:
The instructions here demonstrate the construction of the coil: http://www.stmary.ws/highschool/physics/home/lab/lab3/Electromagnetism_Lab_latest_files/image003.jpg

From these instructions I see that the insulation is only removed on one half of the wire that is touching the conducting supports. This would cause the wire loop to conduct only half of the time in the spin, thus the opposite half of the spin would get no current and thus no forces to oppose that of the conducting half.
Yes, but the instructions at the web site MPH used, say to remove ALL the insulation from the wire. MPH did this, presuming it wouldn't work but it did -- hence the thread.
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King of Men
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What do you want the magnetic field generated by the coil for?
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