## My homopolar motor

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### My homopolar motor

I have a few questions about my little motor.

1. My motor spins clockwise, would it spin counter-clockwise on the opposite side of the world?
2. Is the battery I use to power it loosing "juice" as it spins?
3. Which direction would it spin if I were exactly 1/2 the distance between where it is now and the opposite side of the word (given that I am right about it changing directions if I moved to the opposite side of the world)
4. What force is causing it to rotate, is it the electrical energy or the magnetism?

My homopolar motor,

A battery with a magnet attached to the pos. side and a peace of copper wire shaped like a heart that rests with the min point on the top half of the heart resting on the negative side, here is a youtube link to one that is exactly the same as mine. ( http://www.youtube.com/watch?v=EkU_JmtH3PU )
Vysero
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Joined: 24 Oct 2011

### Re: My homopolar motor

Vysero wrote:I have a few questions about my little motor.

1. My motor spins clockwise, would it spin counter-clockwise on the opposite side of the world?
2. Is the battery I use to power it loosing "juice" as it spins?
3. Which direction would it spin if I were exactly 1/2 the distance between where it is now and the opposite side of the word (given that I am right about it changing directions if I moved to the opposite side of the world)
4. What force is causing it to rotate, is it the electrical energy or the magnetism?

My homopolar motor,

A battery with a magnet attached to the pos. side and a peace of copper wire shaped like a heart that rests with the min point on the top half of the heart resting on the negative side, here is a youtube link to one that is exactly the same as mine. ( http://www.youtube.com/watch?v=EkU_JmtH3PU )

It would spin the same way.

The battery is losing energy.

It would spin the same way.

The battery supplies the energy for the rotation. That's not the same as force, but I suspect you asked the wrong question.
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Joined: 31 Dec 2008

### Re: My homopolar motor

Here a Wippy on the Lorentz force.
http://en.wikipedia.org/wiki/Lorentz_force
Dutchman, Hendrik Lorentz. Good man.
Finally, in 1892, Hendrik Lorentz derived the modern day form of the formula for the electromagnetic force which includes the contributions to the total force from both the electric and the magnetic fields. ...
...Using the Heaviside's version of the Maxwell equations for a stationary ether and applying Lagrangian mechanics, Lorentz arrived at the correct and complete form of the force law that now bears his name.[11][12]

The Lorentz force is the force on a moving charge in a magnetic field and it depends on the direction the charge is going and also the direction of the magnetic field lines. (The complete Lorentz force also includes the effect of static charge, the electric field, but that is not part of this picture.)

The convention (just to keep the directions straight) is that magnetic field lines come OUT OF THE NORTHPOINTING POLE OF THE LITTLE MAGNET in the picture and spread out and circle around and come back in the SOUTHPOINTING end of the magnet.

If you suspend the magnet by a thread you can tell which end is which.

If you put the magnet on the table with its northpointing end upwards, then the magnetic field lines are coming up and spreading OUTWARDS thru where the whirling wire is cutting thru them.

Lorentz said you could use your RIGHT HAND if the moving charge was positive. And that goes for current too since by ancient custom the current is backwards from the flow of electrons, so it is AS IF the current was a flow of positive particles. If you want to be extremely realistic you can you can use your left hand because it is really negative electrons moving from the minus to the plus end of the battery.

1. Thumb in the direction the charge is moving.
2. Index finger in direction of the magnetic field lines (your hand looks like a toy pistol and you now bring out your MIDDLE finger at right angles to both thumb and index)
3. The Lorentz force is in the direction of the middle finger.

What this means is if the northpointing end of the magnet is UP. And if the current is flowing UP (from plus nipple to minus butt end of the battery). Then as you look down on the motor you will see it whirling COUNTERCLOCKWISE.

Actually the real flow is negative electrons from butt to nipple which is down and we should use our left hand but this is inconvenient so use your right hand. Point thumb up. index finger out from central axis, and then middle finger will point in the direction of the Lorentz force. That translates into counterclockwise turning.

In the YouTube, he evidently has the magnet sitting on the table with its SOUTHPOINTING end upwards.
So the turning is reversed. The fieldlines are pointing INWARDS thru the space where the wire is cutting thru them. That is why the motor is whirling CLOCKWISE.

Marshall
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Joined: 17 Oct 2006

### Re: My homopolar motor

Awesome thanks guys, so all i had to do was flip the magnets lol

Now here is my new question, what I have now is I connected a wire to the positive end of the battery. I placed a small magnet on a screw and connected a screw to it so that the sharp end of the screw is dangling from the negative end of the battery. Now, when I complete the circuit from the wire to the screw both the magnet and the screw turn by themselves for a bit then they stop. So, a few questions..

1. Is this the same force that turns my copper wire in the previous motor?
2. Why does it only spin for a few seconds?

Vysero
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Posts: 36
Joined: 24 Oct 2011

### Re: My homopolar motor

Vysero wrote:Awesome thanks guys, so all i had to do was flip the magnets lol

Now here is my new question, what I have now is I connected a wire to the positive end of the battery. I placed a small magnet on a screw and connected a screw to it so that the sharp end of the screw is dangling from the negative end of the battery. Now, when I complete the circuit from the wire to the screw both the magnet and the screw turn by themselves for a bit then they stop. So, a few questions..

1. Is this the same force that turns my copper wire in the previous motor?
2. Why does it only spin for a few seconds?

In the YouTube, the screw is dangling from the plus end of the battery. It is stuck to the nipple, which is the positive end. You say negative. I don't suppose this matters.

How does this work?
When you touch the wire to the side of the magnet, you complete an electric circuit. Current flows out of the battery, down the screw, sideways through the magnet to the wire, and through the wire to the other end of the battery. The magnetic field from the magnet is oriented through its flat faces, so it is parallel to the magnet's axis of symmetry. Electric current flows through the magnet (on average) in the direction from the center of the magnet to the edge, so it flows in the radial direction, perpendicular to the magnet's axis of symmetry. If you took physics at some point, it's possible that you'll remember the effect that a magnetic field has on moving electric charges: they experience a force that is perpendicular to both their direction of movement and the magnetic field. Since the field is along the symmetry axis of the magnet and the charges are moving radially outward from that axis, the force is in the tangential direction, and so the magnet begins to spin. Neat!
==endquote==

This makes it clear that it is the SAME FORCE but the current this time is RADIAL in the magnet and the field lines are VERTICAL in the body of the magnet.

Whenever a charged particle cuts across a field line the direction of the Lorentz force on the charged particle is perpendicular to both the particle's direction and the fieldline direction.

So in this case what direction is perp to both the vertical axis and the radius (of the disk magnet)? the tangential direction.

By the righthand rule, if the current is going IN from the edge towards center (along a radius) and the field lines point UP then the Lorentz force will tend to spin the magnet disk counter clockwise.

========================
I cant think of a good reason why it would only spin a few seconds.

It sounds like the conductive metal jacket of the neodymium magnet might be defective---not cover completely.
How conductive is neodymium magnet alloy itself? Only about 1/10 as conductive as iron, I think
Not sure about this but you might have a kind of half-assed capacitor built into the circuit. Two ordinary conductors separated by an inferior conductor. current flows easily only for a short time and then cuts back to a lower level until capacitance is discharged. Just my speculation.

Canady would know whether this makes sense or not. Engineer. I'm an ex-mathematician and I just speculate. :-D

Or the wire tip is bad? Or it heats up and stops making good contact for some reason?

In any case a lot would depend on the nickel plate jacket of the neodymium magnet, or whatever material magnet it is.

Marshall
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Posts: 7889
Joined: 17 Oct 2006

### Re: My homopolar motor

It stops when you disconnect the wire, i.e., break the circuit, reducing the current to zero. If you have zero current, you have zero force. There may be friction at the tip, there may be air drag, but the biggest loss of energy is the wobble.