# Left Hand Rule Applied to a Winding

## Summary:

If the winding has equal current and directions in each side. They cancel out. So how do you determine the principal force directions?
All illustrations of direction of force in motor has this form.

For the following specifically the rotor. i'm confused where is the direction of force. It uses winding and there seems to be current and force in every side of the winding (applying left hand rule). So where does the principal force really point? I've been figuring this out for days. So hope someone can give aid.

This is video of the motor running.

https://d2y5sgsy8bbmb8.cloudfront.n...rm-Generic-480p-16-9-1409173089793-rpcbe5.mp4

It's from amazon science kit. https://www.amazon.com/gp/product/B073GXWQMV/?tag=pfamazon01-20

BvU
Homework Helper
If the winding has equal current and directions in each side. They cancel out. So how do you determine the principal force directions?
Is the current in CD really running in the same direction as in AB ?

Is the current in CD really running in the same direction as in AB ?
Here is zoom of the center.

Where is CD and AB?

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Merlin3189
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Gold Member

BC and AD rotate in plane of field, so cross product (force ) is always perpendicular to that plane, ie axial and produces no rotation.

AB and CD : currents may be equal in magnitude and direction, but as vectors they are opposite in sense. Field stays the same, current is opposite, so force must be opposite.

If in doubt, I draw in a few field lines then look at them as Faraday describes.

View attachment 273403
BC and AD rotate in plane of field, so cross product (force ) is always perpendicular to that plane, ie axial and produces no rotation.

AB and CD : currents may be equal in magnitude and direction, but as vectors they are opposite in sense. Field stays the same, current is opposite, so force must be opposite.

If in doubt, I draw in a few field lines then look at them as Faraday describes.
no problem about it, but how do you apply it to the following where the winding has 4 sides unlike the above?

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View attachment 273403
BC and AD rotate in plane of field, so cross product (force ) is always perpendicular to that plane, ie axial and produces no rotation.

AB and CD : currents may be equal in magnitude and direction, but as vectors they are opposite in sense. Field stays the same, current is opposite, so force must be opposite.

If in doubt, I draw in a few field lines then look at them as Faraday describes.
I understand the above. No problem with this.

DC Motor, How it works? - YouTube

It's obvious. But in the following, the conductor is in form of windings at all sides, and they seem to cancel so where is the EMF force directions at the right and left?

zoom

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Merlin3189
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Again, the (now) vertical wires give axial forces, so irrelevant.
The now horizontal wires have up and down forces, top up and bottom down. But, that's because they are at the changeover position, when, in fact, there should be no current flowing.
If you imagine the rotor turned 90 deg ACW, so that the coils are top and bottom, and keep the direction of the current as shown here, then the wires on the LHS get an upward force and those on the RHS a downward force, giving a CW torque. If the current is in the same sense in both coils, that applies to all the wires in both coils.

You could of course simply imagine the rotor as an electromagnet. As shown in the pic, viewed from the right, the current is CW, so it looks like a S pole and is attracted to the N pole, which is where it is. From the left current is ACW and looks like an N pole being attracted to the S pole where it is. No reason to turn.

When the rotor is turned 90 deg ACW as above, the S pole is at the top and is attracted to the N pole on the right. Similarly the N pole of the rotor is now at the bottom and attracted to the LH S pole. A CW torque, just as produced by the, up on the left, down on the right, forces above.

TBH I've no idea how to analyse this sort of rotor, with an iron cored rotor and shaped field poles. You probably need and electrical engineer for that. (I expect Jim Hardy would have known). I just knew the basic principle of the motor with wires in a uniform field, as taught in Physics text books. After that I just put my faith in Eric Laithwaite's principle that the more iron and copper, the more efficient the motor! It's actually always puzzled me that the wires in most motors don't seem to be in any magnetic field: nearly all the flux seems to go through the iron, bypassing the wires. The magnetic flux "links" the coils and I've seen analysis of transformers based on that idea, but not motors. It's probably my bad understanding of field and flux.

jake jot
Again, the (now) vertical wires give axial forces, so irrelevant.
The now horizontal wires have up and down forces, top up and bottom down. But, that's because they are at the changeover position, when, in fact, there should be no current flowing.
If you imagine the rotor turned 90 deg ACW, so that the coils are top and bottom, and keep the direction of the current as shown here, then the wires on the LHS get an upward force and those on the RHS a downward force, giving a CW torque. If the current is in the same sense in both coils, that applies to all the wires in both coils.

You could of course simply imagine the rotor as an electromagnet. As shown in the pic, viewed from the right, the current is CW, so it looks like a S pole and is attracted to the N pole, which is where it is. From the left current is ACW and looks like an N pole being attracted to the S pole where it is. No reason to turn.

When the rotor is turned 90 deg ACW as above, the S pole is at the top and is attracted to the N pole on the right. Similarly the N pole of the rotor is now at the bottom and attracted to the LH S pole. A CW torque, just as produced by the, up on the left, down on the right, forces above.

TBH I've no idea how to analyse this sort of rotor, with an iron cored rotor and shaped field poles. You probably need and electrical engineer for that. (I expect Jim Hardy would have known). I just knew the basic principle of the motor with wires in a uniform field, as taught in Physics text books. After that I just put my faith in Eric Laithwaite's principle that the more iron and copper, the more efficient the motor! It's actually always puzzled me that the wires in most motors don't seem to be in any magnetic field: nearly all the flux seems to go through the iron, bypassing the wires. The magnetic flux "links" the coils and I've seen analysis of transformers based on that idea, but not motors. It's probably my bad understanding of field and flux.
In the amazon motor above. Do you think the entire curved blue and Red metal body is the magnet or is the magnet only at the top. I gave the product to a kid and it isn't with me now. But I'll borrow from the kid again later this week.

Merlin3189
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Gold Member
Yes. The iron shell guides the flux from the magnet at the top and produces a horizontal field around the rotor.
Your first diagrams used two magnets one on each side of the rotor. The field can be strengthened by joining the outside faces with iron, completing the manetic "circuit".
Early motors sometimes used horseshoe magnets to have the same effect.
This idea is very economical because you need only one simple magnet, then the cheap shaped piece of iron brings the field to where you want it.

jake jot
BvU
Homework Helper
Hello Jake,

No need to re-post the same image five times. I do wonder who drew the blue current lines on it, though. If they are correct, the motor is in a stable stalled position. The current should be reversed to create a north pole on the righthand side of the coil. Like @Merlin3189 , I suspect the brushes are at or near the point of reversing the current direction.

Did you notice Amazon makes a mess of the pictures by attaching the red iron arc to the south permanent magnet pole in half of them ?

Hello Jake,

No need to re-post the same image five times. I do wonder who drew the blue current lines on it, though. If they are correct, the motor is in a stable stalled position. The current should be reversed to create a north pole on the righthand side of the coil. Like @Merlin3189 , I suspect the brushes are at or near the point of reversing the current direction.

Did you notice Amazon makes a mess of the pictures by attaching the red iron arc to the south permanent magnet pole in half of them ?
I drew the blue current label. After i drew it. I got stuck thinking how it could move. So the secret lies in reversing the current. Thanks for the ideas.

BvU
Homework Helper
secret lies in reversing the current
Yes. Basically you force the poles of permanent magnet and electromagnet to be equal at the moment shown in your picture, so they repel each other and the rotor keeps turning. The brushes take care of that.

Yes. Basically you force the poles of permanent magnet and electromagnet to be equal at the moment shown in your picture, so they repel each other and the rotor keeps turning. The brushes take care of that.
Why is that in most motor teachings. They use the lorenz force in emphasizing it is how motor rotates and not concept of north and south? Which has more contribution to the rotation? See for example

BvU
Homework Helper
Which has more contribution to the rotation?
It is one and the same thing. The Lorentz force is the much more fundamental concept.

It is one and the same thing. The Lorentz force is the much more fundamental concept.
But notice in the following rotor that the north and south pole depicted in the right and left side label is 90 degrees off the lorentz force which comes out of the conductor (remember all the up and down arrows in the conductors depicting the directions of the lorentz force?

BvU
Homework Helper
What are we looking at ? Current in top horizontal part of the coil is coming towards us, lower away from us. Where is the permanent magnet field and which way is it pointing ?

What are we looking at ? Current in top horizontal part of the coil is coming towards us, lower away from us. Where is the permanent magnet field and which way is it pointing ?
Even without the stator or permanent magnet. The rotor has north and south that is perpendicular to the wires. It is the wires which is source of direction in the left hand rule and the lorenz arrows come from wires. This means north, south is not identical to the force direction. Right? Lets say it doesnt produce any force directions. Can the rotor rotate if there is present of stator or permanent magnet around it?

Even without the stator or permanent magnet. The rotor has north and south that is perpendicular to the wires. It is the wires which is source of direction in the left hand rule and the lorenz arrows come from wires. This means north, south is not identical to the force direction. Right? Lets say it doesnt produce any force directions. Can the rotor rotate if there is present of stator or permanent magnet around it?
Here is to illustrate the point. Some demo in youtube only uses north, south and not the direction of force. I think you are saying it's two side of a coin. But let's say we won't use the "direction of force" explanation. Can one explain completely the action of the north, south, commutator in the rotor with respect to the permanent magnet? So the reason the rotor rotates is because the magnetic pole is attracted to the opposite side and this rotates the rotor? But the magnets kinda too far, is the north/south style of explanation able to stand alone (by suppressing the more powerful lorentz force explanation (i drew the violet lorentz force directions in the following)?

How DC Motors Work - YouTube

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BvU
Homework Helper
I think you are saying
Correct, that is what I am saying.
We can go round in circles infinitely, but by now I don't think I still understand what your core question is.
is the north/south style of explanation able to stand alone
Yes, because it is the same thing

Correct, that is what I am saying.
We can go round in circles infinitely, but by now I don't think I still understand what your core question is.
Yes, because it is the same thing
Is it like:

General relativity = lorentz force and left hand rule
Newtonian gravity = Rotor north south

Meaning one is a better explanation. Or like

Space = lorentz force
Time = North and South

Spacetime meaning they occur at the same time?

My core question is. Does a bar magnet necessary have current and lorentz force if it is made to rotate in a permanent magnet?

Imagine you are in the ISS space station or just space that has weightlessness. You put a permanent magnet but instead of using a rotor with commutator. You use a theoretical bar magnet where the polarity can change every half revolution just like having a commutator. This can make the rotor turn indefinitely. But without any lorentz force or left hand rule in the theoretical pole changing magnet rotor (just for sake of illustration).

BvU
Homework Helper
Don't get carried away in space and time. Weightlessness does not come into this anywhere. Stay with feet on ground.

Does a bar magnet necessary have current and lorentz force if it is made to rotate in a permanent magnet?
Magnets tend to align.

sophiecentaur
Don't get carried away in space and time. Weightlessness does not come into this anywhere. Stay with feet on ground.

Magnets tend to align.
Is the velocity to keep it align the same velocity as the lorentz force?

This is the only way they can be equivalent. Any equation or something to equate either has the same value?

sophiecentaur
Gold Member
2020 Award
the same velocity as the lorentz force?
I can't think you meant to say it that way. It doesn't make sense to equate different quantities.

I can't think you meant to say it that way. It doesn't make sense to equate different quantities.

Let us focus the above (let me share it the last time to aid in the discussion).

The velocity of the lorentz force is the speed of light.
I wasn't referring to it. Instead, depending on the value of the lorentz force by way of the current strength, it can make the rotor run faster.

Yet someone explained how the magnet tending to align themselves can stand alone as explanation. But something is not right. First since they are the rotor is solid unit, the alignment of the (electro)magnet is related to the lorentz force acting on the wire. I was asking whether without this lorentz force acting on the wire and pushing the rotor making it rotate, whether the magnet aligning can stand alone as explanation. Someone said yes. So can you share a design where only the magnet is present without any lorentz force acting? This is the only way to prove the aligning alone of the magnets can make rotor rotate as it is. This can happen if there is no current from batteries. But is it not a magnet has some current or electrons align inside. So kindly illustrate some sort of setup where the lorentz force is missing and only the aligning of the magnets make the rotor turn. Thank you.

sophiecentaur