Magnetic field rotation or not?

In summary: He's saying that the system of power (the magnet) and the rays of light (the conductor) are both stationary while the observer (you or me) rotates around them. So he's saying that the magnetic field does not rotate with the magnet.
  • #1
Circlotron
12
0
Suppose for a moment you had a circular copper disc placed concentrically in the air gap between the poles of an upper and a lower circular magnet, the outer faces of the upper lower magnet being connected by a pole piece so as to complete the magnetic circuit. The edge of the copper disc is connected to a first wire and the centre of the copper disc is connected to a second wire that passes through a hole in the centre of the upper magnet and the pole piece so both wires are unaffected by any airgap flux.

Basically a Faraday homopolar motor/generator setup. Nothing new so far.

If now the magnets and pole piece are held stationary and the disc is rotated about the axis of the assembly we will get a small emf induced across the radius of the disc because there is relative motion between the flux and the disc.

Conversely, if the disc is held stationary and the magnets and pole piece are rotated around the axis of the assembly, people more knowledgeable than I say that there will be no emf induced across the radius of the disc because the magnetic flux does not rotate axially with the magnet assembly so there is no relative motion between the flux and the disc.

Let's now switch from generator mode to motor mode and where the disc is held firm and the entire magnet assembly is free to rotate (within the limits of the pole piece going from the outer faces of the upper to lower magnet colliding with the wire connected to the disc edge).

If we now pass a current through the radius of the disc, the unmoveable disc will exhibit a torque in a certain direction and the magnet assembly may or may not exhibit a torque in the opposite direction. If the magnet assembly does exhibit a torque it will rotate until the pole piece hits the wire.

Here is where things get interesting. If the magnet assembly does in fact exhibit a torque and rotate a certain distance then it is capable of doing work. However, established thinking says that the rotating magnet assembly will NOT induce a counter emf across the disc radius because magnetic flux does not rotate axially with an axially rotating magnet as it's source. This means that (ignoring dc resistance) there can be a supply current through the disc radius but no voltage across the disc radius because there is no counter emf generated by the rotating magnet. Watts input = ? amps x zero volts so there would be zero watts input to the system despite torque and motion = some watts output!

The alternative to this of course is that the disc would exhibit a torque in one direction but the magnet assembly would not exhibit a counter-torque. Just what IS going on here?
 
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  • #2
Magnetic fields have a rotational effect on charged particles. If the particles with a certain velocity encounter the B-field perpendicularly then the particles trace out a circular orbit, otherwise helical path.
 
  • #3
Can someone explain why my alternative conclusions would be wrong? There only seems to be two alternatives and both of them are awkward.
 
  • #4
Circlotron said:
Can someone explain why my alternative conclusions would be wrong? There only seems to be two alternatives and both of them are awkward.
The magnetic flux rotates with the magnet. Faraday did a series of experiments that demonstrate it does. These are in his Experimental Researches in Electricity, Series XXVIII, article 3088 and beyond. When a conducting circuit is properly oriented with respect to a cylindrical magnet (which requires a clever, unconventional set up), and that magnet is then rotated about the axis of magnetization, a current is generated. Likewise, when the same circuit is fixed to the magnet and made to rotate along with it, no current is generated. In the former case the conductor is stationary while the magnet is rotated, in the latter case there is no relative motion between conductor and magnet. In other words, the results are exactly what you'd expect.

Now, however, and this is the surreal part, Faraday, himself, is the person responsible for creating and publishing the idea we shouldn't consider the magnetic field to be rotating along with the magnet. For reasons that aren't completely clear to me, he already believed the field doesn't rotate with the magnet, and so, when he explained the above demonstrations, he did so in such a way that his notion was preserved instead of being overturned as it should have been.

That's hard to account for, but there is a possible clue in his wording:

"The system of power about the magnet must not be considered as necessarily revolving with the magnet, any more than the rays of light which emanate from the sun are supposed to revolve with the sun. The magnet may even, in certain cases (3097), be considered as revolving amongst its own forces, and producing a full electric effect, sensible at the galvanometer."
-Series XXVIII, 3090

We know that Faraday harbored the then unprovable suspicion that light was a form of electromagnetic energy. He would, quite likely then, consider a static magnetic field to be a form of light, and would logically suppose the magnetic field around a magnet must be detached from the magnet just like light is detached from the source of light. When current was generated in the above demonstrations, he was prejudiced against seeing it as the result of the rotating magnetic field cutting the stationary conductor, and, instead, conceived of the magnet having a current induced in it by "revolving amongst its own forces."

The set up you describe is different, of course, but the explanation for lack of current is not that the field isn't rotating.
 
  • #5
zoobyshoe said:
Faraday did a series of experiments that demonstrate it does. These are in his Experimental Researches in Electricity, Series XXVIII, article 3088 and beyond.

Got a link? I'm having a hard time finding this.
 
  • #7
zoobyshoe said:
The magnetic flux rotates with the magnet.
Wow!
Back in the late 90's I battled with titans on the old sci.physics.electromagnetics usenet group, having come to this exact conclusion through my own experiments but I was shouted down by all and sundry, some who had Phd's in subjects I didn't know existed and others who were just plain kooks. They couldn't give an explanation at all, some invoking relativity and whatnot. My hope at the time was to try and make a brushless (true) dc motor and eventually a dc motor-generator that would have the same function as a transformer but for dc. I didn't achieve this but I learned a lot along the way.
 
  • #8
I remember finding this interesting when I found it not long after it was published. The author copped a lot of flak from the scientific community at the time IIRC. -> http://www.google.com.au/url?sa=t&r..._DskFw0q6NJ7eVQ&bvm=bv.89381419,d.dGY&cad=rja

Unipolar Experiments
A. G. K ELLY
HDS Energy, Celbridge, Co. Kildare, Ireland
agkelly@eircom.net
ABSTRACT Novel experiments on the relative motion of conductors and
magnets are described. In contradiction of the currently accepted
interpretation, it is shown that the field of a magnet rotates with the magnet
about its North-South axis. Faraday’s Law is shown to be a particular case
 
  • #9
I don't understand how rotating a symmetrical magnetic field could even result in the generation of current. Field lines just represent the direction and magnitude of the vector field, right? How can rotating the magnetic field do anything if there's no change in the field anywhere? You can say that the field lines are moving, but that just seems to be a visual artifact of having a finite number of field lines drawn. The magnitude and direction of the vector at every point in the field stays the same, don't they?
 
  • #10
Drakkith said:
How can rotating the magnetic field do anything if there's no change in the field anywhere?
You don't need a change in the field. Think of a dynamic microphone. As the voice coil moves through the gap the air gap flux density (in a good microphone at least!) remains constant but the voice coil does produce a voltage.
 
  • #11
Circlotron said:
You don't need a change in the field. Think of a dynamic microphone. As the voice coil moves through the gap the air gap flux density (in a good microphone at least!) remains constant but the voice coil does produce a voltage.

You got me there. I was under the impression that electromagnetic induction occurred because the field was changing as you moved a magnet in and out of a coil (or vice versa). I have no idea what happens if you move a coil in a non-changing magnetic field.
 
  • #12
Circlotron said:
Wow!
Back in the late 90's I battled with titans on the old sci.physics.electromagnetics usenet group, having come to this exact conclusion through my own experiments but I was shouted down by all and sundry, some who had Phd's in subjects I didn't know existed and others who were just plain kooks. They couldn't give an explanation at all, some invoking relativity and whatnot. My hope at the time was to try and make a brushless (true) dc motor and eventually a dc motor-generator that would have the same function as a transformer but for dc. I didn't achieve this but I learned a lot along the way.
I want to attach pictures of the pages in question so you and Drakith can read them and see if you agree with my reading of them. However, I can't figure out how to attach images anymore since the last change in PF format.
 
  • #13
Drakkith said:
You got me there. I was under the impression that electromagnetic induction occurred because the field was changing as you moved a magnet in and out of a coil (or vice versa). I have no idea what happens if you move a coil in a non-changing magnetic field.
Also, you must have come across mention of creating current in a conductor simply by moving it between the poles of a horseshoe magnet.
 
  • #14
OK, my scanner is not working so I had to photograph the pages. I uploaded the pics to this site:

https://sites.google.com/site/mrzoobyshoe/faraday1

Go there and you'll see three clickables, Faraday1, Faraday2, and Faraday3.

Some of it refers to stuff on previous pages. The experiments in question get started at the top of column 2 on the first page. The diagrams are schematics of his set up. A more realistic drawing of how he actually embodied this device is on a previous page.
 
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  • #15
zoobyshoe said:
I want to attach pictures of the pages in question so you and Drakith can read them and see if you agree with my reading of them. However, I can't figure out how to attach images anymore since the last change in PF format.
Click the 'upload' button at the bottom right of the reply box.
 
  • #16
You need special relativity to effectively deal with problems like this. Classical physics lead us to wrong conclusions (like that the magnetic field is rotating when we rotate the magnet).
 
  • #17
zoobyshoe said:
Also, you must have come across mention of creating current in a conductor simply by moving it between the poles of a horseshoe magnet.

When I say a 'changing' field, I mean that the magnitude of the field at anyone spot in the conductor changes over time, which happens when you move it through the field of a horseshoe magnet.
 
  • #18
One of the test setups I made way back used the magnet and pole piece assembly from an ordinary loudspeaker. I cut a strip of copper and formed it into a loop and soldered the ends together. I put a length of tape on the inside and the outside of the loop but left the edges bare. Then I placed the copper loop into the speaker magnet air gap far enough to let one edge of the copper loop touch the disc shaped rear steel pole piece so that it had electrical contact. The outer edge of the copper loop had a first wire soldered to it and the disc shaped rear steel pole piece had a second wire connected to it.

Anyway, the idea was that if you held the magnet stationary and rotated the copper loop in the gap you could measure several millivolts from one edge of the loop to the other. What's more, if you held the loop still and rotated the magnet you could also see several millivolts from one edge of the loop to the other. Practically all the flux was across the airgap, through the copper, so very little would be affecting the attached measurement wires like they were in Faraday's experiments.

After that I insulated the inner edge of the cooper loop so that it now didn't contact the the disc shaped rear steel pole piece but instead soldered a wire to the edge and passed it through a hole in the rear pole piece. Then I rotated the assembly on the axis of the centre pole and measured zero millivolts. If the flux was NOT rotating with the magnet I should have measure some slight voltage but I got none.
 
  • #19
Drakkith said:
Click the 'upload' button at the bottom right of the reply box.
Ah! Too late, but now I know.
 
  • #20
Another experiment I did was to get a stack of old magnets from microwave ovens and halfway up the stack put a copper disc. Then I put one wire down the centre hole to the centre of the copper disc and a second wire through a hole in the side to the edge of the disc. Essentially all the flux went from the top of the upper magnet, through the surrounding steel enclosure, and to the bottom of the lower magnet. Again no stray flux to influence the measurement wires. Rotate the entire assembly as a unit and zero millivolts detected. Again, this seems to tell me the flux rotates with the magnet. See attached diagram.
 

Attachments

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  • #21
Circlotron said:
Again, this seems to tell me the flux rotates with the magnet.
I agree, based on Faraday's results. Where a non-rotating field ought to result in current, it doesn't, and where it shouldn't result in current, it does. I have to suppose the field is rotating. Provided you rig the circuit such that only one half of the field "cuts" the stationary conductor, rotating the magnet generates current. (You can't use the whole length of the field because that would just induce the same positive or negative voltage in opposing directions.)
 
  • #22
Although it hasn't been specifically mentioned, this thread seems to be about the "Faraday paradox"? I recently learned about this for the first time in another thread here at PF. You can find a pretty good demo by searching "Faraday paradox" on YouTube. I've noticed from watching the demo that an emf is only produced when the stator (the two connection points) are rotated with respect to the copper disc (or vice-versa). Whether or not the magnet rotates is irrelevant.
 
  • #23
Here's a sketch I did of Faraday's apparatus for this experiment:

FaradayExperimentalApparatus_zpshdkudzxz.jpg


The problem he designed this apparatus to obviate is that you can't cut a conductor twice with the same field line (or lines) and get a current. The second crossing of the conductor by a field line produces an EMF in opposition to the first, and they cancel each other out. So, if you try to generate a current by simply laying a conductor alongside a rotating magnet, it will be cut twice by all the field lines, once as they 'emerge' from the magnet, and a second time as they 're-enter' it.

To get around this problem he routed one of the leads (labeled a b in the drawing) through a hole right up the center of the magnet. It terminates in a slip ring at the "equator" of the magnet. From there conductor c b returns to the meter. c b maintains contact with a b by spring pressure onto the slip ring, and the electrical circuit is complete. Section a d of the conductor is both parallel to the field lines and rotates with the field lines, therefore it is not "cut" by them. Section c b is stationary with respect to the rotating magnet, and is always cut by the field lines. This is a continuous direct current generator; really just a different form of his unipolar dynamo.
 
  • #24
After looking at your drawing for a bit, I think that if the field DOES rotate with the magnet then the length BC would have a voltage induced in it, but if the field does NOT rotate with the magnet then the length BD would have a voltage induced instead. No wonder people have been discussing this wretched thing for so long!
 
  • #25
This setup cannot duplicate the experiments that I referenced in post #22 because the magnet cannot rotate by itself. With this setup the ring commutator always rotates with the magnet. You may think that this would make no difference. But one of the things that makes this problem so puzzling is that it defies intuition. This setup can be duplicated with the experimental setup that I referenced in post #22 by having the copper disc rotate with the magnet. In which case, it too would also produce an emf. It is only when the magnet rotates by itself, and nothing else, that no emf is observed. By the way Zooby, nice job with the sketch.
 
  • #26
Its because one of the postulates of special relativity, that the laws of physics are not necessarily the same between non-inertial frames of reference. A rotating frame such as that of the magnet or the circular disk is not an inertial frame. When we do the experiment you describe in the OP, supplying current to the disc and watching the magnet rotating, an observer in the rotating frame of the magnet will see the disc rotating and hence he will measure voltage on the disc and everything will be fine. But an observer on the frame of the stationary disc will measure no induced voltage. Different non inertial frame of references, different results, absoletuly normal for SR.
 
  • #27
Delta², does SR affect other kinds of electrical machines too? If not, why is this Faraday disc experiment so unique? Why should it be so simple yet so complex for no obvious reason? Can any practical use be made of these rotating frames of reference for everyday machines? Is there an Complete Dummies Guide to Special Relativity?

Edit -> I would love to hear lawyers argue these frames of reference in a court of law...
 
  • #28
A very interesting topic. Here are a few quick thoughts on this problem.

Clearly the rotating copper disk will generate radial electron currents as the orthogonal uniform magnetic field will generate a radial force on electrons in the copper conductor as electron charge x V_theta x Bz. The force is radial and varies with radius as R x Omega x e x Bz in cylindrical polar coordinates where Omega is the disk rotation frequency. Image currents in the disk conductor should be generated as well. Image currents should be transient, on the time scale of the magnetic diffusion time given the electrical conductivity of copper. (The radial electron flow will evolve via magnetic diffusion into the conductor.) Notice a radial electron current will generate a self magnetic field in the azimuthal direction (i.e. the +/- theta direction above and below the disk, and inside the disk too). Notice the radial current does not follow a closed path flowing either outward radially or inward radially. If one spins the disk in the external magnetic field without additional circuitry, the result is an electric charge buildup in regions at the rim and at the center. Hence there will be a radial electric field generated, this field will tend to suppress the radial current. The radial electric field would tend to build up so as to suppress the radial current flow. I think the steady state current and em field distributions in a rotating magnetized disk would be an interesting solvable E&M theory problem, for those of us who know Jackson. The radial electric field buildup can be shorted out with a conductor. For example, some brush electrodes that electrically connect the rim to the center could be installed, Then one could reach a stead state current distribution in the rotating disk by completing the circuit. To complete the electrical circuit, a conducting wire would connect the two brush electrodes. Probably one would loop this wire around outside the magnet gap to avoid induced EMFs in the wire resulting in a generator that could heat up a resistor in series with the wire. So far so good...As to the rotating magnet and stationary disk:I suppose one would be tempted to use a rotating coordinate system to tackle the problem. Maxwell's equations would have to be transformed into the rotating frame with a suitable orthogonal transformation including a rotation axis and rotation frequency. I know that such rotating coordinate systems are used in Geophysics. On the other hand, they are problematic in special relativity. However, the velocities used in the rotating disk experiment are non-relativistic, so the rotating coordinate system approach is probably worth a try. "Fictitious" Inertial forces would be generated in the rotating disk. Perhaps having some electromagnetic consequences. For example: A simple rotating copper disk (no external magnetic field) would see a radially outward body "centrifugal force."

Would that cause mobile conduction electrons to tend to flow radially and build up negative charge on the rim of the disk?

Like water in a spinning bucket climbs the walls. That's all for now.This post is pretty much casual stream of consciousness, so there may be some misstatements. If you catch one please post a reply.

TY for all the great comments here.
 
  • #29
Circlotron said:
After looking at your drawing for a bit, I think that if the field DOES rotate with the magnet then the length BC would have a voltage induced in it, but if the field does NOT rotate with the magnet then the length BD would have a voltage induced instead. No wonder people have been discussing this wretched thing for so long!
This is a good point. Length b d is quite short, but it would "cut" the most intense possible part of the field if revolved with respect to that field. If the field were not rotating we would indeed expect a current to be generated there when the magnet is rotated.

I've been pondering this, and I think there must be a way to rule it out. I believe that if you substitute a thicker wire for c b you should get more current: more field lines will be cut per unit time. So, if that suspicion is correct, and there is a difference between the current with the present wire and that of a thicker wire, we could rule out the generation of current in length b d. (The difference between the two wires would be that it would require more force to rotate the magnet at the same speed. Conservation laws, etc.)

However, I'm not rock solid certain a thicker wire should produce more current, and I haven't been able to find any information that clears this up in my mind.

TurtleMeister said:
This setup cannot duplicate the experiments that I referenced in post #22 because the magnet cannot rotate by itself. With this setup the ring commutator always rotates with the magnet. You may think that this would make no difference. But one of the things that makes this problem so puzzling is that it defies intuition. This setup can be duplicated with the experimental setup that I referenced in post #22 by having the copper disc rotate with the magnet. In which case, it too would also produce an emf. It is only when the magnet rotates by itself, and nothing else, that no emf is observed. By the way Zooby, nice job with the sketch.
I'm vaguely familiar with the set up you're talking about from a discussion of it here some years ago. My point in focusing on this set up I've introduced here is that it strikes me as easier to work with. If we can use this to prove the magnetic field rotates with the magnet, it will require people to explain the odd results of any other set up in some way other than proposing a non-rotating field.
 
  • #30
TurtleMeister wrote: "This setup cannot duplicate the experiments that I referenced in post #22 because the magnet cannot rotate by itself. With this setup the ring commutator always rotates with the magnet."

So remove the disc, the left-hand part of the magnet, and the slip ring joints. The field where the disc used to be is now nonuniform but that doesn't matter. The circuit is now from one pole of the meter, along the rotation and field axis to the bent wire bit where the disc used to be, and out through the field lines back to the other pole of the meter.

We're all agreed that if the meter + wire are rotated around the axis of symmetry we'd read current, yes?

If the meter and wire are held still and the magnet handle cranked we should see no current.

This is considered paradoxical because the relative motion between the circuit and magnet is assumed to be symmetrical but it isn't. Rotation is different from linear motion because linear motion is relative but angular motion is absolute; there's a "preferred frame" for rotation that doesn't exist for linear motion.

Consider Einstein's elevator but it isn't about linear acceleration this time. Can we do an experiment to show whether the elevator car is rotating, even if it's really slow? Of course- Foucault's pendulum comes to mind. There just isn't any analogous way to tell if the elevator car is in constant motion.
 
  • #31
MarkPercival said:
TurtleMeister wrote: "This setup cannot duplicate the experiments that I referenced in post #22 because the magnet cannot rotate by itself. With this setup the ring commutator always rotates with the magnet."

So remove the disc, the left-hand part of the magnet, and the slip ring joints. The field where the disc used to be is now nonuniform but that doesn't matter. The circuit is now from one pole of the meter, along the rotation and field axis to the bent wire bit where the disc used to be, and out through the field lines back to the other pole of the meter.
If you're talking about my drawing, there is no disk in it anywhere.
 
  • #32
I don't know whether this will help, but I found it to be a useful way of looking at a Faraday Homopolar machine when the disk and magnet are moving together.

For reasons unclear to me, there appears to still be discussion as to whether the magnet field rotates with the magnet when the disc rotates.

To remove this possibility in this scenario, do not use a disc magnet but a simple small magnet.

https://www.physicsforums.com/threads/faradays-paradox.763966/page-2
 
  • #33
Voltage Drop said:
I don't know whether this will help, but I found it to be a useful way of looking at a Faraday Homopolar machine when the disk and magnet are moving together.

For reasons unclear to me, there appears to still be discussion as to whether the magnet field rotates with the magnet when the disc rotates.

To remove this possibility in this scenario, do not use a disc magnet but a simple small magnet.

https://www.physicsforums.com/threads/faradays-paradox.763966/page-2
Just to be clear, let me paraphrase your explanation.

It sound like you're saying that when the disk and magnet rotate together, the leads connecting the center and edge of the disk to the meter remain stationary in the lab frame, creating a situation where there is relative motion between the magnetic field and at least one of the leads. The magnetic field, therefore, cuts this lead and generates a current that is detected by the meter.

The second scenario you describe is where the magnet and disk are held stationary in the lab frame, but the leads and meter are rotated around the magnet. Here again, the magnetic field "cuts" the leads and generates a current that can be read at the meter.

Did I understand your post correctly?
 
  • #34
Perfect! You explained it far better than I.
 
  • #35
@VoltageDrop
It seems like what you are describing is not a true homopolar generator (simple small magnet?). The type of generator used to demonstrate the Faraday paradox is designed in such a way that cancellation prevents the magnet from inducing current in the lead wires. There is a very good demo for this on YouTube. The demo allows the magnet, disc, and commutator to rotate independently. Just go to YouTube and search for Faraday paradox.
 
<h2>1. What is the cause of magnetic field rotation?</h2><p>The rotation of magnetic fields is caused by the movement of charged particles, such as electrons, within the Earth's core. This movement creates currents that generate the magnetic field, and as the currents change direction, the magnetic field also rotates.</p><h2>2. How does the rotation of magnetic fields affect our daily lives?</h2><p>The rotation of magnetic fields is essential for the Earth's protection against harmful solar radiation. It also plays a crucial role in navigation, as compasses use the Earth's magnetic field to determine direction. Additionally, some animals, such as birds and sea turtles, use the Earth's magnetic field for navigation during migration.</p><h2>3. Can magnetic fields ever stop rotating?</h2><p>No, the rotation of magnetic fields is a continuous process caused by the constant movement of charged particles in the Earth's core. However, the direction and strength of the magnetic field can change over time.</p><h2>4. Is the rotation of magnetic fields the same all over the Earth?</h2><p>No, the rotation of magnetic fields varies depending on the location on Earth. This is due to the Earth's uneven distribution of magnetic materials and the movement of charged particles in the core, which can be influenced by factors such as the Earth's rotation and the sun's activity.</p><h2>5. Can humans manipulate the rotation of magnetic fields?</h2><p>While humans cannot directly manipulate the rotation of magnetic fields, we can indirectly influence it through activities such as mining and drilling for natural resources. These activities can disrupt the Earth's magnetic field and potentially affect its rotation. However, the Earth's magnetic field is a complex and dynamic system that is not easily manipulated by human actions.</p>

1. What is the cause of magnetic field rotation?

The rotation of magnetic fields is caused by the movement of charged particles, such as electrons, within the Earth's core. This movement creates currents that generate the magnetic field, and as the currents change direction, the magnetic field also rotates.

2. How does the rotation of magnetic fields affect our daily lives?

The rotation of magnetic fields is essential for the Earth's protection against harmful solar radiation. It also plays a crucial role in navigation, as compasses use the Earth's magnetic field to determine direction. Additionally, some animals, such as birds and sea turtles, use the Earth's magnetic field for navigation during migration.

3. Can magnetic fields ever stop rotating?

No, the rotation of magnetic fields is a continuous process caused by the constant movement of charged particles in the Earth's core. However, the direction and strength of the magnetic field can change over time.

4. Is the rotation of magnetic fields the same all over the Earth?

No, the rotation of magnetic fields varies depending on the location on Earth. This is due to the Earth's uneven distribution of magnetic materials and the movement of charged particles in the core, which can be influenced by factors such as the Earth's rotation and the sun's activity.

5. Can humans manipulate the rotation of magnetic fields?

While humans cannot directly manipulate the rotation of magnetic fields, we can indirectly influence it through activities such as mining and drilling for natural resources. These activities can disrupt the Earth's magnetic field and potentially affect its rotation. However, the Earth's magnetic field is a complex and dynamic system that is not easily manipulated by human actions.

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