Can a magnetic fields/forces do work on a current carrying wire?

[vanhees71]

Let's do your simple example first. Just a DC circuit with a resistor. To be more realistic we take into account also the self-inductance of the system. We can describe it as a series of an ideal resistor and a ideal coil. Then we use the equations for circuits, which are nothing else than a quasistationary approximation of Maxwell's equations.

The differential equation describing this system is then given by
$$L \dot{i}+R i=U(t),$$
where $L$ is the self-inductance, $R$ the resistance, $i$ the current, and $U(t)$ the battery. Being switched on at $t_0=t$, we have
$$U(t)=U_0 \Theta(t).$$
Now we simply solve this equation with the initial condition $i(0)=0$. We get for $t>0$
$$i(t)=\frac{U_0}{R} \left [1-\exp \left(-\frac{R}{L} t \right ) \right ].$$
Now we calculate the total power for $t>0$
$$P(t)=U(t) i(t)$$
and the total amount of work done at time $t$
$$E_{\text{tot}}(t)=\int_0^t \mathrm{d}t' P(t')=\frac{U_0^2}{R} t+\frac{L U_0^2}{R^2} \left[1-\exp \left(-\frac{R}{L} t \right) \right].$$
The amount of work converted into heat is given by the Ohm loss
$$E_{\text{loss}}(t)=\int_0^{t} \mathrm{d} t' R i^2(t')=\frac{U_0^2}{2 R} \left [2 R t -3L - L\exp \left(-\frac{2 R t}{L} \right )+4 L \exp \left (-\frac{R t}{L} \right ) ) \right ].$$
The reminder is used to build up the magnetic field
$$E_{\text{mag. field}}(t)=\frac{L U_0^2}{2 R^2} \left [1-\exp \left (-\frac{R t}{L} \right ) \right ]^2=\frac{L}{2}i^2(t).$$
The only work done on the charges obviously is the Ohmian loss, which is not due to the magnetic field built up and counteracting the builtup of the current (Lenz's Law).

In the stationary limit, one has the asymptotic behaviour
$$E_{\text{tot}}(t) \simeq \frac{L U_0^2}{2 R^2}+\frac{U_0^2}{R} t = \frac{L i_{\infty}^2}{2} + U_0 i_{\infty} t.$$
Of course the total energy is conserved (heat + magnetic-field energy) and comes from the battery.

 

[Dale]

You claim that E.J > conduction loss, but rather it is conduction loss plus mechanical energy spinning the loop. Yet a dot product is involved. If E spins the loop as well as provide loop current, here is the flaw in such logic. ...

E.J cannot be the work done spinning the loop, in whole nor in part.

Yes, I understand that you disagree with my claim that E.j > conduction loss. That is not what I was asking. I was asking whether or not you ALSO disagree with my logical conclusion from that flawed premise. Perhaps you have not studied logic, but an argument can be wrong for 2 reasons: 1) the premises are wrong 2) the logic is wrong. The two things are separate ways that an argument may be wrong and they can be evaluated separately. I know already that you believe 1), but despite asking something like 4 times now I still don't know if you believe 2).

My premise is that E.j > conduction loss. My logic is that IF the premise is true then B cannot also do the mechanical work because then energy would not be conserved. Specifically, the conduction loss would account for any increase in thermal energy, the B would account for any mechanical work, and an amount of work equal to E.j - conduction loss would be unaccounted for.

Again, I know that you think the premise is false, but do you also think that the logic is wrong?

The reason that I am asking (and asking and asking) is because I am trying to determine if our only disagreement is a disagreement about fact, or if there is also a disagreement about logic. The two types of disagreement need to be addressed differently. I believe that your premise is wrong, but I accept your logic that IF E.j = conduction loss then there must be some other form of energy to account for the mechanical work, and the only other available form seems to be B.

The work done setting up loop current is done tangent to the loop.

Work is a scalar, it doesn't have a direction and is not tangent to anything.

 

[cabraham]

Yes, I understand that you disagree with my claim that E.j > conduction loss. That is not what I was asking. I was asking whether or not you ALSO disagree with my logical conclusion from that flawed premise. Perhaps you have not studied logic, but an argument can be wrong for 2 reasons: 1) the premises are wrong 2) the logic is wrong. The two things are separate ways that an argument may be wrong and they can be evaluated separately. I know already that you believe 1), but despite asking something like 4 times now I still don't know if you believe 2).

My premise is that E.j > conduction loss. My logic is that IF the premise is true then B cannot also do the mechanical work because then energy would not be conserved. Specifically, the conduction loss would account for any increase in thermal energy, the B would account for any mechanical work, and an amount of work equal to E.j - conduction loss would be unaccounted for.

Again, I know that you think the premise is false, but do you also think that the logic is wrong?

The reason that I am asking (and asking and asking) is because I am trying to determine if our only disagreement is a disagreement about fact, or if there is also a disagreement about logic. The two types of disagreement need to be addressed differently. I believe that your premise is wrong, but I accept your logic that IF E.j = conduction loss then there must be some other form of energy to account for the mechanical work, and the only other available form seems to be B.

Work is a scalar, it doesn't have a direction and is not tangent to anything.

But the dot product depends on direction. I've been saying that & some just aren't listening. Work is indeed a scalar, but a dot product operates on 2 vectors, both having direction. The relative direction between the vectors is all important. Since E acts tangentially (E is a vector having direction), it cannot provide torque. But it does the work replacing resistive heat loss in the loop. B acts radially & it provides torque.

The direction of E wrt J is all important. The dot product is zero when 2 vectors are normal. The torque on the loop is normal to current density J. You claim that E.J > conduction loss, & that a part of E.J accounts for mechanical work = torque * angle. But the dot product excludes that from being the case. If there was a component of E acting radially & producing torque, that component of e dotted with J is ZERO. This is not debatable.

Claude

 

[cabraham]

Let's do your simple example first. Just a DC circuit with a resistor. To be more realistic we take into account also the self-inductance of the system. We can describe it as a series of an ideal resistor and a ideal coil. Then we use the equations for circuits, which are nothing else than a quasistationary approximation of Maxwell's equations.

The differential equation describing this system is then given by
$$L \dot{i}+R i=U(t),$$
where $L$ is the self-inductance, $R$ the resistance, $i$ the current, and $U(t)$ the battery. Being switched on at $t_0=t$, we have
$$U(t)=U_0 \Theta(t).$$
Now we simply solve this equation with the initial condition $i(0)=0$. We get for $t>0$
$$i(t)=\frac{U_0}{R} \left [1-\exp \left(-\frac{R}{L} t \right ) \right ].$$
Now we calculate the total power for $t>0$
$$P(t)=U(t) i(t)$$
and the total amount of work done at time $t$
$$E_{\text{tot}}(t)=\int_0^t \mathrm{d}t' P(t')=\frac{U_0^2}{R} t+\frac{L U_0^2}{R^2} \left[1-\exp \left(-\frac{R}{L} t \right) \right].$$
The amount of work converted into heat is given by the Ohm loss
$$E_{\text{loss}}(t)=\int_0^{t} \mathrm{d} t' R i^2(t')=\frac{U_0^2}{2 R} \left [2 R t -3L - L\exp \left(-\frac{2 R t}{L} \right )+4 L \exp \left (-\frac{R t}{L} \right ) ) \right ].$$
The reminder is used to build up the magnetic field
$$E_{\text{mag. field}}(t)=\frac{L U_0^2}{2 R^2} \left [1-\exp \left (-\frac{R t}{L} \right ) \right ]^2=\frac{L}{2}i^2(t).$$
The only work done on the charges obviously is the Ohmian loss, which is not due to the magnetic field built up and counteracting the builtup of the current (Lenz's Law).

In the stationary limit, one has the asymptotic behaviour
$$E_{\text{tot}}(t) \simeq \frac{L U_0^2}{2 R^2}+\frac{U_0^2}{R} t = \frac{L i_{\infty}^2}{2} + U_0 i_{\infty} t.$$
Of course the total energy is conserved (heat + magnetic-field energy) and comes from the battery.

I've been saying the same all along. You've just shown that E does work replacing heat losses. I've also acknowledged that w/o E there is no magnetic force due to currents in the loops. You've shown that the energy inputted equals that lost as heat plus that stored inductively. Nobody has suggested otherwise. But you have not even touched the issue of which force spins the loop. For the 3rd time, can you sketch a diagram showing how E provides torque to spin the loop? Thanks.

Claude

 

[vanhees71]

Ok, on the weekend, I'll try to do a little calculation of a simple loop in a magnetic field. That's not so easy, and for sure I'm not able to make a drawing that helps, simply because I'm not good at that ;-)).

 

[truesearch]

I've said it before but someone is just not listening ! There is NO energy gap between Valence band and Conduction band in a conductor. This is a physics principle, it has nothing to do (directly) with "wound rotor induction motors".
It is a narrow (I would say irrelevant) issue in this post but it has been used as part of an explanation relating to the original post.
This is wrong physics used to support an explanation... what else might be wrong and not come to light?
I wonder if the Hall effect has caused some confusion?...hasn't been mentioned as far as I can see.
It is not too important, this post has just about run its course, don't waste time explaining anything more for me but try to get the physics correct (all of it)

 

[cabraham]

I've said it before but someone is just not listening ! There is NO energy gap between Valence band and Conduction band in a conductor. This is a physics principle, it has nothing to do (directly) with "wound rotor induction motors".
It is a narrow (I would say irrelevant) issue in this post but it has been used as part of an explanation relating to the original post.
This is wrong physics used to support an explanation... what else might be wrong and not come to light?
I wonder if the Hall effect has caused some confusion?...hasn't been mentioned as far as I can see.
It is not too important, this post has just about run its course, don't waste time explaining anything more for me but try to get the physics correct (all of it)

That was rude. You have not provided any proof to counter my claims. I already said that the bands in a conductor overlap. But I emphasized since windings have resistance some E field is needed to replace I²R losses. Band gaps don't change that fact. I am only asking you to acknowledge that in a conductor which is not superconducting, there is a non-zero E field. Even if every electron is already in the conduction band, E field still exists.

You keep harping on this point so as to cast doubt on my facts I've presented. My case is built on spatial directions of E & B, & torque direction. Please refute my findings if you can. BR.

Claude

 

[Dale]

cabraham, YOU STILL HAVE NOT ANSWERED MY REPEATED AND RE-REPEATED QUESTION. THIS IS POSITIVELY IRRITATING. WHY ARE YOU AVOIDING THE QUESTION THAT I HAVE SO CLEARLY ASKED SO MANY TIMES?

 

[Dale]

But the dot product depends on direction.

This is correct. Both E and j have directions, and their dot product, E.j, does depend on the angle between them. However, there is no sense in which E.j has a direction or is tangent to anything even if both E and j are.

 

[andrien]

I'm just confused...

I know that magnetic fields can do work only on pure magnetic dipoles like a bar magnet. Based on the formula, the magnetic force on a charge is qv⃗ ×B⃗ which is identically perpendicular to v⃗ and that's why it does no work. However, forces on magnetic dipoles and more general objects don't have the form v⃗ × - they're not perpendicular to v⃗ , so they do work in general.

But still when I look at this picture I get confused:

http://ecommunity.pwsd76.ab.ca/file.php/1030/moddata/resource/23766/physics_30/images/m4/103_dc_motor.jpg

In this picture aren't magnetic force causing the rotation of the loop! Aren't the magnetic forces in a motor on of the key! Factors of motion within it? I mean it makes no sense to me why in this cause magnetic force can't do work on an electric charge... Could someone pelase help me out with this!

at least you agree that magnetic field can do work on a dipole.you can see clearly that the current loop is a magnetic dipole and a magnetic dipole has potential energy -m.B in magnetic field and their is also a torque on it which is m*B.To see that it is a dipole ,when you look at loop from one side current is flowing clockwise and watching from the other side it is anticlockwise.so both poles are here it is dipole ,however thin you make the loop you can't separate poles.

 

[Dadface]

The energy stored in the B field of the self inductive circuit discussed above(posts pre 214) is drawn primarily from the supply when the circuit is switched on.When the circuit is switched off the field collapses and dumps its energy,but things are now far more complex.Not only can there be a current surge but also there can be sparking and the attendant transmission of em radiation(which also occurs during switch on).There are so many variables to consider at switch off that any detailed analysis becomes very challenging.

If mutual induction is considered the changing B fields slicing through any surrounding conductors/circuits can induce emfs and corresponding currents and,in accordance with Lenz's law result in movement.

The point I am trying to make here is that whatever circuit I consider,even the simplest, I can see that B fields feature in work done and energy conversions.

Back to one of my earlier points

1.With a motor the magnetic force is instrumental in the creation of the torque(for a coil of area A we can write T=BIA).Some people here seem to agree with this.
2.The torque can result in the rotation of the coil.I think every respondent agrees with this.
3.The rotation is such that the force and motion on each side of the coil act in the same direction(both being colinear in a suitable radial field.The structure of the circuit restricts the motion of the charge carriers and prevents the curved paths which would be followed by,for example,by free electrons in a vacuum)I don't recall reference being made to this but I will have another scan through the thread.
4.by definition it follows that work is done due to the torque.

So,I ask again, is work by the magnetic force in the situation where there is a wire carrying current in a B field.If not where does my reasoning break down.

The point I am making is illustrated nicely in the sketch above which appeared in the opening post.It's simple,the magnetic force seems to be doing work.Why is this being overlooked or denied by some?Or is it?
(Thanks for the reminder andrien)

 

[cabraham]

This is correct. Both E and j have directions, and their dot product, E.j, does depend on the angle between them. However, there is no sense in which E.j has a direction or is tangent to anything even if both E and j are.

But if E & J are normal then their dot product is zero. The work is thus zero. That is my point. E.J is indeed a scalar having no direction. But any component of E normal to J results in zero work. Zero is zero, regardless of direction.

You've been saying I'm not answering your question. But I've directly addressed every point you've made. What have I not answered? This has become nothing but an attempt to save face by blowing smoke. Instead of my critics presenting data to support their claim, they relentlessly attack my facts in an attempt to discredit me. Even if someone can discredit me, that does not make their position correct.

Nobody is assumed correct by default. If I've erred regarding torque & work done on loop, state why & how, as well as the real source of work on the loop. I will assist anyone who is out to find the truth. If I'm wrong, the laws of physics can easily be applied to refute me. Nobody has done that thus far. The facts presented are valid, but are not germane to the torque. I have not disputed the laws & math thus presented. They just don't apply to the torque. Cheers.

Claude

 

[Dadface]

But if E & J are normal then their dot product is zero. The work is thus zero. That is my point. E.J is indeed a scalar having no direction. But any component of E normal to J results in zero work. Zero is zero, regardless of direction.

You've been saying I'm not answering your question. But I've directly addressed every point you've made. What have I not answered? This has become nothing but an attempt to save face by blowing smoke. Instead of my critics presenting data to support their claim, they relentlessly attack my facts in an attempt to discredit me. Even if someone can discredit me, that does not make their position correct.

Nobody is assumed correct by default. If I've erred regarding torque & work done on loop, state why & how, as well as the real source of work on the loop. I will assist anyone who is out to find the truth. If I'm wrong, the laws of physics can easily be applied to refute me. Nobody has done that thus far. The facts presented are valid, but are not germane to the torque. I have not disputed the laws & math thus presented. They just don't apply to the torque. Cheers.

Claude

Hi Claude. I am saying that work is done by the magnetic field.Of course there are energy conversions before that starting with the power supply but the magnetic field feaures in the energy conversion chain.If forced to give a yes no answer to the original question my answer would be yes.What is your take on this?

 

[cabraham]

Hi Claude. I am saying that work is done by the magnetic field.Of course there are energy conversions before that starting with the power supply but the magnetic field feaures in the energy conversion chain.If forced to give a yes no answer to the original question my answer would be yes.What is your take on this?

Agreed. The energy in B cannot exist w/o E, whose energy cannot exist w/o the input power supply (battery, ac mains, car alternator. etc.). E & B, under dynamic conditions, are mutually inclusive. I have overstated my point that although E force is not what turns the rotor, E is indispensable for motor operation, as is B, as is J (current density), as is V (input voltage source), as is cemf (counter emf generated by motion), as is SN (strong nuclear) force.

It just happens that B force is oriented in a direction so as to produce torque on the rotor, where E is not. But E provides other functions, I've already detailed. Likewise for SN (strong nuclear) force. Any motor/generator reference text will go into the details & math along w/ illustrations.

You seem to have given this a proper examination, & I agree w/ your final conclusion. Best regards.

Claude

 

[Dale]

You've been saying I'm not answering your question. But I've directly addressed every point you've made. What have I not answered?

What you have not answered is whether you think that the logic of my energy conservation argument is sound. I.e. Do you agree that IF E.j > resistive losses THEN B cannot also do the mechanical work because then energy would not be conserved?

Instead of answering that you have repeatedly explained why you disagree with my premise that E.j > resistive losses. Do you understand the difference between logic and a premise?

 

[cabraham]

What you have not answered is whether you think that the logic of my energy conservation argument is sound. I.e. Do you agree that IF E.j > resistive losses THEN B cannot also do the mechanical work because then energy would not be conserved?

Instead of answering that you have repeatedly explained why you disagree with my premise that E.j > resistive losses. Do you understand the difference between logic and a premise?

Yes I know the difference. Here is your answer to that question - no. If E.J > resistive loss, that is no guarantee that B is doing no work. Energy is inputted to the magnetic fields. An increase in input energy can take place w/o increasing mechanical output if stored field energy increases. I'll finish later, I have a 1:00 pm meeting.

Claude

 

[Dale]

Here is your answer to that question - no. If E.J > resistive loss, that is no guarantee that B is doing no work. Energy is inputted to the magnetic fields. An increase in input energy can take place w/o increasing mechanical output if stored field energy increases.

Thank you, this is exactly what I have been asking for.

Your objection to my logic is correct, I do need to account for the field energy of the rotor. Since the energy in the B field of the rotor depends only on the current through the rotor then my argument only holds if that current is constant. Would you agree to my logic in the special case where the current is held constant? I.e. if E.j > resistive loss in a rotor driven by a constant current source then the B field cannot do all of the work or energy would not be conserved.

Obviously, such a rotor would not be terribly practical since it could only do 1/2 of a revolution, but it could still do mechanical work during that 1/2 revolution and we can still analyze it.

 

[cabraham]

Thank you, this is exactly what I have been asking for.

Your objection to my logic is correct, I do need to account for the field energy of the rotor. Since the energy in the B field of the rotor depends only on the current through the rotor then my argument only holds if that current is constant. Would you agree to my logic in the special case where the current is held constant? I.e. if E.j > resistive loss in a rotor driven by a constant current source then the B field cannot do all of the work or energy would not be conserved.

Obviously, such a rotor would not be terribly practical since it could only do 1/2 of a revolution, but it could still do mechanical work during that 1/2 revolution and we can still analyze it.

But even with a constant current source input, CCS, some inductive energy is still stored. An R in series with an L is CCS driven. The heat loss is I²R, & the stored energy is LI²/2. Again, just as with the CVS input (constant voltage source), only part of the total input energy is heat loss.

Before I ran off to my meeting, I was going to say that the increase in loop inductance energy takes place due to E field. But as the rotor turns, energy is given up. It is replenished by the input source & E field. So E.J > heat loss. But the difference is the increase in inductive energy stored. This increases B field & force, which turns rotor, losing energy in the process, then replenished by CVS & E.

So E does indeed transfer energy to B. I have been consistent with my premise that B would not exist w/o E, & vice-versa. You can't have one w/o the other. Whatever 1 is doing, the other is lending a helping hand. But E energizes the inductance & the heat loss, whereas B mechanically energizes the rotor. Energy is exchanged. What B gives up to mechanical rotor motion energy, E replaces as inductive energy & B.

Wow, is that easy or what?

Claude

 

[Dale]

But even with a constant current source input, CCS, some inductive energy is still stored.

Clearly, but with a CCS the rotor's B-field (inductive) energy is constant. And work is a change in energy. Therefore with a CCS none of the work goes into the rotor's field. Do you agree?

 

[truesearch]

That was rude. You have not provided any proof to counter my claims. I already said that the bands in a conductor overlap. But I emphasized since windings have resistance some E field is needed to replace I²R losses. Band gaps don't change that fact. I am only asking you to acknowledge that in a conductor which is not superconducting, there is a non-zero E field. Even if every electron is already in the conduction band, E field still exists.

You keep harping on this point so as to cast doubt on my facts I've presented. My case is built on spatial directions of E & B, & torque direction. Please refute my findings if you can. BR.

Claude

No ruder than you are, I think my phrasing is similar to yours.
I am only asking you to acknowledge that in a conductor there is no energy gap between valence and conduction bands. This is basic textbook stuff. What is the problem?
I keep harping on because you stated this false fact in your explanation. In my language that is 'wrong' and should be corrected.

 

[cabraham]

No ruder than you are, I think my phrasing is similar to yours.
I am only asking you to acknowledge that in a conductor there is no energy gap between valence and conduction bands. This is basic textbook stuff. What is the problem?
I keep harping on because you stated this false fact in your explanation. In my language that is 'wrong' and should be corrected.

It depends on how you define "gap". The valence band overlaps the conduction but not completely. For all those e- in the overlap region, there is zero energy gap. These e- conduct w/o the need for work from an external source. The e- at the top, i.e. in conduction band region not overlapping valence, also have zero energy gap as above.

That covers the majority of e-. I have always acknowledged the energy gap for the above e- as being zero. Now here is where you will likely dispute me. Based on textbook diagrams, there is a small region in the valence band that does not overlap conduction band, down at the "bottom" energy state. These e-, minority in number, would require external work to get them into conduction, i.e. there is a non-zero energy gap.

I think I have made my position clear, & acknowledged your belabored point for the most part, albeit not entirely. Did I clarify my position regarding this energy gap issue that hopefully will conclude now? BR.

Claude

 

[cabraham]

Clearly, but with a CCS the rotor's B-field (inductive) energy is constant. And work is a change in energy. Therefore with a CCS none of the work goes into the rotor's field. Do you agree?

No. Remember there is mutual inductance between rotor & stator. Each receive energy from the other. Although I is constant, LI²/2 still changes, since inductance changes. The total inductance seen at the rotor changes as the rotor turns. When the poles are aligned, L is maximum, but at 90 electrical degrees, when rotor poles are midway between stator poles, L is minimum.

So energy, W, is varying. Picture the flux path seen from the rotor. With aligned poles, magnetic path through air is minimized. The flux path consists of rotor iron, small air gap, plus stator iron. The small air gap dominates the reluctance & hence determines L value. When the rotor poles are in between stator poles, the path has more air & higher reluctance, so that L is smaller, & energy is minimized. But mechanical energy is maximum in this case. With poles aligned, L is maximum, mechanical energy is minimum.

Conservation of energy is upheld beyond a doubt. Even a CCS exciting the motor has to conserve energy. If I've erred or omitted something by all means ask for clarification. Best regards to all.

Claude

 

[Miyz]

wow, every time something new is popping up this discusion...

I guess everyone's using their ammunition to the fullest...

 

[truesearch]

Cabraham: I think you have defined 'gap' for us : "for all those electrons in the 'overlap region' there is zero energy gap". The Valence and conduction bands do not need to 'overlap completely' there just needs to be a 'contact' so that all energies in the conduction band are available to the valence band.
We seem to agree on the physics.
Does this mean that the statement from post 127:
"So far the naysayers have produced nothing. They talk a big game about Einstein, reference frames, etc., but cannot show me the fields working in a simple induction motor. Show me, please, how it is E force, & not B force that spins the rotor. So far all I get is people blowing smoke. Not 1 naysayer has addressed the motor operation question.
In a motor, we are not simply moving electrons from valence to conduction. We are exerting forces on wire loops resulting in torque & work being done. "
can be discounted as part of your explanation?...it seems logical to do so.
What did you think of my reference to the Hall effect in the conductors? Is this the physics explanation behind your 'tethering' analogy?
I have never met this 'tethering' analogy before.
Must dash to ebay now...

 

[cabraham]

Cabraham: I think you have defined 'gap' for us : "for all those electrons in the 'overlap region' there is zero energy gap". The Valence and conduction bands do not need to 'overlap completely' there just needs to be a 'contact' so that all energies in the conduction band are available to the valence band.
We seem to agree on the physics.
Does this mean that the statement from post 127:
"So far the naysayers have produced nothing. They talk a big game about Einstein, reference frames, etc., but cannot show me the fields working in a simple induction motor. Show me, please, how it is E force, & not B force that spins the rotor. So far all I get is people blowing smoke. Not 1 naysayer has addressed the motor operation question.
In a motor, we are not simply moving electrons from valence to conduction. We are exerting forces on wire loops resulting in torque & work being done. "
can be discounted as part of your explanation?...it seems logical to do so.
What did you think of my reference to the Hall effect in the conductors? Is this the physics explanation behind your 'tethering' analogy?
I have never met this 'tethering' analogy before.
Must dash to ebay now...

Maybe my tethering analogy is much ado about nothing. When I pick up a pencil, I grab only a part of it, yet the whole mass & structure of the pencil is moved via "tethering". The E & SN forces internal to the pencil are apparent here. So with the motor I was just reminded in the link I provided way back in the early pages of this thread that the B force cannot move the stationary lattice protons. I concurred that the E force "tethers" the e- & p+ together. Hence B yanks on the e-, then tethering accounts for the p+ moving along with the e-.

I then added, well, for those who insist that "E force does everything", my counterpoint was "the neutrons are not affected by E force, they get tethered via SN force". So that is how this tether stuff got started.

As far as "discounted goes", I don't believe I spoke wrongly. The naysayers have given me their conclusions w/o proof, then when I offer proof for my side, it gets attacked by focusing on semantics & trying to bait me into typing something that can be shot down hoping to discredit me.

Let me just convey how I feel a debate should be conducted, feel free to comment. If a defendant is charged with a crime, the default judgment by a jury is "not guilty" until the DA proves guilt beyond a doubt. That is American jurisprudence. The DA has the burden of proof, not defense counsel, herein "DC". DA must present solid evidence establishing defendant being guilty, but DC is not obligated to prove client's innocence, nor to produce actual guilty party if defendant is innocent.

If the DA presents evidence, i.e. witness, circumstances, documents, etc. that implicate the defendant, the DC gets to cross examine then present his case as well. Should the DC manage to successfully refute prosecution evidence fully, jury must acquit. DC is not obligated to prove innocence, but DA must prove guilt. The default state is not guilty, period.

But in other circumstances, like a debate, 2 parties are trying to establish their findings as valid & wish to persuade the public, or a professional community, customers, whatever, that their data is the more reliable. There is no default state. If Peter & Paul are the opponents, they both have equal burden of proof, neither one more or less than the other. If neither side is persuasive enough, the audience has the option of casting a verdict of indeterminate, pass, not enough info to decide, etc.

But if Peter presents evidence to support his position, then Paul does a great job of refuting Peter's evidence, that casts doubt on Peter's thesis for sure, but it does not prove Paul's thesis as correct. To do that Paul must present facts supporting his claim, that can withstand scrutiny from Peter & the audience. Even if I have a memory lapse, & err in judgment on an isolated fact, that does not refute everything I've presented.

To "win" a debate you have to actually win it, not take it by default. It is not enough to play it safe & simply "not lose the debate". You have to win by proving your point beyond a doubt. That is what I have been striving for. I not only provided theory, & diagrams, I also refuted what I found to be in conflict with published laws dating to the 19th century & repeatably verified. When I erred, be it a sign reversal on the B force, or anything, I quickly acknowledged my error, thanked the person who caught it, then move on.

Hopefully I've made myself understood. Happy ebaying. I buy a lot from ebay, & have not had any issues at all for over 10 years.

Claude

 

[Dale]

No. Remember there is mutual inductance between rotor & stator. Each receive energy from the other. Although I is constant, LI²/2 still changes, since inductance changes. The total inductance seen at the rotor changes as the rotor turns. When the poles are aligned, L is maximum, but at 90 electrical degrees, when rotor poles are midway between stator poles, L is minimum.

Oh, I forgot about the mutual inductance. I was trying to stay away from having to do the full energy conservation equation with some simplifications, but I can't see a way around it. More details later.

 

[cabraham]

Oh, I forgot about the mutual inductance. I was trying to stay away from having to do the full energy conservation equation with some simplifications, but I can't see a way around it. More details later.

There's certainly a lot going inside a motor, more than meets the eye. If I fail to convince some of the point I am trying make, at least I can convince them that motors are no trivial matter. I believe that anybody who has participated in this thread had to have come to the realization that they don't know as much as they think they do. It's a lot to digest.

I will be the first to admit that my motor skills need more development. Anyway, it's an interesting thread to say the least. It has helped me sharpen my knowledge on the subject, hopefully others feel the same. BR.

Claude

 

[Per Oni]

No. Remember there is mutual inductance between rotor & stator. Each receive energy from the other. Although I is constant, LI²/2 still changes, since inductance changes. The total inductance seen at the rotor changes as the rotor turns. When the poles are aligned, L is maximum, but at 90 electrical degrees, when rotor poles are midway between stator poles, L is minimum.

So energy, W, is varying. Picture the flux path seen from the rotor. With aligned poles, magnetic path through air is minimized. The flux path consists of rotor iron, small air gap, plus stator iron. The small air gap dominates the reluctance & hence determines L value. When the rotor poles are in between stator poles, the path has more air & higher reluctance, so that L is smaller, & energy is minimized. But mechanical energy is maximum in this case. With poles aligned, L is maximum, mechanical energy is minimum.

Conservation of energy is upheld beyond a doubt. Even a CCS exciting the motor has to conserve energy. If I've erred or omitted something by all means ask for clarification. Best regards to all.
Claude

Can you clarify: I hope you realize energy LI2/2 has nothing to do with the energy that drives the motor? The power P which drives the motor (as previous people have said) = back emf x current I. The energy during a semi cycle is given by W = P x t (semi cycle).

Further: A permanent stator magnet does not give energy to this motor since it would soon run out of magnetic field.

Also: Mechanical power is highest when force F=BIl is highest, which is the case when B is max, which is when the conductor is just under the pole, so with poles aligned as you call it. As for “mechanical energy” that is a strange concept in this context. But whatever it is, it will be max with poles aligned.

Moreover: I also agree that talking about an energy gap is totally out of context for this subject.

On the plus: you have made some valid points as well in previous posts.

 

[cabraham]

Can you clarify: I hope you realize energy LI2/2 has nothing to do with the energy that drives the motor? The power P which drives the motor (as previous people have said) = back emf x current I. The energy during a semi cycle is given by W = P x t (semi cycle).

Further: A permanent stator magnet does not give energy to this motor since it would soon run out of magnetic field.

Also: Mechanical power is highest when force F=BIl is highest, which is the case when B is max, which is when the conductor is just under the pole, so with poles aligned as you call it. As for “mechanical energy” that is a strange concept in this context. But whatever it is, it will be max with poles aligned.

Moreover: I also agree that talking about an energy gap is totally out of context for this subject.

On the plus: you have made some valid points as well in previous posts.

1st bold - LI²/2 has everything to do with motor energy. The input source current & the back emf are involved but how does energy get to the rotor? Torque is produced through the interaction of B fields. But B gets its energy from the input source current & voltage. The input source voltage minus back emf quantity divided by L is the rate of change of inductive current, & energy changes. The LI²/2 energy is transferred. In ac motors, some is reactive power. This energy can result in lagging power factor, or for an over-excited synchronous generator, leading power factor.

2nd bold - When the poles are aligned, I said that the torque is minimum. There is a force but the moment is zero. Look at a diagram, or draw one. The force is radial & does not spin the rotor.

As far as a permanent magnet motor goes this is a straw man you invented. Nobody said a permag "gives energy" to the motor. The input power source does that. Nobody in this thread would ever believe what you just presented. We're not that dumb. Please don't treat us like we are. BR.

Claude

 

[Miyz]

There's certainly a lot going inside a motor, more than meets the eye. If I fail to convince some of the point I am trying make, at least I can convince them that motors are no trivial matter. I believe that anybody who has participated in this thread had to have come to the realization that they don't know as much as they think they do. It's a lot to digest.

I will be the first to admit that my motor skills need more development. Anyway, it's an interesting thread to say the least. It has helped me sharpen my knowledge on the subject, hopefully others feel the same. BR.

Claude

You and I both! Motor's are really not studied properly and the phenomena that causes it's torque + motor is simply not understood properly. Main proof this thread .

But anyways so far I honestly due believe based on what Claude and the other member's presented that magnetic fields/forces in the motor indeed can do work.

Hope @DaleSpam that you started to take back you're older conclusion "superconductor" one.
It seems that the most logical answer is that the magnetic fields do work. Based on multiple effects...

Anyways thanks again everyone! I think we can safely say that the OP question... IS ANSWERED.

If others still don't believe magnetic fields/force do no work in a motor please please review the detailed work of Claude in this whole thread! Made a lot of point and used a lot of good examples to back them up!

Hope the best of luck to all,

Miyz,(Be back with another interesting thread (HOPEFULLY)).

 

[Miyz]

To all:

Have we reached to an agreement? That magnetic field do indeed do work?

Miyz,

 

[vanhees71]

No, I strictly disagree with the statement that magnetic fields do work on particles or macroscopic bodies. I think Maxwell's theory and particularly its quantized version (Quantum Electrodynamics) is a correct description of electromagnetism!

 

[Drakkith]

I feel this whole thread could be answered like this. Can magnetic fields/forces do work on a current carrying wire? Yes, but they are only one aspect of electromagnetism, and it is the combination of the electric and magnetic aspects that ultimately results in work being done. Without either one no work would be done in this manner.

 

[truesearch]

I am happy with it and furthermore would like to dissociate myself from such statements as
We're not that dumb. Please don't treat us like we are. BR.


No one has any authority to express opinions on my behalf

 

[Dadface]

I feel this whole thread could be answered like this. Can magnetic fields/forces do work on a current carrying wire? Yes, but they are only one aspect of electromagnetism, and it is the combination of the electric and magnetic aspects that ultimately results in work being done. Without either one no work would be done in this manner.

Well put Drakkith.That's my thoughts exactly.I think it all boils down to semantics.

vanhees do you agree that the force F=BIl is instrumental in doing work in motors and the like? If so would you describe F as being a magnetic force or an electric force or otherwise?
In my opinion(at this moment)F is best described as being an electromagnetic force.