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

DaleSpam

Sorry, I missed that, I must not have read as carefully as I had thought. Where was that?

No, at least I probably won't do any such analysis unless it seems likely to simplify things. I was merely responding to your question about what was meant by the term "EM field", you seemed unaware of what was being refered to and that the distinction between E and B depends on the choice of reference frame. But I am not recommending a full-blown covariant analysis from multiple reference frames, etc. I am having a hard enough time thinking it through in a single frame.

Miyz

Really looking forward for you're conclusion

Miyz

+ Its amazing how this thread turned out to be huh? :)
Magnetic fields! Interesting phenomena!

vanhees71

I'm a bit puzzled how this can happen more then 112 years after Einstein's famous paper on "the electrodynamics of moving bodies" (my translation from German).

Khashishi

A sailboat is in a lake with the wind parallel to the sail. It doesn't accelerate. Now a person pulls the sail at an angle to the wind. This causes the sail to deflect the wind and the boat starts to move. In this case, the wind does work on the boat. The person requires force to move the sail, but he isn't doing any work to move the boat.

A wire in a magnetic field is like the boat. The electric current acts like the wind. But the electrons move parallel to the wire so there's no work on the wire. Applying the magnetic field deflects the electrons into the side of the wire, causing the wire to move. It is the motion of the electrons which does work on the wire, not the magnetic field. The electrons slow down when they ricochet off the side of the wire and push the wire. The magnetic field is exerting a force on the electrons, but not doing work.

pgardn

What do the electrons ricochet off of?

What do they touch?

Miyz

The B field and the electric current are both proportional to one another. If you'd think about it... When electric current creates a magnetic fields that will interact with other magnetic field of a magnet.

Magnetic forces does the work and the electric current to is doing work both forces would add up to do work in a sense as I said before they add um as total work done on an object. I don't really agree that one is doing work while the other is not.
They both are.

Miyz

+

Based on the motor effect: Electricity flow to the wire as a "force" then another "force" is acted upon it that result in ANOTHER FORCE.

Electricity & magnetism and related to on another and are each forces acted upon each other in that causes of a "loop" were electricity flows through it, magnetic force is applied and torque is generated as a consequence to the interaction of both forces together. If you'd like to understand lets break each step into forces? The flow of electrons within the wire is caused due to EMF, due to that force the charges will be in motion through the conductor. Due to that motion of charge a magnetic field is created throughout the whole loop. Once a magnet is introduced to the system. Its magnetic field will interact with the loop and would attract it or repel it in a sense electricity(force) flowing through that loop will be a temporary"based on the flow of charge" dipole. Thus there will be a magnetic force applied on the loop and motion + torque are created.

Now if a freely moving charge moved through a magnetic field as Claude said: "e- (electron) has a velocity & a mag field is present, then a Lorentz force acts on the e- in a direction normal to its present velocity, & normal to B. Thus a mag field can change an e- momentum value, but not its kinetic energy value. Hence a mag field does no work on a charge." That is true.

But in cause of the motor effect! Where both charges & magnetic field's are present in a different orientation work is done and the value of kinetic energy will change. Because the charges are moving throughout a conductor and due to their motion a magnetic field is created and the magnet's field would interact with it. That interaction would be a force! The e- to move and due to its strong nuclear force it moves the p+ and n0 again Claude perfectly clarified that:"Now we have a current loop, 2 of them in fact. Mag field 1, or B1, exerts a force on the electrons in loop 2, normal to their velocity. In accordance with the above, B does not alter the e- energy value, only its momentum value, by changing the e- direction. But as these e- move in a new direction, the remaining lattice protons get yanked along due to E force tethering. But did the E actually do the work? The stationary lattice was moved acquiring non-zero KE (kinetic energy) when it started at zero KE.

Likewise, the neutrons got yanked along by strong nuclear force, which tethers the n0 (neutron) to the p+ (proton). A mag force in a direction normal to the loop deflects e- in a radial direction, resulting in p+ & n0 getting yanked radially. The force due to B accounts for all motion & work. But B cannot act on p+ as they are stationary, nor on n0 since the are charge-less. Did E do the work? E cannot act on n0 since they are charge-less. Did SNF do the work? SNF does not act on e-.

The work done by E appears to me a near zero. E exerts force no doubt, but when integrated with distance I compute zero. The E force between e- & p+ does move the p+, but the e-/p+ system energy is not changing. If an E force changed the distance between e- & p+, then E did work. Likewise for SN force."

Still they say electrical force does the work? Not really... Magnetic field then? Not really... Strong nuclear forces? Nope. Then what?! The total net force of all three interacting with one another.

As I said this before and I will say it again.

Magnetic field's can do work under certain circumstances.ONLY in the presence of both Electrical forces + Nuclear forces can then magnetic fields do work.

Now again: Electricity & magnetsim are related forces. Interacting together would cause this effect.

As Me & Claude finally agree that Magnetic force/field DOES work in this system. There all proportional to each other without the presence of E forces + SN forces the mag field/force can't do anything.

Miyz,

(Please give me you're opinion or you're contradiction to this idea because so far nothing is against it.)

& Appreciate all you're efforts to this very very strong thread! almost 4,000 views!

NOTE: "Hope whom ever reads this post could say I agree or disagree backing up their claims with proper illustration of the motor effect and any formula's that my support their claims"

vanhees71

Again I must stress, I don't understand your insistance on a statement which contradicts very basic calculations within the system of Maxwell's equations. You find this in any serious textbook of classical electromagnetics under the name "Poynting's Theorem".

I can only repeat that this no-brainer gives the clear answer that the electric components of the electromagnetic field do the work on any distribution of matter
[tex]P=\int \mathrm{d}^3 \vec{x} \vec{E} \cdot \vec{j}.[/tex]
The current density [itex]\vec{j}[/itex] has to be understood as containing both the flowing charges [itex]\vec{j}_{\text{charges}}=\rho \vec{v}[/itex] and the equivalent current for any kind of magnetization (through ring currents or through generic magnetic moments of elementary particles associated with their spin, which is a semiclassical picture of a quantum phenomenon), [itex]\vec{j}_{\text{mag}}=c \vec{\nabla} \times \vec{M}.[/itex]
As in all my postings I use Heaviside-Lorentz units (rationalized Gaussian units).

DaleSpam

Definition of work: a transfer of energy other than through heat.

None of the "tethering" stuff is relevant. Those are internal forces, and internal forces cannot do work on a system. Internal forces can only change a system's configuration, not its energy.

I am not yet convinced that the magnetic field cannot do work in the case of permanent magnets, but I am convinced that the magnetic field does not do work in a motor.

In a motor the integral of E.j that vanhees71 has posted fully accounts for the energy transfer in all situations. The B field is not relevant. If you double E.j then you double the work done on the motor regardless of B. If you double B then you do not change the work done on the motor. If you have E.j=0 then no work is done on the motor, regardless of B, but even if you have B=0 the work done on the motor is still given by E.j which then goes rapidly to thermal energy.

cabraham

I've already refuted that argument. E dot J is the dot product of 2 vectors acting tangential to a current loop. No torque is incurred on the rotor. I'll draw a diagram & post it later. Without a diagram showing the forces, it's hard to visualize.

Claude

Miyz

"In physics, mechanical work is a scalar quantity that can be described as the product of a force and the distance through which it acts in the direction of the force."


"If a constant force of magnitude F acts on a point that moves a distance d in the direction of the force, then the work W done by this force is calculated as: W= Fd"

http://en.wikipedia.org/wiki/Work_(physics)


I do know that work is the transfer of energy. However, in our case what would you like to envision? Forces, not energy.(I personally don't and can't imagine the kinds of energy I just break it down to work then the forces involved in the system to have a better idea of whats going on.)

Eventually we know energy has been transfered from point A to B, or conserved as heat.

Darwin123

Unfortunately, there is a lot of ambiguity in the jargon of physics. Units alone do not completely specify the important units alone. For instance, "potential difference" and "electromotive force" are completely different concepts, even though both have units of volts. Sometimes, "heat" means "energy" and sometime "heat" means entropy. Although these two definitions of "heat" have different units, they sometimes flow together. Sometimes they don't flow together.
That last sentence of yours can cause a lot of confusion if the physical concepts aren't specified, either explicitly or by the context. Students just starting can be thrown by the least bit of confusion.
There is a book that helps me a great deal with ambiguous physics concepts. It is:
"The Teaching of Physics" by J. W. Warren.
I have an edition published by Butterworth's in 1965.
Unfortunately, I don't know where or even if the book is still published anyplace. I don't know if there is a link to the book somewhere on-line. I certainly hope so.
It is a small book but it clears up a lot of basic questions students ask about physics. It discusses ambiguities in the jargon of electrodynamics, thermodynamics, calorimetry, classical mechanics and atomic physics.
This book emphasizes deficiencies in the way physics was taught in 1965. Judging by the questions people are still asking, I don't think the situation has improved since then. Most of the concepts clarified in this book are still taught the same way. Badly!

DaleSpam

The concept of thermodynamic work (the definition I cited) is a generalization of the concept of mechanical work (the definition you cited). The thermodynamic definition is the one that is typically used for fields, since it can be applied in situations where the mechanical definition is hard or impossible to use.

http://en.wikipedia.org/wiki/Work_(thermodynamics)
http://www.lightandmatter.com/html_b...ml#Section13.1
http://zonalandeducation.com/mstm/ph...work/work.html

Miyz

Fair enough. However, isn't it more fit for our situation to use the mechanical model? Since were dealing with a lot of forces?

Miyz

Looking forward for that diagram!

Miyz

@DaleSpam

What happened to you're conclusion?