How does magnetic field induce current?

[A Dhingra]

how does magnetic field induce current??

can you explain why only VARYING magnetic field induces current ...or can say generates potential difference...
simply also a magnetic filed causes alignment of domains in the conducting wire , as such forming regions of regular and irregular alignment ... this appears to be like a difference in electric field of the regions... this can also lead to generation of potential difference... thought the magnetic field is not time varying... how far is this interpretation correct ... if possible explain what causes the electron drift due to varying magnetic field...
thnaks in advance...
()

 

[mrspeedybob]

magnetism is a smoke and mirrors illusion created by an electric field + Special relativity
See link. http://physics.weber.edu/schroeder/mrr/MRRtalk.html

If there is no relative motion between the "magnetic field" and a charged particle then you loose the illusion of magnetism.

 

[jtbell]

can you explain why only VARYING magnetic field induces current

Because Maxwell's Equations for electric and magnetic fields say so.

Why are Maxwell's Equations true? Because the universe has a local U(1) gauge symmetry.

Why does the universe have a local U(1) gauge symmetry?

Uh... because a giant turtle made it so?

 

[Phrak]

Why are Maxwell's Equations true? Because the universe has a local U(1) gauge symmetry.

Can you really get Maxwell's equations from this?

 

[bongas]

Thats the exact same question i asked a few days a ago, just read this:

https://www.physicsforums.com/showthread.php?t=441407

 

[jtbell]

Can you really get Maxwell's equations from this?

http://en.wikipedia.org/wiki/Gauge_theory#An_example:_Electrodynamics

 

[Phrak]

Can you really get Maxwell's equations from this?

http://en.wikipedia.org/wiki/Gauge_theory#An_example:_Electrodynamics

I don't see U(1) symmetry (phase invariance) in classical propagating electromagnetic fields, if that's what your talking about.

Perhaps you've just overreacting from 'why question' overload, however, I've seen U(1) deferred to often enough by others lately, and I fail to understand the significance.On another note, simply supplying the spacetime manifold with a real vector field guarantees Maxwell's equations emerge, so long as charge is defined as a second derivative of the vector field.

But I remain curious about U(1),

 

[cabraham]

magnetism is a smoke and mirrors illusion created by an electric field + Special relativity
See link. http://physics.weber.edu/schroeder/mrr/MRRtalk.html

If there is no relative motion between the "magnetic field" and a charged particle then you loose the illusion of magnetism.

According to A Einstein himself, magnetism is just as valid as electric fields. Neither is more basic, neither is the "illusion". An E field w/o an H field, when viewed from a differenr ref frame can yield both E & H. Likewise, a frame w/ H only & no E, will look different in another frame, having E & H fields.

Nobody has ever proved that E is more basic than H. Nor the converse. They mutually coexist, & they are inter-related via relativity. But magnetism is not an illusion. Motors wouldn't work if H was just an illusion.

Claude

 

[Born2bwire]

According to A Einstein himself, magnetism is just as valid as electric fields. Neither is more basic, neither is the "illusion". An E field w/o an H field, when viewed from a differenr ref frame can yield both E & H. Likewise, a frame w/ H only & no E, will look different in another frame, having E & H fields.

Nobody has ever proved that E is more basic than H. Nor the converse. They mutually coexist, & they are inter-related via relativity. But magnetism is not an illusion. Motors wouldn't work if H was just an illusion.

Claude

I was going to mention this too. While they are interconnected via special relativity, I have yet to see any proof that allows for any general magnetic field to be reproduced by a purely transformed electric field.

As for the OP, I would say take a quick look at the Lorentz Force. If we have a static magnetic field, then the field is only going to apply a force onto an existing current. Thus, if we have a loop of wire then the charges will be at rest with respect to the field and thus there is no force induced. So a static magnetic field on a stationary wire is not going to induce any forces. But if we have a changing magnetic field, then there is both a magnetic field and electric field. The electric field will provide a force on stationary charges and this can "jump start" the electrons so to speak so that once they begin moving they can be influenced by the magnetic field component.

We can also induce a changing magnetic flux by changing the area enclosed by our wire loop. If we have a static magnetic field but we move the wire into and out of the field then we will induce a current. This can be thought of as using moving the charges with respect to the field (by pulling the wire along) and thus these moving charges can be influenced by the magnetic field.

 

[K^2]

I was going to mention this too. While they are interconnected via special relativity, I have yet to see any proof that allows for any general magnetic field to be reproduced by a purely transformed electric field.

General magnetic field cannot be, because it's not a global transform. Yes, if you take an electric field, in a different frame it can become a sum of electric and magnetic fields. But that does not mean that for any state you can find an inertial system where magnetic field goes to zero.

The transformations have to make sense only locally. That is, for any point in space you should be able to place a test charge with arbitrary momentum so that in the rest frame of that charge the effects of electric field completely describe acceleration of the charge.

In a coordinate system other than particle's rest frame, you may end up with discrepancy. One way to fix that discrepancy is to introduce Lorentz Force. It is then possible to show that this results in a field that must obey Maxwell's laws for magnetic field. (This proof you should be able to find.)

This, however, isn't the only way to fix the "problem". You can start with a 4-vector potential and avoid the hassle with electric and magnetic fields all together.

 

[sophiecentaur]

With an appropriate experiment, you can detect a magnetic field, an electric field or any other of the phenomena we know and love. That doesn't mean that they are, in any way, more special than anything else. We use the idea of fields etc. when making models of our world.
Why do people keep wanting more than that? Do you really think that there are 'ultimate answers', lurking out there and that some things are 'really' while others aren't? The best we can hope for is to know a bit more tomorrow than we do today. Be pragmatic.

 

[Bob S]

Can you explain why only VARYING magnetic field induces current ...simply also a magnetic filed causes alignment of domains in the conducting wire ......

We can separate these questions into two categories: 1) The generation of electric fields and potential by a varying magnetic field, and 2) the generation of currents by electric fields and potentials. The latter category is electrical engineering. I will discuss the former.

Maxwell's equations lead directly to the integral form of Faraday's law:

$$\oint_C\bold{E\cdot}d\bold{\ell} = -\frac{\partial}{\partial t}\int_S \bold{B\cdot n}\;\;dS$$

This is often written in a simpler form:V = -A dB/dt, where V is the loop voltage (per revolution or turn), and A is the area of the surface of the integration on the right side (and enclosed by the loop on the left). This electric field E and potential V are the direct result of Faraday's Law, and do not require any conductors or currents.

The best (but not well known) illustrative example of this is the betatron (electron particle) accelerator developed during the 1940's and 1950's. The largest of these was the betatron built at the University of Chicago, with an iron cross section about A = 1 m² and an excitation field of B(ωt)~1.4 sin(ωt) Tesla. With 60 Hz excitation, the betatron could produce about 500 volts per turn; V(t) = A ωB₀ sin(ωt) = ~ 500 peak volts per turn. Free electrons in a vacuum chamber surrounding this iron could be accelerated up to energies ~300 million electron volts in about ~15 milliseconds (in ~ 600,000 turns). So the induced electric field exists in vacuum, without any conductors. For a comprehensive discussion of betatron accelerators, see chapter 11 in the free downloadable book "Principles of Charged Particle Acceleration" by Stanley Humphries, at http://www.fieldp.com/cpa.html

Electrical engineering applications include ubiquitous electrical transformers.

Bob S

 

[mrspeedybob]

According to A Einstein himself, magnetism is just as valid as electric fields. Neither is more basic, neither is the "illusion". An E field w/o an H field, when viewed from a differenr ref frame can yield both E & H. Likewise, a frame w/ H only & no E, will look different in another frame, having E & H fields.

Nobody has ever proved that E is more basic than H. Nor the converse. They mutually coexist, & they are inter-related via relativity. But magnetism is not an illusion. Motors wouldn't work if H was just an illusion.

Claude

The link I gave explains every effect of magnetism in terms of electric field and relativity, and yes, this explanation will explain the operation of an electric motor. If neither field is more fundamental then you should be able to start with the assumption that the magnetic field is fundamental and derive the existence of the electric field. I haven't seen that derivation. If it exists I'd love to see it.

 

[K^2]

It's going to be exactly the same derivation.

The only reason E field is treated as more fundamental is because we have electric charges and no magnetic charges. A "true" field without associated charges would be suspicious, especially if it induces apparent charges in its companion field.

 

[mrspeedybob]

It's going to be exactly the same derivation.

The only reason E field is treated as more fundamental is because we have electric charges and no magnetic charges. A "true" field without associated charges would be suspicious, especially if it induces apparent charges in its companion field.

I can't seem to wrap my mind around the reverse derivation. It makes no sense to me. An electric field makes sense as the interaction between charged particles. If I'm going to start with a magnetic field and derive the interaction and the charges in terms of a magnetic field how do I first describe the magnetic field?

 

[K^2]

I think you can get a descent description going if instead of monopole charges you introduce dipole charges. These allow B field to have zero divergence and non-zero curl. You could most certainly build any static B field out of these. You should then be able to use that to derive a companion field that has non-zero divergence, id est, the E field.

I'll think about it a bit more and see if I can come up with a simple way to show this.

 

[Phrak]

Electric and Magnetic fields enter, on an equal footing, as elements of the electromagnetic
tensor.

http://en.wikipedia.org/wiki/Electromagnetic_tensor"

The origin of F is a real vector field A. This definition, breaks the symmetry between electric and magnetic fields in such a way that the magnetic field must be everywhere divergence free. This means no magnetic monopoles. The electric field is not necessarily divergent free. It says we can have sources of the electric field, like charged particles.

 

[cabraham]

The link I gave explains every effect of magnetism in terms of electric field and relativity, and yes, this explanation will explain the operation of an electric motor. If neither field is more fundamental then you should be able to start with the assumption that the magnetic field is fundamental and derive the existence of the electric field. I haven't seen that derivation. If it exists I'd love to see it.

Regardless, H is just as real as E, not an "illusion". The fact that length contraction takes place giving rise to an additional force, does not make said force an "illusion" If a force known as E exists in the rest frame, & an additional force known as H exists in the test charge reference frame, neither is an illusion.

An example which comes to mind is the radioactive decay of muons as they fall from the atmosphere upper belt to the earth. Without taking relativity & length contraction into account, the half life of the muons computes so that a very small percentage of muons survive the trip all the way to the Earth surface. But observation shows that a large majority of muons make it to the Earth surface.

The reason is that time dilation & length contraction due to relativity take place. The long distance from the atmosphere upper belt to the surface got contracted since muons move very fast, comparable to light speed. Or you can view it as time dilation of the half life.

Either way, this is not an illusion. The muons behave contrary to classical physics, but consistent with relativistic physics.

Back to E & H now. The fact that length contraction gives rise to an additional force known as magnetic, does not make it an "illusion". Regarding the derivation of E from H using relativity, there are no magnetic monopoles discoved as of now. It's harder to visualize a static H due to particles which relativistically transform to E. But not all E fields are due to discrete charges.

When induction takes place, the induced E field is not conservative. It has "H-like" nature. It is solenoidal & path dependent. When a motor winding set is energized, we are dealing not with a free charge being attracted to a wire, but two wires, or windings being attracted magnetically. A thread a few years back covered this motor issue. Here is the link, & feel free to ask me to elaborate.

https://www.physicsforums.com/showthread.php?t=347539

BR. Claude

 

[Phrak]

Regarding the derivation of E from H using relativity, there are no magnetic monopoles discoved as of now. It's harder to visualize a static H due to particles which relativistically transform to E.

It's not harder, but one and the same thing. q_e is to E as q_m is to H (B,actually).

Either by positing nonzero magnetic charge or invoking the vacuum, there is a cyclic symmetry of the Maxwell Hertz equations, under discrete pi/2 rotations of the E-B plane.

 

[jnorman]

this seems to be one of those "why is it like that?" kinds of questions, and the answer is that we really do not know.

for me, i am going with jtbell's "giant turtle" explanation...

 

[cabraham]

It's not harder, but one and the same thing. q_e is to E as q_m is to H (B,actually).

Either by positing nonzero magnetic charge or invoking the vacuum, there is a cyclic symmetry of the Maxwell Hertz equations, under discrete pi/2 rotations of the E-B plane.

Please elaborate. I agree with you that neither electric nor magnetic field is more basic (at least that's what you seem to suggest). A superconducting loop carries all B with zero E. A co-moving charge sees B & E. Also, I was under the impression that the Maxwell eqns were asymmetric due to absence of magnetic monopoles. That was my point.

Claude

 

[Phrak]

OK. I see your point. I don't think it's meaningful to ask which of these is more fundamental but to note the differences.

To elaborate, start with Maxwell's equation with variable magnetic charge and current density. Call the current density 'k'.

Construct the complex quantities
j + ik
rho_e + rho_m
E + iB

For the electromagnetic field tensor and 4-vector potental,

F+iB
A+iZ.

Z is a new field If it's a problem, ignore the last.

Perform a transformation of each of these by multiplying by exp(i theta), for some arbitrary theta. For the fields B and E you get a B' and an E'. Nothing physical has really been done. A ninety degree rotation gives us back Maxwell's equations with the variable symbols swapped around. If you started with zero magnetic current and charge, now the electric current and charge are both zero. A 180 degree rotation returns Maxwell's equations with negative signs put where they normally don't appear.

 

[A Dhingra]

thanks to every one...
but i would request you to talk about this without making use of the laws like that of Faraday and maxwell or Lorentz force..., because i am not very through with them...

 

[A Dhingra]

As for the OP, I would say take a quick look at the Lorentz Force. If we have a static magnetic field, then the field is only going to apply a force onto an existing current. Thus, if we have a loop of wire then the charges will be at rest with respect to the field and thus there is no force induced. So a static magnetic field on a stationary wire is not going to induce any forces. But if we have a changing magnetic field, then there is both a magnetic field and electric field. The electric field will provide a force on stationary charges and this can "jump start" the electrons so to speak so that once they begin moving they can be influenced by the magnetic field component.

loreentz force is:
$$\vec F = q ( \vec E + \vec v \times \vec B )$$ right?

here as you said , when magnetic field is static ... means still B exists... but v = 0 ...so no force acts on the charges...
is this what you meant ??

can you explain this without using Lorentz law... simply in the physical or say visual form...

 

[Born2bwire]

loreentz force is:
$$\vec F = q ( \vec E + \vec v \times \vec B )$$ right?

here as you said , when magnetic field is static ... means still B exists... but v = 0 ...so no force acts on the charges...
is this what you meant ??

can you explain this without using Lorentz law... simply in the physical or say visual form...

v is the velocity vector for the charge that that fields are acting upon. When I say that the magnetic field is static I mean that the field is time-invarient. However, in magnetostatics we allow for time-invariant currents. That is, charges that move to create currents that do not vary over time (DC). So if we have a static magnetic field then unless we have moving charges (currents) the field will not induce any forces on our system. If we have a time-varying magnetic field, then we must also have a time-varying electric field. The electric field will exert a force on stationary charges. These charges will move under the influence of the electric field and thus then experience a secondary force from the magnetic field.

However, contrary to what you stated in the OP, we do not need a time-varying magnetic field to induce currents. Specifically, we need a time-varying magnetic flux. This can obviously be induced by a time-varying magnetic field but we can also induce this by changing the total flux through our surface. Take for example a static magnetic field produced between two bar magnets. This field is largely localized in the volume directly between the two magnets. If I were to take a wire loop and place it in the plane perpendicular to the field lines, then I would have a magnetic flux through the loop. If I move this loop into and out of the space between the magnetics the strength of this flux will change since I am moving it through a spatially varying magnetic field. The time varying flux will induce a current in the wire loop. This occurs because by physically moving the loop I am physically moving the charges on the wire. And thus now we have charges with a velocity that can be acted upon by the magnetic field.

But I need to have a spatially varying magnetic field for a current to be induced otherwise nothing will happen. The reason for this is that unless the field is spatially varying then the charges, while experiencing a Lorentz force from the magnetic field, will end up experiencing no net force.

 

[sophiecentaur]

thanks to every one...
but i would request you to talk about this without making use of the laws like that of Faraday and maxwell or Lorentz force..., because i am not very through with them...

I think you should realize that Maths and existing theories sometimes need to be used because there is no substitute. There may well just not be a simple, pictorial, explanation for something like this. To get a good understanding of something it may actually be necessary to learn the language used to describe it. You can't always take a short cut to some areas of knowledge - there is the risk of getting hold of the wrong end of the stick.

 

[Dale]

thanks to every one...
but i would request you to talk about this without making use of the laws like that of Faraday and maxwell or Lorentz force..., because i am not very through with them...

How can you possibly expect to talk about physics without reference to physical laws? You should learn the laws, without them there is no conversation possible on this topic. In fact, the whole idea of the magnetic field is meaningless without them.

 

[A Dhingra]

ok .( let me be straight and mention that i know these laws... though i am not able to logically reason them ...so i said i am not thorough with them...)
rather i should start from the Faraday's law...here...
according to this law time varying magnetic field induces current...and has the formula
where emf = E and flux is O
so ( - E = do/dt)...
right?

now i want to ask that... Faraday came to this conclusion according to the observations he found...but there must be some way to explain this effect without saying that this is so because Faraday's law says so... i mean there must be some other way where more of reason is explained...
please explain this to me ...

 

[Dale]

Faraday came to this conclusion according to the observations he found...but there must be some way to explain this effect without saying that this is so because Faraday's law says so... i mean there must be some other way where more of reason is explained...
please explain this to me ...

Faraday's law is a fundamental law in classical electromagnetics, so it has no explanation within that theory. So the only way to answer this question is in terms of another theory, in this case that theory is Quantum Electro Dynamics (QED). All of the results of Maxwell's equations can be derived in the classical limit of QED essentially from the fact that there exists a U(1) gauge symmetry.

So, now we come to a similar situation. Feynman (et al.) came to this conclusion about U(1) gauge symmetry according to observations he found. It works for all electromagnetic phenomena observed to date so we accept and use it even though there is currently no "way to explain this effect".

It may be that QED is later "explained" by another more general theory, but at some point if we ever develop a complete theory of everything then that theory will have no other explanation than the fact that it matches the observations we have found. That is the most important feature of a scientific theory, that it match observation, not that it be explainable in terms of another theory. Our fundamental theories never have that "explainable" property, by definition.

 

[A Dhingra]

so the conclusion is nature can never be well explained...

anyways... can you suggest me some books about Quantum Electro Dynamics (QED)that i can read to get more idea of it...

thanks

 

[Dale]

so the conclusion is nature can never be well explained...

Not if your idea of "well explained" requires that the most fundamental explanation always have another explanation, then another explanation, then another ...

This is what jtbell was alluding to way back in post #3. But realistically, that type of requirement is rather immature and non-scientific. It is like my 4 year old boy who has discovered that he can always ask "why" over and over and over and over ... Scientifically, a phenomenon is considered well explained if we have a theory which accurately predicts the result of all of the empirical observations made to date.

anyways... can you suggest me some books about Quantum Electro Dynamics (QED)that i can read to get more idea of it...

I would start with Feynman's lecture series at the University of Auckland (Vega). It is a nice overview. Also see:
https://www.physicsforums.com/showthread.php?t=408608

 

[sophiecentaur]

so the conclusion is nature can never be well explained...

"Well" or satisfactorily, but not perfectly.

 

[Born2bwire]

Not if your idea of "well explained" requires that the most fundamental explanation always have another explanation, then another explanation, then another ...

This is what jtbell was alluding to way back in post #3. But realistically, that type of requirement is rather immature and non-scientific. It is like my 4 year old boy who has discovered that he can always ask "why" over and over and over and over ... Scientifically, a phenomenon is considered well explained if we have a theory which accurately predicts the result of all of the empirical observations made to date.

I would start with Feynman's lecture series at the University of Auckland (Vega). It is a nice overview. Also see:
https://www.physicsforums.com/showthread.php?t=408608

Or even just read Feynman's book, "QED" ( https://www.amazon.com/dp/0691024170/?tag=pfamazon01-20 ), which was directed at laymen.

 

[A Dhingra]

thanks...