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Thread: Electromagnetism Questions

  1. #1 Electromagnetism Questions 
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    How are electric and magnetic fields the same thing, how do they interact with each other, and why does an electric current flowing through a wire create a magnetic field?
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    Quote Originally Posted by Scheuerf View Post
    How are electric and magnetic fields the same thing, ..
    They're not the same thing. They are merely two parts of one thing, the electromagnetic field. The electric and magnetic field are parts of the electromagnetic field just as space and time are part of spacetime, i.e. t (time) and [b]x are part of the 4-position (ct, x, y, z) and energy and momentum are part of the energy-momentum 4-vector

    Quote Originally Posted by Scheuerf View Post
    how do they interact with each other,
    That's complicated and as far as I know you need the language of math to describe it. It's described by Maxwell's equations

    Quote Originally Posted by Scheuerf View Post
    .. and why does an electric current flowing through a wire create a magnetic field?
    I'm not aware that that anybody knows. That's a law and as such cannot be derived from something simpler.
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    Quote Originally Posted by Scheuerf View Post
    ...why does an electric current flowing through a wire create a magnetic field?

    Relativity provides an answer. Here's a simplified, incomplete explanation: Consider a test charge moving toward a current-carrying wire. The wire contains moving and stationary charges (from the point of view of the test charge). Thanks to relativity, length contraction causes the densities of the positive and negative charges in the wire to differ, leading to a net force on the test charge. So, the magnetic force may be viewed as the consequence of the electrostatic force undergoing a relativistic transformation.

    As I recall, Edward Purcell has a nice discussion -- with helpful pictures -- in his book on E&M (part of the Berkeley Physics Series, vol. 2).
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    Whether an observer measured an electric or a magnetic field just depends on the state of motion of the observer relative the charge. They are one in the same thing, just seen from differing viewpoints!
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    Quote Originally Posted by tk421
    Relativity provides an answer. Here's a simplified, incomplete explanation:
    A more dynamic version of that explanation has been posted on another thread. Here it is again.

    But like every SR explanation, once you consider the wire as a closed loop, it became tough to explain the same cable have two different length, or the same length have two different densities of charge.

    So I decide using GR and thus thing moving things spending a greater part of their 4D path in time, are taking less space. Why come the extra charges ? Well, it's kind of borrowed in the time dimension. Moving on please.

    Now I can fall asleep, a kitty on my lap.
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    Quote Originally Posted by Boing3000 View Post
    But like every SR explanation, once you consider the wire as a closed loop, it became tough to explain the same cable have two different length, or the same length have two different densities of charge.
    There is indeed a subtle consideration that one must take into account when dealing with other than infinitely long, straight wires. It's certainly more involved, but it's not a fundamental problem for SR.

    So I decide using GR and thus thing moving things spending a greater part of their 4D path in time, are taking less space. Why come the extra charges ? Well, it's kind of borrowed in the time dimension.
    GR is unnecessary here. And if your use of GR has charges being "borrowed", I fear that a serious lack of rigour is involved.
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    Quote Originally Posted by tk421
    GR is unecessary here. And if your use of GR has charges being "borrowed", I fear that a serious lack of rigour is involved.
    You fear well. But I need to fill the gap, one way or another.
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    As long as the electric field has too high of a resitance to flow through, there will not be caused a magnetic field. When the potential energy is high enough and or the restance is low enough it will cause a flow of electrons from one atom to another. This will cause a magnetic field to form. These magnetic fields can be used to change the potentials of the electric fields, such in step up or step down transformers. It all is caused by the electric potentials (voltages) The basic formula of ohms law is that if you want to determine current (which creates the magnetic fields) you divide voltage potential by resistance. If you want to determine resistance, you divide voltage by current. If you want to determine Voltage potential, you multiply current times resistance. It is all interrelational.
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    Quote Originally Posted by mayflow View Post
    As long as the electric field has too high of a resitance to flow through, there will not be caused a magnetic field. When the potential energy is high enough and or the restance is low enough it will cause a flow of electrons for one atom to another. This will cause a magnetic field to form. These magnetic fields can be used to change the potentials of the electric fields, such in step up or step down transformers. It all is caused by the electric potentials (voltages) The basic formula of ohms law is that if you want to determine current (which creates the magnetic fields) you divide voltage potential by resistance. If you want to determine resistance, you divide voltage by current. If you want to determine Voltage potential, you multiply current times resistance. It is all interrelational.
    I'm sorry, Mayflow, but you shouldn't post on subjects you know nothing about. None of us is omniscient, but we do have a duty to know our limits and to post accordingly. The above is almost entirely wrong, either grossly or subtly. A small sampling:

    1) An electric field can easily give rise to a magnetic field even if there is a "high resistance." Maxwell's displacement current term reflects this. You might know this effect in the context of capacitive current. And yes, you will observe a magnetic field even between the plates of a capacitor, where there is a very high resistance indeed.

    2) In view of 1), we see that magnetic fields do not rely on a flow of electrons from one atom to another. And this is without considering the microscopic magnetic fields associated with spin; this consideration is important in understanding permanent magnets, in which there are no electrons flowing about.

    3) You seem to have, at best, only passing familiarity with Ohm's law. However, you seem totally unaware that Ohm's law is not a law at all. It is a small, quasistatic subset of Maxwell's equations. For example, you cannot explain antennas with Ohm's law, so that by itself should tell you that there's a problem with Ohm's law.

    4) Even in a linear, quasistatic world, voltage is not simply current times resistance. Cyclotrons, for example, depend on gross violations of that simplistic equation.

    5) etc.

    I implore you to acquire at least enough education so that you properly understand what your limits of knowledge are. Your limitations are much, much more severe than you acknowledge.
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    Ohm's law is true in is DC circuits, and in AC as well, but in AC circuits you have to change pure rsistance to impedence, since there are introduced capacitances and inductances which also can create time delays and other sorts phase shifts. This is still related to electron (or if you prefer, hole flow) which is cause by electric potentials as far as I have seen. Magnetism - Wikipedia, the free encyclopedia

    Now if you do not mind contsantly calling me stupid and uneducated, I think both I and the forum overall would appreciate it.
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  11. #11  
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    Quote Originally Posted by mayflow View Post
    As long as the electric field has too high of a resitance to flow through, there will not be caused a magnetic field.
    Mff, Maxwell says that you are totally wrong:

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    Quote Originally Posted by mayflow View Post
    Ohm's law is true in is DC circuits, and in AC as well, but in AC circuits you have to change pure rsistance to impedence, since there are introduced capacitances and inductances which also can create time delays and other sorts phase shifts. This is still related to electron (or if you prefer, hole flow) which is cause by electric potentials as far as I have seen. Magnetism - Wikipedia, the free encyclopedia
    And you haven't seen nearly enough, which is my point.

    I repeat, you have only a very severely limited understanding of the subject, which by itself is not a serious problem. What is serious, and that which disqualifies you from offering answers to others, is that you have no clue about where your knowledge ends. That leads you to make statements that are just plain wrong.

    Ohm's law is not generally true in AC circuits, even if you change "resistance" into "impedance." That you think so is, ipso facto, a demonstration of my very point. That you ignored two examples that I cited is frustrating. You have an unhelpful -- and ultimately self-defeating -- habit of ignoring sources of education. Stop resisting knowledge. There really is nothing to fear.

    I will repeat the examples here. You cannot explain antennas with Ohm's law whether or not you replace resistance with impedance. If you wish to disagree, show me an Ohm's law-based derivation of the radiation resistance of an antenna.

    Similarly, on the basis of Kirchhoff's voltage "law" subset of Ohm's "law", show me how the voltage can be nonzero around a closed path (this is what a cyclotron depends on, so I'm not invoking some obscure, contrived nonphysical example).
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    Quote Originally Posted by mayflow View Post
    Now if you do not mind contsantly calling me stupid and uneducated, I think both I and the forum overall would appreciate it.
    I have never called you stupid. If you disagree, please show me where I have done so, and I will apologise.

    As to calling you uneducated, that is an empirically demonstrable truth, and I have demonstrated it repeatedly -- with specific examples, multiple times -- in an effort to get you to reduce the considerable noise level that you contribute. As I said, no one is omniscient, but we all have a responsibility to know where our knowledge stops being reliable and to post accordingly. I wouldn't think of posting on, say, a sports forum and giving advice to footballers. They would properly dismiss me as ignorant.

    I again implore you to get a real education about the matters that you post on. It would save the rest of us a lot of trouble trying to clean up after you.
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    No problem you answer the others and I will stay out of this now. Have fun.
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    Quote Originally Posted by tk421 View Post
    As to calling you uneducated, that is an empirically demonstrable truth, and I have demonstrated it repeatedly -- with specific examples, multiple times -- in an effort to get you to reduce the considerable noise level that you contribute.
    Unfortunately it's a common misconception that when someone uses the terms uneducated or ignorant it's taken to mean stupid rather than their real meaning or as its meant in the context in which it's used.

    Ignorant - Definition and More from the Free Merriam-Webster Dictionary
    : lacking knowledge or information

    : resulting from or showing a lack of knowledge
    Uneducated - Synonyms and More from the Free Merriam-Webster Dictionary
    lacking in education or the knowledge gained from books
    Mayflow - Please do not take those terms to mean that someone's calling you stupid, okay my friend?
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    Quote Originally Posted by Physicist View Post
    Unfortunately it's a common misconception that when someone uses the terms uneducated or ignorant it's taken to mean stupid rather than their real meaning or as its meant in the context in which it's used.
    Yes, thank you very much for that reminder. Too many think that "ignorant" is a pejorative. All of us are ignorant about a great many things. Simply acknowledging that truth goes a long way toward guaranteeing that it doesn't become a permanent condition!
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    Oh good. I am only uneducated and ignorant according to TK and his bud xOx. I can only be thankful that the people I work for and with think differently. I am an electronics tech specializing in measuring electromagnetive waves.
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    Quote Originally Posted by mayflow View Post
    Oh good. I am only uneducated and ignorant according to TK and his bud xOx. I can only be thankful that the people I work for and with think differently. I am an electronics tech specializing in measuring electromagnetive waves.
    I'm sorry that your pride is hurt, but I have pointed out -- with specifics -- where your knowledge is woefully inadequate. I have not, to my knowledge, called you stupid. Indeed, I have implored you to get educated, which I would not do if I truly believed you to be incapable of learning. But as am I and is every other human, you are ignorant in certain areas. You have not challenged the correctness of that assessment (nor would you succeed were you to try). Your qualifications as an electronics tech unfortunately mean very little when it comes to the posts in question. I work with many fine techs, but they don't know beans about Maxwell's equations. The important thing is that they know that they don't know, and don't go around offering ill-informed opinions about the subject.
    Last edited by tk421; 09-16-2014 at 07:02 AM.
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    Dude, all I'm gonna say is even though he was wrong, you didn't have to say it in a way that would make him feel bad about it. Sure you can go all physics and say its his perception but I will say it's always best that we empower people in whatever way we know how. Cheers.
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    Quote Originally Posted by mayflow View Post
    Oh good. I am only uneducated and ignorant according to TK and his bud xOx. I can only be thankful that the people I work for and with think differently. I am an electronics tech specializing in measuring electromagnetive waves.
    I'm so sorry to see that you feel insulted. I'm sure that the people who used those terms never meant them to be taken as such. Let it be known from hereon in that if I ever use those terms they only mean what they're quite literally defined to mean in the dictionary as I posted references. I myself will make an effort not to use those terms when conversing with you.

    It appears that we have a bit in common, Mayflow. I used to be an electronics technician in the early 80s and still know enough to be able to get work in the field if I should ever try. In the mid 2000's I applied for a job which required a solid understanding of electronics. I didn't get the job because of my physics background, i.e. I was over qualified. I hate it when that happens! Blech!
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    Quote Originally Posted by kavkav View Post
    Dude, all I'm gonna say is even though he was wrong, you didn't have to say it in a way that would make him feel bad about it. Sure you can go all physics and say its his perception but I will say it's always best that we empower people in whatever way we know how. Cheers.
    Dear kavkav,

    Welcome to the forum!

    When I read this post I was unable to determine who it was you were addressing it to. Can you tell me who it was who you directed these comments towards please? Thank you.
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    Quote Originally Posted by Physicist View Post
    Dear kavkav,

    Welcome to the forum!

    When I read this post I was unable to determine who it was you were addressing it to. Can you tell me who it was who you directed these comments towards please? Thank you.
    Right, I thought quick reply was to the person.

    It was meant for tk421.
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    Quote Originally Posted by tk421
    Relativity provides an answer. Here's a simplified, incomplete explanation: Consider a test charge moving toward a current-carrying wire. The wire contains moving and stationary charges (from the point of view of the test charge). Thanks to relativity, length contraction causes the densities of the positive and negative charges in the wire to differ, leading to a net force on the test charge. So, the magnetic force may be viewed as the consequence of the electrostatic force undergoing a relativistic transformation.
    That doesn't answer his question though. He want's to know why an electric current flowing through a wire creates a magnetic field. We know how it does it. We just don't know why. E.g. if you have a long straight wire carrying a steady current I then the magnetic field created by the current carrying wire at a distance r from the wire will be




    What your response explains is the relativity of the fields and not why current creates a magnetic field. In essence he's asking why the field created by current exerts a velocity dependent force on the charged particles that are moving in the field. Why that happens is unknown. It can only be described at this point in history. Perhaps quantum field theory (QFT) has the ability to prove it but I'm not versed in QFT.
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    Quote Originally Posted by Physicist View Post
    That doesn't answer his question though. He want's to know why an electric current flowing through a wire creates a magnetic field. We know how it does it. .
    That's true, but fairly useless pedagogically, so I elected to provide something useful. There are never true answers to any "why" questions in physics. Every answer to a why question is properly "we don't know." Think about it.

    So rather than simply giving that standard response, I offered the OP something of educational value. I have found that most students are fairly comfortable (although unjustifiably so) with the notion of electrostatic repulsion/attraction, so explaining magnetic force as a relativistic consequence of electrostatic forces is a way to ease the student gradually into the more general notion of how forces transform under relativity.
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    Quote Originally Posted by kavkav View Post
    Dude, all I'm gonna say is even though he was wrong, you didn't have to say it in a way that would make him feel bad about it. Sure you can go all physics and say its his perception but I will say it's always best that we empower people in whatever way we know how. Cheers.
    Normally I would agree with you, dude. However, you are unaware of a long history of Mayflow's behaviour. Suffice it to say that a stronger-than-usual tone was not only warranted, but in fact necessary. Subtlety has been completely ineffective in the past, so empirical evidence compels the tone that you are objecting to.
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  26. #26  
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    Quote Originally Posted by tk421 View Post
    That's true, but fairly useless pedagogically, so I elected to provide something useful. There are never true answers to any "why" questions in physics. Every answer to a why question is properly "we don't know." Think about it.
    I disagree. "Why is the sky blue"? has an answer. Any question which starts out with "why" that we can answer by a derivation is just such an example too. It's necessary to explain to students which questions physics doesn't have an answer for and which do. Those which we can't answer are those which are the laws of physics. E.g. "Why is the speed of light invariant" and "why are the laws of physics the same in all inertial frames?" are questions that we don't know the answer to. It's important that we don't give students the impression that we know everything because they'll miss something genuinely important if we do.
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    Quote Originally Posted by Physicist View Post
    That doesn't answer his question though. He want's to know why an electric current flowing through a wire creates a magnetic field. We know how it does it. We just don't know why. E.g. if you have a long straight wire carrying a steady current I then the magnetic field created by the current carrying wire at a distance r from the wire will be



    Err, this is very wrong, what you want is:




    There are two gross mistakes in your formula, E instead of B and "1" instead of the current "I" , you would do well to check your posts before you try teaching others.
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    Quote Originally Posted by Physicist View Post
    I disagree. "Why is the sky blue"? has an answer. Any question which starts out with "why" that we can answer by a derivation is just such an example too. It's necessary to explain to students which questions physics doesn't have an answer for and which do. Those which we can't answer are those which are the laws of physics. E.g. "Why is the speed of light invariant" and "why are the laws of physics the same in all inertial frames?" are questions that we don't know the answer to. It's important that we don't give students the impression that we know everything because they'll miss something genuinely important if we do.
    You are completely missing the point, and also happen to be (nearly) completely wrong. You are also curiously inconsistent in the application of your criteria. For example, you state that an answer consisting of a derivation is an answer to a "why" question. My answer, to which you object so strongly, is a verbal derivation of how the magnetic force is merely a relativistically transformed electrostatic force. So you are being inconsistent.

    But that doesn't matter much, because you are wrong at a much more fundamental level.

    One can always provide an answer to any why question (and I did, in fact, provide one; whether you liked it or not is irrelevant to the truth of that statement). My statement contained the important qualifier "real". Here's what I mean: In your example, one could talk about Rayleigh scattering, say, and call that an answer. But is it really "the" answer? I could ask, "Well, why does scattering happen at all, and why does it depend on the fourth power of frequency?" And the answer to that kicks off another, and so forth, until one ultimately has to say, "That's just the way it is."

    If you do not understand the simple truth of what I am saying, you simply haven't thought hard enough.

    Of course it's important that we not give the students the wrong impression. I would argue that your answer of "there is no answer" to what causes magnetism gives the wrong impression by providing a trivially true answer (but selectively; you would have to provide the same answer to your "blue sky" question, if you were honestly to apply the same criteria), which is why I provided an answer that goes beyond your dead-end reply.
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    Quote Originally Posted by tk421 View Post
    You are completely missing the point,
    .
    Not at all. I strongly disagree with you. That's not the same thing as missing the point. And I disagree to the point that I think that you're dead wrong.

    I don't see the point of discussing this if you're going to take the attitude that I'm just too dense to understand you. So I will no longer respond to another comment that you make in this thread.
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    Quote Originally Posted by Physicist View Post
    Not at all. I strongly disagree with you. That's not the same thing as missing the point.
    In general, that is true, of course. However, in this case, you are disagreeing with me because you are missing the point.

    And I disagree to the point that I think that you're dead wrong.
    It's too bad that you are so closed-minded about this. I note that you did not address the examples I raised in my argument. It is a petty response simply to re-assert your statement without addressing the core of my argument. That's an unfortunate un-scientific attitude.

    I don't see the point of discussing this if you're going to take the attitude that I'm just too dense to understand you. So I will no longer respond to another comment that you make in this thread.
    You seem to be the one reluctant to discuss; I note for the record that I'm not the one picking up my marbles and going home in a huff. I have certainly not said anywhere that you are too dense to understand me. I have only said that you haven't thought deeply enough about the fact that all explanations must ultimately end in "that's just the way it is." I have given my examples, and you haven't rebutted them.

    If you wish to stop there, that is of course your choice. But if you are only interested in pushing your opinions on others, I am happy to discontinue this attempt at discourse.
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