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Alternating current and magnetic fields

  1. Mar 17, 2013 #1
    We know that a direct current generates a magnetic field which surrounds the wire in circular patterns (right-hand rule). However when alternating current runs through a wire, no actual electrons are transported from one end to the other but rather they "vibrate" so to speak and transmit this vibration throughout the wire. Now, since the electrons are not "moving", does this mean that no magnetic field is generated surrounding the wire just as it would do if the current was direct?
  2. jcsd
  3. Mar 17, 2013 #2
    Please read your question again as currently it is self contradictory.

    There is a magnetic B field whenever there is a current in any form, line current, surface current or volume current. The electron concept shouldn't be used in classical EM as it is meaningless in the context.
    If you have an alternating current running through a wire, of course there will be a magnetic field, and possibly also an electric field.
  4. Mar 17, 2013 #3


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    I endorse that. Whoever has told you that you need to include electrons in any explanation of Electricity was clearly not very well informed. It is a popular view that, to sound authoritative about matters electrical, you need to use the word "electron" as much as possible. The UK National Curriculum doesn't help either - but that was written by a committee who had no idea of its actual purpose.
    No one (but No one) who knows what they're talking about, talks about electrons in the context of classical EM or even electronics (above the level of internal operation of individual devices).
  5. Mar 17, 2013 #4
    It is nice to see that people other than myself share this opinion.
    Personally I die a little on the inside every time they say things like that in textbooks and in the popular media.
  6. Mar 17, 2013 #5
    What have you people got against electrons and good standard text books?
    Connect a wire to a battery.....a current flows....the current is a flow of electrons.....physics knowledge enables me to calculate the drift velocity of electrons......where am I (and my books) going wrong?
    Where do electrons come in your explanations of physics phenomena.
    The poster in this thread has received nothing but negative feedback......basically told what not to do with no meaningful references as to why .
    Only opinions
    Why keep knocking the British education system? The great prof Brian cox is a product of this system and most of the PF experts would love to meet him.
    Consider an average student of physics coming here for guidance: the most common advice is;
    Your text books are no use
    Your teachers are no use
    Your education system is no use
    "Shun advice at any price, that's what I call good advice". (Piet hein)
    Last edited: Mar 17, 2013
  7. Mar 17, 2013 #6


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    In the wire, yes (assuming it's a metal wire). Inside the battery, no. Inside most other types of conductors, also no.

    And even for the current in the metal wire, calculating electron drift velocities is hardly the most practical thing you will even need to do, except when answering exam questions.

    To the OP: you can't have something that "vibrates" but "doesn't move". Either it moves, or it doesn't. End of paradox.
  8. Mar 17, 2013 #7
    Than you.....you have confirmed that electrons play a role in explanations of physical phenomena.
    There is no need to 'die a little inside' when you have read a physics text book.
    Calculating the drift velocity may not be the most practical thing to do but it is physics knowledge and increases understanding of physics.
    Surely we all want that.
  9. Mar 17, 2013 #8


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    OK, if you are really convinced that there is a valid way to answer the OP's question, using electrons and drift velocity, feel free to produce an answer - either with your own efforts or with a good reference at a suitable level.

    The issues of 'explaining' electricity in terms of particles (electrons) and 'explaining' EM in terms of particles (photons) is problematic. Unfortunately, the model that is used for both these two particles when discussed by elementary Science texts and teachers is very questionable. The pictures that students are given for both electrons and photons is, essentially, classical ones but the particles are Quantum Particles and neither of those popular pictures yields reliable predictions for most common phenomena. If you read a 'good', intermediate text book with care, you will usually find that it commits itself far less to these simplistic models than you probably assume. The popular Science media frequently overstep the mark and stray into fantasy on a regular basis.
    When someone comes onto a forum like this, one surely wants to do better than just the popular and simplistic ideas and to advance a bit. There is nothing wrong with using the 'school' models to deal with 'school' problems but you can't expect 'school' models to take you any further. It is strange that people are humble enough in their attitude towards Maths to acknowledge that one level of the discipline just can't be used to deal with higher levels. Why do people think that Physics should be any different?
    Do you think Brian Cox has a Noddy picture of an electron in his mind when he considers a problem in EM or Cosmology? He (and all other high achieving Scientists) has progressed through the education system to a level where he is capable of extending knowledge but he uses an appropriate level of knowledge and he will never be making a glib assumption that it's all based on Year 10 Science.
  10. Mar 17, 2013 #9
    .......has progressed through the education system......is the most relevant thing you have said.
    I am convinced that the great professor (like all great teachers) has a 'noddy' picture of an electron in his mind when he wants to explain cosmology etc.....that is why he is on telly and is known. I imagine he would use year10 assumptions when speaking to year10 level students.
    We all start with noddy explanations
  11. Mar 17, 2013 #10


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    We all start with noddy explanations but, when we know enough, we don't try to use them where they don't apply. That's my point. There's an expression, involving angels fearing to tread, which is very applicable when we stray into quantum phenomena. Valid explanations of electrical phenomena do not involve electrons going right through a circuit - or even moving at all. Did you ever work out how far they will drift with a moderate (audio) AC frequency? It's a salutory exercise for people who picture them rushing up and down a wire in their personal view of electrical current.

    On the subject of teachers. They are (very unreasonably) required to deliver some very sophisticated ideas without the knowledge for it. This is the fault of the Curriculum that they are expected to deliver and the fact that they cannot be paid enough to expect them to be able to do so. Taking your example of the lovely Briab Cox - he is quite capable of giving a well informed and appropriate answer to more or less any random question that any GCSE or A level Science student might ask about his field and probably well outside his envelope. Your average Science teacher is not in that position and I guess, even Prof Cox might be hard pressed to answer, in a subsequent lesson, a searching question about Biology (which most state school teachers also have to do).

    For some reason, there is a general idea, expressed by politicians, promulgated by the popular media and believed by the general public, that Science is easy peasy and approachable by everyone. We know this isn't true. It needs to be treated with respect if we are to get anywhere with it.
  12. Mar 17, 2013 #11
    Can someone tell me how to ascertain the power dating of a generator? Like there must be some equation or something....
    And also what are the main components of a rotor in a generator ? ( all the parts and components)
  13. Mar 17, 2013 #12
  14. Mar 18, 2013 #13
    The issue with talking about electrons in classical EM is that electrons themselves are not part of the theory, in th same way that one shouldnt talk about space time curvature in QM, one should not talk about electrons in classical EM
  15. Mar 18, 2013 #14
    Also, the drift velocity of an electron is meaningless in the context of QM.
  16. Mar 18, 2013 #15


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    “60 Hz (AC) magnetic fields are naturally emitted by current-carrying electrical conductors and devices. The AC magnetic field strength emitted by electrical circuits is directly proportional to the magnitude of electrical current.”

    and here is an interactive demonstration project from Wolfram that helps visualize the AC-caused magnetic field:

    Cheers, Bobbywhy
  17. Mar 21, 2013 #16
    Hi. SweatingBear, the electrons don't move much at all during their 'oscillation', I agree, but they all move together. Their electric fields therefore move together. Since it is a moving electric field that creates the magnetic field, you still get an induced magnetic field with AC current.
  18. Mar 30, 2013 #17
    So AC through a wire would influence a compass in the vicinity of the wire, just as DC through the same wire would?
  19. Mar 30, 2013 #18


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    Can you give one reason why it would not? (Assuming it has very low moment of inertia.)
  20. Mar 30, 2013 #19
    The forces on the compass are identical for ac and DC for the same instantaneous current. So the only difference in the response is the inertia of the compass. For high frequencies the compass inertia will reduce the compass response dramatically.
  21. Apr 9, 2013 #20
    Hey does anyone know how much will a 250MW generator cost?
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