1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about magentic induction

  1. Apr 5, 2010 #1
    i have a question about induction and how it works, what i want to know is the why, not the what or the formulas i want to know the why, why when a changing magnetic flux is nearby a electrical conductor it induces a current? you know a changing magnetical flux but not only caused for movement but too for an intensity change, so basically i want to know the why of the induction of currents, did i made myself clear?
  2. jcsd
  3. Apr 5, 2010 #2
    Im not exactly sure but, my understanding of it is that the magnetic flux pushes the electrons through the wire by the Lorentz force which creates a current.
  4. Apr 5, 2010 #3
    "[URL [Broken] law[/URL] states that when the magnetic flux around a conductor changes, the conductor will create a magnetic flux to oppose this change. "[URL [Broken] law[/URL] shows the relationship between the changing magnetic flux around a conductor and the induced emf in the conductor: the induced emf in the conductor will create magnetic flux in the opposite direction of the change in magnetic flux around the conductor. Lenz's law follows from the Law of Conservation of Energy.

    "To understand the implications for conservation of energy, suppose that the induced currents' directions were opposite to those just described. Then the north pole of an approaching magnet would induce a south pole in the near face of the loop. The attractive force between these poles would accelerate the magnet's approach. This would make the magnetic field increase more quickly, which in turn would increase the loop's current, strengthening the magnetic field, increasing the attraction and acceleration, and so on. Both the kinetic energy of the magnet and the rate of energy dissipation in the loop (due to Joule heating) would increase. A small energy input would produce a large energy output, violating the law of conservation of energy."

    Some of the kinetic energy of a magnet moving through a solenoid is converted into the electrical energy of the induced emf in the solenoid.
    Last edited by a moderator: May 4, 2017
  5. Apr 5, 2010 #4
    First off, Science does not answer the question "why". That is a religious question. What science can answer is "how".

    So as to "how" the first thing you must realize is that your thinking with regard to induction is all wrong. A magnetic field does not create an induced E field and hence does not induce a current into a conductor. If one looks very carefully at Maxwell's equations with regard to causality, one discovers that equations which are commonly thought to be causal actually are not. For example does a changing magnetic field induce a current in a nearby wire? No it does not! The magnetic field does NOT cause that induction. The reason is that if one has a changing current two fields are propagated out from that current at the speed of light. One is a magnetic field and the other is an electric field. Since they come from the same source they are related! And therefore one can use Maxwells equations to take one of them and calculate the value of the other one! Doing so gives the correct value, but it does not explain "how" the fields arise! But since the magnetic and electric fields happen at the same time, the magnetic field cannot create the electric field because if that were to happen, then superluminal energy transfer could occur which is regarded as impossible.

    What is known about "how" is that if one has a changing current, it creates about itself an E field. When that E field happens to be inside a conductor, it created motion of charges which is to say a current! That current and E field is retarded (which is to say delayed) from the timing of the source current. So the "how" of induction is that if one has a changing current element, about that current element is created an E field parallel to or anti-parallel to the source current. If that E field is in a conductor it can create a current. The E field is there whether a current is created or not. The E field is pretty easy to calculate from the rate of change of an "A" (vector magnetic potential) field that also propagates outward from the current source. This operation can easily be seen from the Neumann equation for mutual inductance. There the E field at a distance is calculated by an integration over the entire source current and then the induced EMF is found by a second integration over the secondary path. This dual integration then relates the source current to the induced potential.

    That said. your real question is actually, "what kind of physical model can explain the way a varying current can create an E field in the same direction at a distance from itself"? The bad news is that as far as I know, nobody has even suggested such a model beyond the early "stress in the aether models" which are now discredited. Even then models didn't go beyond vague suggestions. So the bottom line is that you've asked a question that has stumped modern physics. Yes, one can calculate the values of such situations, but beyond that there are no models that make "how" totally obvious. OK?
  6. Apr 6, 2010 #5
    thanks, but don't be so rude it's just a question, pardon me if i didn't made myself clear but i am not very good speakin' or putting my ideas together
  7. Apr 6, 2010 #6


    Staff: Mentor

    Hi ciberboy5004, welcome to PF!

    I don't think that bjacoby intended to be rude. He just has very strong opinions on the subject which are not shared by most people and which he therefore advocates in somewhat overdone language.

    However, he is correct in his assessment that science is very bad at answering "why" questions. What we have is a set of 6 equations (4 Maxwell's equations, Lorentz force equation, and the continuity equation). These 6 equations allow you to accurately predict the behavior of any classical EM system, and they agree with the results of all experiments where quantum behavior is not significant.

    Fundamentally, that is all that science is capable of doing, giving us equations which accurately predict the results of experiments. Induction is a feature of one of these equations. The only other thing that science can do is to describe one model in terms of another more general model. Are you perhaps interested in how we go from Quantum Electrodynamics to Maxwell's Equations?
  8. Apr 6, 2010 #7
    Who is being rude? You asked a question and I answered as best I could. You know I could have interpreted your statement "have I made myself clear?" as being rude, but truth is I understood that you were having trouble expressing yourself. No problem. You know one of the great things about these forums is that you can keep asking questions or asking for clarifications as long as you like and the idea is that with enough back and forth misunderstandings get understood!

    I'd say you asked an EXCELLENT question. It's one of the mysteries of induction that people often ponder. Namely just what is the model or mechanism that allows a magnetic field in one part of space to create a force (E field) in another distant part? How can an E field be generated from a B field at a distance even in regions where the B field is apparently Zero (outside a torroid)? How can B be connected to E when B = 0? Shows you are thinking about the problem. And you are not expected to know the answer. Most people don't seem to know the answer. As DaleSpam notes there are quite a few who think my explanation is not correct. I have asked them to explain in detail precisely where my logic has fallen down, but so far nobody has. I'm not saying that I and the other physicists upon whom I've based these opinions have no possibility of being in error, but if it's so obvious that this explanation is nonsense, then it should be an easy matter (like proving perpetual motion wrong) to say where the problems are.

    I suspect you felt I was rude because I suggested that asking "why" was a religious question. But that wasn't rude. It was truth about the scientific method. Why implies motive. For any of us to access the motives of the universe in a way other than by faith just isn't going to happen. "HOW" on the other hand tells the WAY or MANNER in which things happen. And that is what science studies! I hope this explains it better. If it doesn't you are allowed to ask for greater detail.
  9. Apr 6, 2010 #8
    well pardon me, you weren't rude i have my answer everyone is happy
  10. Apr 6, 2010 #9


    Staff: Mentor

    I think your statement here is a little disingenuous. I have addressed your main errors (as I see them) on two or three other threads all of which you stopped responding to. I understand that the things we discussed may require some thought before responding, so I am content to let you work things out. But in the meantime I feel that it is inappropriate to make claims like this in other threads as though your logic is somehow unassailable or unrefuted.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook