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Fundamental and basic physics questions on flux lines

  1. Mar 11, 2012 #1

    Please correct me if I'm wrong;

    Flux lines are a sort of mathematical visualisation of magnetic fields, they do not exist. As such, there still exists a magnetic field between flux lines (?), just in calculations we use flux lines because they are simpler (?). In voltage transformers, we use an iron core because flux travels better in iron than air (if this is true, why is this?). Also, since flux lines (i.e. magnetic field) can still exist in air, we can have a transformer without an iron core...? (just won't work as efficiently?).

    If the above is correct then I have some doubts;

    What does 1 line of flux represent exactly? Is it arbitrarily chosen at point of calculation or it means something? An amount of magnetic field strength or something along those lines? If it is set, then I'd like to know who set it?

    Then lastly, I'd like to know how exactly does magnetic induction work? This is what I understand and please correct me if I'm wrong;

    A magnetic field passes through a piece of electrical wire, the charged particles (another doubt, the charged particles are just electrons--> right?) in the wire move to minimize the effect of the magnetic field (like magnetisation?) and this is why if you have a DC input to a transformer, you get a jump in voltage when you initially turn it on then goes to zero? (I am not sure about this too, does this happen?). And this is why if you have AC input to a transformer, you get an AC output because the magnetic field keeps reversing and the charges keep going back and forth?

    Many thanks for reading, I would greatly appreciate if you would answer my questions. These few fundamental questions have been bugging me for far too long, I have tried researching the answers to these but can't seem to find a black and white answer from anywhere as such I felt it would be best to just ask.

    Thanks again...
  2. jcsd
  3. Mar 11, 2012 #2


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    welcome to pf!

    hi ihateblackbox! welcome to pf! :smile:
    it's a bit like the contour lines on a map …

    the number of lines is arbitrary …

    one map of a hill might show far more contour lines that another amp of the same hill, if eg one is measuring in feet and the other in metres …

    the closeness of the magnetic field lines shows the strength of the field

    the flux through any surface is proportional to the number of field lines crossing it

    that strength is measured in amp-turns per metre (A.m-1): that's easiest to understand inside a solenoid, where the H field is exactly the currrent (amps) times the pitch (turns per metre) …

    increase the current, or increase the pitch, and the field lines inside the solenoid are closer together (so there's more of them) …

    increase the diameter (while keeping current and pitch the same), and you have the same field strength, so the field lines are the same distance apart, but of course there's more of them! :wink:
    if the magnetic field is constant, there's no force unless there's already a current …

    then the electrons (in the current) are a moving charge in a magnetic field, and the field forces them sideways :wink:
    yes, if the magnetic field is changing, then electrons which would otherwise be stationary do feel a force :smile:
  4. Mar 11, 2012 #3
    Re: welcome to pf!

    Firstly, thank you for your detailed answer :). It answered most of my questions but still some things are lingering...

    1)Just to be definitely sure, there exists a magnetic field between the flux lines correct?

    2)I am still not 100% sure how magnetic induction works. In the case of a magnet and a piece of wire, is it that the charges in the wire get attracted/repelled to one side? If so then this whole transformer thing; is simply attracting and repelling charges in the wire to create an AC current?

    3)Passing a DC voltage into a transformer (EDIT:that is off to begin with) should create a spike, correct? (even for a split second). And this is because the charges in the secondary are being attracted/repelled to another position. Correct/wrong?

    EDIT: 4) Why does flux travel better through an iron core than through air?
    Last edited: Mar 11, 2012
  5. Mar 11, 2012 #4
    If they don’t exist then the hill in tiny-tim’s post doesn’t exist either.

    Magnetic flux is the magnetic field. Or better: magnetic field is the magnetic flux per unit area.
  6. Mar 11, 2012 #5
    Yeah, I understand that, but I was talking about magnetic flux lines. Not magnetic flux. If I understand it correctly, flux lines do not exist, same as how contour lines do not exist. It is a visualisation which makes it easier for us to do calculations/whatever.
  7. Mar 11, 2012 #6
    Yep, flux lines are just lines drawn to represent the magnetic field.
  8. Mar 11, 2012 #7
    One fundamental question is still yet to be answered. How does magnetic induction exactly work? In the case of a magnet next to a piece of wire, is it that the charges are attracted/repelled by the magnetic field and so move position? And this is why AC creates a current because the charges are constantly moving position?
  9. Mar 11, 2012 #8


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    there's no simple everyday explanation :redface:

    it's because the electromagnetic field (E;B) is the "gradient" of a 4-vector (A,φ), and the "gradient" of a "gradient" is zero

    ie (E;B) = ∂ Λ (A,φ),

    and since ∂ Λ ∂ Λ (A,φ) = 0

    that means ∂ Λ (E;B) = 0, which is the same as x E + ∂B/∂t = 0 (the Maxwell-Faraday equation),

    so a changing magnetic field causes an electric field :smile:
    no, the charges are not attracted/repelled by the magnetic field, only by a changing magnetic field
  10. Mar 11, 2012 #9
    Ok, what you are saying confuses me and contradicts my understanding.

    Let me reiterate and please correct me if I am going wrong somewhere; by attraction I mean if a magnet is brought next to a piece of wire, the electrons in it try to move to minimize the effect of the magnetic field. If they were connected in a loop (i.e. closed circuit), the electrons would move, albeit only amount x, (EDIT:i.e. they would move to another position and then stop moving.) but would move--> correct?

    If we were to on the other hand keep moving the magnet, the electrons would continuously try to move, this would create a continuous current. Correct?
  11. Mar 11, 2012 #10
    Last edited: Mar 11, 2012
  12. Mar 11, 2012 #11


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    no, a stationary magnet does not cause a current, because it does not cause free electrons to start moving

    it does cause "bound" electrons to move, because a spinning electron is a tiny loop of current … those tiny loops in eg iron are attracted by the magnet, and (since they're not free to move, ie they're not current electrons), they drag the whole iron with them

    free electrons … current

    bound electrons … no current​
    yes, the free electrons would move :smile:
  13. Mar 11, 2012 #12
    This may possibly be true, but I want the most simple explanation, in terms of what is happening, because I find it easier to visualise in my mind, and the easier I can make it, the more I will, but not any simpler.

    When I am explained to in terms of equations, I don't really understand much at all. Because to me, equations are simply models of what we think will happen.

    For example I can see that the equation V=IR has an obvious fault. It implies that if there is a current, there has to be a voltage. What if I accelerated the electrons (somehow) without using a potential difference? If I learnt electronics using that equation and someone told me that the current creates a voltage, I wouldn't know they were wrong. However, since I know how the equation was come upon, the "physics behind it"; Potential difference creates current, not the other way round.

    EDIT: I understand, its probably impossible to do what I described, but I'd rather know "that" than just learn the equation.
    Last edited: Mar 11, 2012
  14. Mar 11, 2012 #13
    OK, yes this clears up a lot. Just one more thing; when you say "bound" referring to electrons in the iron bar, do you mean one of the delocalised electrons or one of the electrons in orbit around the nucleus of Fe atoms (ions?). If you didn't mean delocalised electrons, why don't they attract the delocalised electrons?

    EDIT:I read your post again, and I think you meant the electrons in the atoms/ions by "bound". This doesn't make sense to me, why wouldn't it attract the free electrons?
    Last edited: Mar 11, 2012
  15. Mar 11, 2012 #14
    How can electrons move and a current not be present?
  16. Mar 11, 2012 #15
    disregard my posts, tiny-tim is who you should listen to :)
  17. Mar 12, 2012 #16


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    hi ihateblackbox! :smile:

    (just got up :zzz:)
    technically, it does, but it has no effect …

    they're free, so they don't drag the iron with them …

    also, the free electrons in iron are like the bound electrons in wood …

    they're pointing in random directions (not lined-up as in iron), so the effect is random
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