Magnetic flux and C-EMF

  1. A friend of mine told me that in order for Back-EMF to be generated,
    The flux lines has to be parallel. Is that true?
    This question is based on a electromagnet - magnet set up. The flux lines must be parallel?
    At least C-EMF can be generated but if the flux lines are perpendicular it will not be significant as if they were in parallel.
  2. jcsd
  3. mfb

    Staff: Mentor

    What has to be parallel to what?
  4. A magnet's flux lines and an electromagnets flux lines.
  5. mfb

    Staff: Mentor

    You don't need any current in the electromagnet to get a voltage there, so the magnetic field of the electromagnet does not have to exist, if the magnet is moving.
  6. But generally does it matter if the flux lines are parallel or perpendicular?
  7. mfb

    Staff: Mentor

    It does not even matter if there is a relative orientation, therefore: No.
    The geometry of the magnet and the coil of the electromagnet matters, of course, but not in a way that you can say "x parallel to y = good!" or "x perpendicular to y = good!".
  8. Then in what way does the geometry matters? So the simplest way to answer my question is: No.
    I need to ask my friend use to back this claim.
  9. mfb

    Staff: Mentor

    In general, in a complicated way.
  10. Why don't I need any current? Even if the magnet is moving? Could you explain.
  11. Drakkith

    Staff: Mentor

    Hold on. If you set up an electromagnet - magnet combination, the flux lines from both magnets will either reinforce or cancel each other out. If you have like poles facing each other the lines get "flattened" at the pole since they can't cross each other, whereas if you have opposite poles facing each other the lines simply go from one magnet straight to the other magnet.

    Let's say you turn your power to the electromagnet off. Then you move the permanent magnet in and out of the coil. You will generate both an EMF and a C-EMF in the coil based on the number of turns and the strength of your magnet. This is because the EMF induced in the coil causes current to flow. The current flow creates a magnetic field that cuts across the coil and induces a counter EMF that is opposite to your original EMF.

    If you leave the power on and move the magnet in and out of the coil the same thing happens, as the magnetic field of the coil changes when you move the magnet in and out. This change in the field again induces EMF in the coil itself (seperate from the EMF from your power supply of the electromagnet), which causes a change in current flow, which causes a change in the magnetic field, inducing a counter EMF.

    The key here is that no matter how your field lines are meeting, moving the magnet around changes the magnetic field in the coil in some fashion. This change generates all the EMF and C-EMF, which again is separate from the EMF provided by your power supply to the electromagnet. The EMF from moving the magnet around may work with OR against your power supply, and the C-EMF always works against that induced EMF.

    I'm sure that's about as clear as mud, so let me know if it doesn't make sense.

  12. In what why can it work WITH and in what why can it work against?
    Could you explain more?

    Aside from the question above everything is clear.
  13. Drakkith

    Staff: Mentor

    Let's say you have DC current flowing through a coil creating a magnetic field. Moving a permanent magnet in and out will cause a voltage to be applied that opposes the current flow when the magnet is moved one way, and will work with the current flow the other way. It's exactly the same as if you had zero current flowing through the coil in that moving the magnet in and out produces an AC current. But since you had current flowing already you would just cause less or more to flow.
  14. Very interesting point, this can be used to help me with my project! I can learn to work with C-EMF and the induced EMF, certainly I must study a lot of configurations...

    Another thing, when a magnet goes in and out of a coil. It generates a higher quantity of EMF or C-EMF (depending if the coil was a electromagnet or not...).
    But if the magnet was only near by, moved only to the tip of the coil/electromagnet it will not generate a significant EMF/C-EMF.

    And I doubt the flux lines being parallel or perpendicular would matter at all...
  15. Drakkith

    Staff: Mentor

    No, both EMF and C-EMF are always generated.
  16. Ow sorry, had to re-read your finally post and figured out what I forgot to add " BOTH emf and c-emf are generated."

    But still, they are highly generated if the magnet's field passes fully through the coil? In other words, if the magnet passes through the coil completely. But if it only comes to the tip, its less than going through.
  17. Drakkith

    Staff: Mentor

    Of course. If you just wiggle the magnet around 5 feet from the coil you'll barely do anything since the strength of the field has dropped off significantly.
  18. Or to be more specific here is what I mean by close by not going through the coil.
  19. Ok, after reviewing my friend statement. He didn't mean that C-EMF will not be generated, but it will. However, it will not have maximum effect.

    His statement:
    "Its not that the lines need to be parallel but the field applied to the solenoid must be perpendicular to the cross sectional area of the solenoid to have maximal effect. So that if there is a flux in the solenoid to start with the applied field will then be parallel to that field."

    Does this make any sense? And is it true? "Maximum effect" part?
    I'm still reviewing the basics, so I can't determine anything.
  20. Drakkith

    Staff: Mentor

    I'm actually not certain. I'm barely a beginner myself. I think he's saying the field lines need to run perpendicular to the coil's wires and parallel to the field lines that run through the coil if you have current flowing. The only way for that to happen is to have your magnet inside the coil, so that would make sense that it would have the maximum effect since your magnet literally couldn't get any closer if it wanted to.
  21. I think so too.

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