Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic Fields questions

  1. Jan 22, 2012 #1
    I got this electromagnetic field detector that can measure magnetic field (B-fields) of Alternating Current of 60 Hz. Supposed after getting the 2 wires together and the magnetic field cancelled and reading lowered from 10 milligauss to 1 or 0 milligauss. Does it mean getting a meter that measures steady magnetic field would get reading of the original 10milligauss in the same setup? That is.. when the AC magnetic field cancelled, it becomes DC magnetic field or would the magnetic field turn zero (or close to 0 owing to not all cancelled)?

    This is because I heard when magnetic fields cancelled in the AC wires of the same circuit. They are not gone. The energy density is still there and only the frequency got cancelled. Thanks.
     
  2. jcsd
  3. Jan 22, 2012 #2
    No, it does not become a DC field. For that you need a DC current in the wire.
     
  4. Jan 22, 2012 #3
    But isn't it that for the zone where magnetic fields canceled, they were not annihilated.. but simply suppressed... it's like two water waves coming from opposite directions.. when they meet, the zone would be calm, but they would continue at others sides.. much like superposition. Isn't it this is also what happened to magnetic fields cancelled?
     
  5. Jan 22, 2012 #4
    It doesn't work like water. The fields add each as if the other wasn't there.
     
  6. Jan 22, 2012 #5
    Can you please elaborate what you meant by "The fields add each as if the other wasn't there"? When magnetic fields from alternating current cancel. Isn't it that the fields still exist only they are not in sine wave anymore but steady? Hope someone with more words and descriptions can explain. Thanks.
     
  7. Jan 22, 2012 #6

    Drakkith

    User Avatar

    Staff: Mentor

    What is the polarity of each wire? The same or opposite?
     
  8. Jan 22, 2012 #7
    Just imagine you wire single wire AC cords to a light bulb and plug it into 60 Hertz 110 Volts AC. Separately, the wire would have big magnetic field like 10 milligauss.. but when you put the wires close to each other (the wire before and after the bulb), the magnetic fields cancelled. My question is. Isn't it that energy is conserved. So when two EM waves cancel each other means that the two EM waves are somehow hiding underneath the covers becoming DC magnetic field. If not true, where did the magnetic field go?
     
  9. Jan 22, 2012 #8

    Drakkith

    User Avatar

    Staff: Mentor

    The fields don't cancel, they exert a force on each wire and try to force the two wires apart. At least that's what I think my basic electronics book was saying. I'll try to get a reference.
     
  10. Jan 22, 2012 #9

    Drakkith

    User Avatar

    Staff: Mentor

    Actually, I think my first question was wrong. I don't think the field has a polarity between 2 wires. But I definitely remember that two wires in a circuit would be pushed apart.
     
  11. Jan 23, 2012 #10
    No this won't happen. The magnetic field of the currents in a power cord would not be strong enough to push the wires apart. What were you thinking? I hope others can comment too of what happened to the magnetic field when they are cancelled. Do they just hide in the zone or completely annhilated.. please elaborate
     
  12. Jan 23, 2012 #11

    Drakkith

    User Avatar

    Staff: Mentor

    My apologies, I didn't mean to imply the wires would actually be pushed apart in a power cord, obviously this does not happen. I only meant that the wires would feel a force that would want to push them apart.
     
    Last edited: Jan 23, 2012
  13. Jan 23, 2012 #12
    But what happened to the magnetic field when the meter reads 0? Please describe what happens. Thanks.
     
  14. Jan 23, 2012 #13

    Drakkith

    User Avatar

    Staff: Mentor

    Here we go, take a look here about current in a loop: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html#c1

    As you can see, at the bottom and top of the loop the magnetic field is "around" the wire. This corrosponds to the magnetic field in a single wire as shown here: http://en.wikipedia.org/wiki/File:Electromagnetism.svg

    Since the current is travelling opposite in the return line in your power cord the two fields are oriented differently. One would be clockwise and the other counterclockwise if you were looking down the wires. These fields oppose each other in that they exert a force on each wire, attempting to force them apart. However, they also "add" together to produce a basic electromagnet, as the first link shows.

    The only difference in your AC circuit would be that the current and orientation of the magnetic fields switch at 60 hz. I believe two wires next to each other instead of in a loop would experience the same effects as the loop. So the fields don't cancel or annihilate, it simply switches poles with the electric frequency.
     
  15. Jan 23, 2012 #14

    Drakkith

    User Avatar

    Staff: Mentor

    Do you have a link to the type of detector you have?
     
  16. Jan 23, 2012 #15
  17. Jan 23, 2012 #16

    Drakkith

    User Avatar

    Staff: Mentor

    So are you actually getting a reading near zero? What exactly are you doing when you get this reading?
     
  18. Jan 23, 2012 #17
    When the AC line before and after the bulb is split apart, the reading is higher. When they are close together. It got very low (but not zero). What happened to the magnetic field. It got cancelled but does it get annihilated or is it just hiding in the cover or what....
     
  19. Jan 24, 2012 #18
    Or let me reformulate the questions in more clearer terms.

    Suppose we have two photons that are 180 degrees out of phase. We can assign one unit of energy to each photon. When we superpose the two photons, the combined electric and magnetic field goes to zero as does the Poynting vector and the energy density. So where does the energy go?
     
  20. Jan 24, 2012 #19

    Drakkith

    User Avatar

    Staff: Mentor

    I don't know how your meter detects the magnetic field so I can't really say much on that. I don't think two photons would interact like that though. If anything they would interfere and would act accordingly.
     
  21. Jan 25, 2012 #20
    Or just consider a powerline in the street. Some are designed such that the magnetic fields cancelled or attenuated a lot so it won't reach the houses below. In such cancellation, what happened to the magnetic field? Or just consider any general scenerio where magnetic field cancel, where does the energy go. Hope others can comment too. Thanks.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Magnetic Fields questions
Loading...