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I Confusion about the magnetic field

  1. Mar 15, 2017 #1
    I'm having some confusion about the creation of magnetic field lines. How I understand it so far, is that a magnetic field is basically an electric field viewed from a relative point. I don't have a great understanding so far, as I just started to learn about it, but I've learned that objects traveling fast condense together (something due to special relativity) relative to non-moving objects, so there would be a higher charge density on the moving object.

    Starting at 1:10 in this video, he explains why the magnetic field exists.


    He goes on to show that the magnetic field will exist and exert a force on the proton when the proton is moving. BUT what I don't understand is when the proton is stationary in that video, and the electrons are flowing, why don't the electrons condense and exert an attractive force on the proton?

    This kind of goes along with my first question. I've also watched a couple other videos explaining that when an object is not moving, no magnetic force is acted upon it (I'm hoping that's true, and I that my first question covers this). So when a wire with a current flowing through it is placed over a compass, why does the compass needle move? It moved, so that should mean that there was a force applied to it. But the compass needle wasn't moving, so what force caused it to move? I thought that there should be no magnetic force acting on a stationary object? So if the compass needle was stationary, how did a magnetic force act on it.

    So I guess my fundamental question is this: If you create a magnetic field along a wire, by inducing a current along a straight wire, will a positively charged object experience a magnetic force? Why or why not? If it does, will it be attracted or repelled? If it doesn't experience a force, then how and why does a compass needle move when it is near a wire that has current flowing through it?

    When I say that a stationary object should experience no magnetic force I'm referring to this video at 1:20:


    Hopefully my questions weren't too confusing, and thanks in advance for any help!
     
    Last edited: Mar 16, 2017
  2. jcsd
  3. Mar 16, 2017 #2
    If there is a loop shaped wire near a straight wire, and there is electric current in both wires, what happens?
     
  4. Mar 16, 2017 #3
    I have no idea, and I'm not sure how this pertains to my questions. If there is some insight I'd love to here.
     
  5. Mar 16, 2017 #4
    Magnets have microscopic current loops in them.



    EDIT: Oh yes Welcome to the forum. Let us consider a rectangle-shaped current 'loop' near a straight wire. The youtube video tells us what forces the segments of the rectangle that are parallel to the straight wire experience. Right?
     
    Last edited: Mar 16, 2017
  6. Mar 16, 2017 #5
    If it is parallel to the straight wire then that means it would be orthogonal to the magnetic field. But if it the object isn't moving how would it experience a magnetic force?
     
  7. Mar 16, 2017 #6
    I actually think I figured out my own question guys. The reason the compass is deflected near the wire is not due to the stationary charges of the iron, but their self-induced magnetic field (with moving electron on atoms) that interact with the moving charges in the wire. That's the reason a "stationary compass" will move near a wire with current flowing, but say something like a ball of carbon will not move near the wire.
     
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