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Moving Rod within a circuit

  1. Sep 29, 2014 #1
    So when you have a regular circuit with a voltage source on one end and a conductive rod that is free to move laterally on the other and you turn on the current, the rod will move either left or right depending on the current direction. What is causing this movement? Lenz's Law? So is the rod trying to generate a current that opposes the current of the circuit that it is in, for the purpose of negating the change in magnetic flux? What is going on there? Usually the moving charge object and the external magnetic field affecting it, through F = qvB, are separate entities, but in this case they are fused together

    Thank You
  2. jcsd
  3. Sep 29, 2014 #2


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    What is the set up that you are describing? If there is just the circuit with no external magnetic field then the direction of movement should, in a simple case, be independent of the current direction - just on the circuit layout.
    http://heory.uwinnipeg.ca/physics/mag/node10.html [Broken], amongst many others on 'force between two wires' etc. which shows the way it works, depending on the relative directions of the currents in the two wires. In a simple circuit (loop), the currents will be in opposite directions.
    Last edited by a moderator: May 7, 2017
  4. Sep 29, 2014 #3
    You seem confused. You mixed up unrelated concepts, and you haven't mentioned whether there's an external magnetic field or not.
    Check your question again. :)
  5. Sep 29, 2014 #4
    Ah no there is no external magnetic field. Hmm I can't really upload the picture but I'll try to draw it out with text. Alright so the vertical lines are not broken, of course. There is no external magnetic field, however, when I turn on the voltage source, a magnetic field is induced by the current. If it is conventional current we are talking about moving counterclockwise from the voltage source, then the magnetic field will be pointing out of the screen. My question is what exactly, in this setup, is causing the rod to move? Is it the magnetic force that the current is producing? The rod is part of the circuit producing the magnetic field, and at the same time, it is being moved by the magnetic field it itself is part of producing. I am sure I am mixing concepts here but don't know how to detangle them, particularly Lenz's law and F = qvB.
    | ''''''''''''''''''''''''''''''''''|
    V''''''''''''''''''''''''''''''''''| <------ The rod that is free to move laterally, I was talking about in my original post.
    Last edited: Sep 29, 2014
  6. Sep 29, 2014 #5
    Also, when the rod is moving back or forth, isn't the magnetic flux changing and therefore a counter current is being produced? Or is this only considered when the magnetic flux is from an external magnetic field? Sorry, I know I've going on in there. Pretty confused at the moment.
  7. Sep 29, 2014 #6
    You are taking about two different topics, magnetic forces and electromagnetic induction.
    I'll clarify some points that seem unclear to you, first.
    It's not the magnetic field of the wire itself which causes a magnetic force, it's the interference between two magnetic fields. The magnetic field of both of the vertical wires interfere
    producing a force on the movable one.
    Google 'Magnetic force between two wires'.

    When you say that you move the wire that's a totally different thing, you are talking about EM induction.
    Lenz's rule isn't related to magnetic forces.
    Lenz's rule is used to determine the direction of an INDUCED current.
    Induction is due to a change in the magnetic flux linked to the conductor.
    When you say that , we should be talking about motional
    Try googling motional induction.
    After googling, If you have any more problems, come back her and post it. :):)
    Last edited: Sep 29, 2014
  8. Sep 29, 2014 #7
    Ahh, I understand why it moves now. I knew of the two wire example already but, for some reason, didn't see its application in the circuit.
    However, for Lenz's rule and motional induction, the examples of motional induction I found online involve a circuit with no voltage source that is being induced by an external magnetic field. In the circuit example I provided, the circuit is producing the magnetic field via its current. When the movable rod is moving in such a circuit, wouldn't there be a change in magnetic flux over time as well, which would produce an induced current, according to Lenz's rule, in the opposite direction of the current provided by the voltage source?

    Also, Thanks for your response!
  9. Sep 30, 2014 #8
    Pardon me, it's more proper to use the concept 'motional emf'.
    By the way, I'm not sure what the motional emf would be like since there's already a current in the circuit,
    I think it won't matter, it will just be like another source, but I'm not sure.
    By the way the direction of the voltage and current depends on the direction of the magnetic field and the direction of the motion of the wire, using Fleming's right hand rule, you could determine the direction.
  10. Oct 1, 2014 #9
    Ah icic, thanks for the input, I'm undoubtedly over-thinking this. Also, you mean to replace the use of the wording 'induced current' with 'emotional emf' as a more apt descriptor for the concept in question, correct?

    Thanks again!
  11. Oct 8, 2014 #10
    Correct !
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