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Can spinning an electromagnet increase its strength?

  1. Apr 29, 2009 #1
    We've heard of the water pipe analogy for electric circuit. It says that the current produces magnetic field and that the "pipe" is irrelevant - only the amp-turns are relevant. So I was wondering, is it possible to increase the amp-turns of a electromagnet by rotating it in the same direction that the current is travelling? Let's say the velocity of the charge in the circuit is 5 millimeters per second. Then let's say we decide to rotate that charge faster by spinning the solenoid with an outer velocity of 5 meters per second in the direction of the flow of charge.

    1) Would this increase the amp-turns?
    2) Would this increase the magnetic field?
    3) Would there be any inherent resistance in turning the solenoid?
    4) Would this increase the strength of the electromagnet?

    Question: What are the theoretical implications IF:
    1) The amp-turns don't increase? My answer: The water pipe analogy does not remain valid.
    2) The magnetic field doesn't increase? My answer: The magnetic field does not come from the current.
    3) There is resistance inherent in turning the solenoid in this manner? My answer: An unlikely explanation would be need to show why resistance to motion would be generated.
    4) The electromagnet did not increase in strength? My answer: Increasing the relative motion of charges did not increase the magnetic field.

    I think this deserves an experiment to see if the water pipe analogy for electric circuits holds true.
    Last edited: Apr 29, 2009
  2. jcsd
  3. Apr 29, 2009 #2


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    (I'm assuming you're thinking of charge in a loop. and spinning the loop about its axis)

    Spinning changes neither the charge nor the current, so nothing electromagnetic should change. Yes, spinning the loop speeds up all of the electrons in the wire... but it speeds up the protons by the same amount.
  4. Apr 29, 2009 #3
    Wouldn't having more electrons than protons in the charge loop make a difference though? Doubling the speeds of both would make positive and negative contributions to the magnetic field, but if the electrons outnumber the protons, then the fields should not cancel out entirely should they?
  5. Apr 29, 2009 #4
    What is the water pipe analogy here? I don't see it. If you rotate a conductor in a magnetic field, you might get a current, but why would there be no resistance to this?
  6. Apr 29, 2009 #5
    For a start: Imagine that you have a uniform magnetic field. Then let's say you have a wire that is oriented perpendicular to this field. Then move the wire in a direction parallel to itself. So we have:

    B oriented upwards
    | <----------------------- E moves left (perpendicular to the direction of the magnetic field B)

    Does the uniform field B induce a current into E? It doesn't seem like it here.

    Keep in mind that increasing the voltage and current along line E will ultimately produce it's own B.



    Let's say you have a merry-go round. Now let's put a coil in the center oriented vertically. Then let's take a battery resting on the merry-go-round and connect it to coil. Now we have current flowing inside the coil. Let's say that the coil has a net negative charge. Let's then spin the merry-go-round at 10,000 rpm. If the magnetic field of a solenoid, B, is from the current, then the field produced by the solenoid, B, should increase in strength the faster you spin the solenoid. If the magnetic field is truly from the current, we should be able to produce arbitrarily high "effective" current densities without burning the coil. The electrical resistance, which is obviously in the system, no longer provides an upper limit to the "effective" current. The magnetic field of the electromagnet extends at right angles to ground. The rotating force required to spin the merry-go-round and the solenoid is therefore at right angles to the solenoid field, B, that develops as a result.

    The question is this:

    Using the laws of electromagnetism, explain the mechanism that resists the rotational acceleration parallel to the curling electric field inside the windings of the electromagnet and perpendicular to field it generates....

    B of coil
    | <----------------------- coil winding (moving perpendicular to B)
    | <----------------------- coil winding (moving perpendicular to B)
    | <----------------------- coil winding (moving perpendicular to B)

    Note, this is not a homework question, but I am surely stumped on this one. As far as I know, there is no clear connection between imparting kinetic energy into the solenoid in the described manner versus the resulting generation of the magnetic field (especially because they occur at right angles to each other). They seem like separable functions to me; in other words, the energy of the solenoid's field does not seem to be derived from the kinetic energy of the solenoid. What I am asking for is the correct understanding of this system.
    Last edited: Apr 29, 2009
  7. Apr 29, 2009 #6

    Vanadium 50

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    But the electrons don't outnumber the protons.
  8. Apr 29, 2009 #7
    That's true for at the scale of the universe (presumably).

    Where is the scientific argument that "proves" that ALL coils carrying current must have exactly zero net charge? Coils that have stray capacitance may certainly have net negative charge when subjected to an external electric field.

    I chose not the gloss over this fact because it has direct implications for the question I am asking. The point is that if a coil is negatively charged for these reasons:

    1) Stray capacitance
    2) Exposure to an electrical field

    That rotation of such a charged object in a direction parallel of the wire would result in a magnetic field whose gradient falls along a direction perpendicular to the force that generated that rotation.


    | <----------------- rotational force on solenoid

    What is lacking is understanding of the source of the potential energy in the field:

    1) Is it derived from the energy used to spin the solenoid?
    1a) If this is the case, it would imply some type of inertial effect for which I have found no generally accepted explanation or description. Q: Does a description of this exist?
    1b) If this is not the case, it would imply that some form of potential energy is being derived that is not from the input of kinetic energy. Q: What potential energy would that be?
    Last edited: Apr 29, 2009
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