|MadSci Network: Physics|
The absolute easiest way to reverse the polarity of a magnet is to turn it around. This takes mechanical energy. However, the easiest solution can sometimes be impractical. You can also change the polarity of a magnet by wrapping a coil around it (so that the coil goes in the direction of polarization) and then passing a very large current across the coil momentarily. With a high coercivity magnet like the ones you have you must create a magnetic field significantly larger than the field you expect to remain on the magnet after the process is over. You will need to know the exact magnetization curve for your magnet. Creating this magnetic field requires holding a driving voltage across the coil for a specific length of time. For some reason I get the feeling you are trying to build a perpetual motion machine, and I feel tempted to dismiss your question by saying it's just not possible, that the law of conservation of mass/energy prohibits such a machine. Although that's all true, it is much more interesting to go step by step and analyze your machine to see what would really happen. First of all, it should not surprise you that the bearing assembly rotated past the neutral point after you released it. This is the result of inertia, the tendency of moving objects to continue to move until an external force is able to stop them or otherwise change their velocity. Another case where that happens is the pendulum. It repeatedly misses the neutral point because potential energy is converted back into kinetic over and over. This behavior in your bearing assembly is nothing out of the ordinary. If I understand correctly, you have a ball bearing with an attached magnet so that the line that goes across the poles of the magnet lies on the plane of rotation of the bearing: There is also a series of magnets held stationary. For simplicity of this analysis I have assumed that each of the stationary magnets in the series is a cycle being repeated in your machine, thus, if we understand one cycle, we understand the machine's overall output. The amount of energy required to invert the magnetic field of a neodymium magnet far surpasses the amount of energy needed to power an electromagnet to generate the ever changing magnetic field you desire. Consider a coil wrapped around the rotary permanent magnet: According to Faraday's law, the rate of change of a magnetic field over time is proportional to the electromotive force applied to produce the change. E=-dB/dt (E is the electromotive force (that is voltage) and B is the field) Since you would be applying a voltage to the coil in order to change the magnetic field, the result is that momentarily, an equal oposing voltage is produced in the coil (impeding the flow of current) and then the magnetic field is gradually changed over a period of time proportional to the overall change in magnetic field. Neodymium magnets have a very high coercivity, this means that to reverse the magnetic field you must use a magnetic field just barely larger in magnitude and oposite in direction to the field previously used to saturate it. Because of this, the amount of work required to "flip" the field of a permanent magnet is roughly twice the amount of work required to magnetize a coil with an air core (or a soft magnetic core). And the magnetization remanant in the magnet is lower than the field used to magnetize it (remanence is always lower than unity in a permanent magnet) So the best way to create an alternating field in your rotor assembly is to use an electromagnet instead of a permanent magnet, because it takes less energy. The end result is an electric motor. The work that your rotor assembly will produce due to the electric current applied will not be larger than the amount of energy applied at the input. This is because of losses due to friction and induced eddy currents in the bearing assembly by the stationary magnets. If you could somehow manage to minimize or avoid eddie currents, friction, and electrical resistence in the coil(s) you would aproach 100% efficiency and would have made a machine capable of sustaining itself for a very long time once started. But no energy could be extracted from it without bringing it to a stop. -Aurelio
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