MadSci Network: Physics

Re: How do i reverse the polarity of a neodymium magnet?

Date: Thu Dec 12 19:06:16 2002
Posted By: Aurelio Ramos, Grad student, Computer Engineering
Area of science: Physics
ID: 1038438350.Ph

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 

E=-dB/dt (E is the electromotive force (that is voltage) and B is the 

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 

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.


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