On 12/7/2007 1:33 PM, Horace Heffner wrote: > > On Jul 12, 2007, at 9:53 AM, Harry Veeder wrote: > >> >> However, I just realised the shield in my drawing >> >> http://web.ncf.ca/eo200/generator.html >> >> is also subject to its own weight -- without the magnets it will >> tend to >> orient itself so the gaps are aligned horizontally. > > If built symmetrically as shown, i.e. well balanced, it will no more > have a preferential position than a propellor or fan.
You are correct. (for some reason I was thinking in terms of a liquid seeking its own level). > >> So with gravity and the >> right combination of magnet strength and shield weight distribution >> you >> can minimise the energy required to keep the shield rotating. > > The main energy required to keep the shield rotating can be viewed as > coming from the need to compress flux. Magnetic flux has a magnetic > pressure: > > P = B^2/(2 mu_0) > > When the hole is cut off the flux is compressed on the magnet side of > the shield. When it expands into the core the energy from that > expansion is reduced by the counter emf, i.e. the current, in the > coils producing an opposing field. Similarly, by Lenz law, the energy > required to close the hole and compress the flux is increased by the > current induced in the coils. Theoretical signs which say "no escape". > If you have no core at all, the shield in your diagram will > experience a strong cogging effect when it rotates, and the net drag > is due to heating of the magnet and or shield via induced eddy > currents that also, by Lenz law, oppose the motion. Aren't these more like technical challenges, rather than theoretical signs? Harry
