Okay, I’m no genius or physics guy but someone tell me if this is possibe.
If you were to spin up a generator in space, say on the space station or something… (typical magnetic field broken by copper wire = current… sorta thing)
Once you spun it up, wouldn’t it theoretically provide power forever? No friction, no contact with other parts…initial energy would be to spin it up as fast as you want, and it would provide power at that rate forever…
Where is my thinking wrong? Why not spin up a generator and let it go rather that rely on solar power?
I’m sure I’m missing some elemental point here, but that’s why I read Straight Dope.
Lenz’s law is the right answer. As the generator spins, it induces a current in the wire, and that same current creates a magnetic field that is opposite of the magnet, which will eventually cause the generator to stop spinning.
Nitpick: This is true in theory but in reality, on most dynamos the extra work is dominated by mechanical friction. You can tell because truly high-quality dynamos (like the $250 dynamos made by SON in Germany) require far less work to generate the same amount of power. Almost imperceptible, in fact.
I’m not sure that I agree, in fact I’m pretty sure that I encountered the phenomenon before I learned about Lenz’s law (on a cycle that had a switch in the light circuit, leaving the dynamo in contact with the wheel rim all the time.
For this statement to be true, you need to show that most dynamos have an efficiency of less than 50%. I have no off-hand knowledge of that, but if you substitute the word “generator”, or better yet, “alternator” for “dynamo”, then 90% or better is the rule.
In the Heinlein juvenile novel Space Cadet they use the “braking” effect of Lenz’ law to stop rotation of the training ship, putting the energfy into storage batteries. Later, they restore the spin by working the system in reverse.
This page shows efficiency measurements of 9 bicycle dynamos. Of those, only two have 50% or better efficiency at 20 km/h. Both are $100+ products.
Although perhaps “dominated” was too strong a word; I can believe that under some circumstances, the added Lenz’s Law drag is noticeable. I just haven’t been able to feel it myself. (And I have tried; I have a SON dynamo on one of my bikes, and I can reach the switch while riding.)
I’ve definitely noticed it myself (although anecdotes aren’t all that useful, admittedly).
Simple experiments using small electric motors as generators can demonstrate the effect of Lenz’s law - taking a small DC motor such as that found in a CD or cassette player, the shaft can be spun and will remain spinning for quite a few seconds. Short the terminals of the motor together (or connect them to a lamp) and the shaft won’t spin freely at all, indeed, it will be noticeably much harder to set in motion.
Of course, that proves nothing about bicycle dynamos - the small DC motors in consumer electronics are so finely engineered that you can connect the terminals of one to the terminals of another and a gentle spin of the shaft of one of them will cause the shaft of the other to spin.
You can even set up an (admittedly inefficient) electric flywheel - place the motors facing each other and connect the terminals together in such a way that spinning one shaft causes the other motor to spin in the same direction. Couple the shafts and give the thing a spin - it works much like a flywheel, except that only part of the energy is stored in the rotating parts - some of the energy is stored as the circulation of current in the system.
It’s unclear to me why you think putting a ‘typical’ generator on the space station would eliminate all friction in the first place.
Putting it in a vacuum would eliminate air resistance, but the same applies to a vacuum on Earth. You could come up with a design to minimize contact in zero G, but a space station generator would be a magnetic device with significant angular momentum in a elliptical orbit, subject to frequent powered orbit and attitude correction, and have to tap power from it somehow. Each of these could be handled individually, but your final design would be almost completely unlike any typical generator, except in principle. I suspect you’d inevitably going to need some sort of bearings [even magnetic bearings have known losses on Earth] to maintain alignments. The electrical contacts you mentioned, alone, would be sources of friction.
Space is a very different environment from the one we’re used to on Earth’s surface, and clever designs can operate in ways that would be difficult or impossible on earth - but just because motion through space itself is frictionless doesn’t mean that anything you put in space becomes frictionless (or the astronauts would’ve gone mad by now. Friction is essential in innumerable ways.)
Certainly the conversion of rotational and orbital energy to electrical energy has many potential yses in space; tether technology is in its infancy now, but will no doubt ’take off’ if and when carbon buckytube fibres are available to avoid breakages…
From this site http://www.tethers.com/microPET.html
quote*
Electrodynamic tethers can provide long-term propellantless propulsion capability for orbital maneuvering and stationkeeping of small satellites in low-Earth-orbit. The µPET™ Propulsion System is a small, low-power electrodynamic tether system designed to provide long-duration boost, deboost, inclination change, and stationkeeping propulsion for small satellites. Because the system uses electrodynamic interactions with the Earth’s magnetic field to propel the spacecraft, it does not require consumption of propellant, and thus can provide long-duration operation and large total delta-V capability with low mass requirements.*
If you wanted to exploit Jupiter’s magnetic field for energy, for example, you could use the material in the planet’s rings;
Once the useful water and even more useful deuterium was extracted, the solid residue could be used as ballast in electrodynamic tether generators; as the magnetic field of Jupiter produced electricity for consumption, it would deorbit the generators and the rocky ballast until they enter the gas giant’s atmosphere.
This process would also remove the rings themselves, which represent a (minor) navigation hazard near that useful planet.
Also notice that the energy you put into the system to “spin it up” would be the maximum amount of energy you could get out of the system, in this case, as electricity.
Except in the case of existing orbiting objects such as the rings of Jupiter, Saturn, Uranus and Neptune, which would convert their existing orbital speed into free energy (but only once).