The Naked Scientists Forum
On the Lighter Side
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19/07/2009 23:18:12 »
Ok so your typical rotating fan's purpose is to move air from one spot to another, thus doing work. But at the same time, it wastes alot of energy moving the blades of the fan. Theoretically could you like the inside of the fan's cage with a ring of copper wire perpendicular to the blades, and then stick magnets on the blades, allowing the mechanical energy of the blades to also create more electricity to power itself? making the whole thing more efficient?
Reply #1 on:
20/07/2009 12:51:48 »
Perpetual Motion Machine ALERT!!!!
Neilep Level Member
Reply #2 on:
20/07/2009 13:03:58 »
I think what Sophie's trying to say is...
The fan would have to do extra work in order to generate the electricity, and as there would be losses involved, this would actually decrease the efficiency of the fan.
I think the only way to increase the efficiency would be to reduce losses elsewhere - less friction in the bearings, perhaps?
Reply #3 on:
20/07/2009 13:20:35 »
You can probably design better blades and cowling to reduce turbulence in the air, which would reduce losses and improve efficiency.
Indeed a generator becomes mechanically "stiffer" to turn the greater the electrical load you put on it. This is a natural effect of "conservation of energy".
You can demonstrate this with a small DC motor from a toy (such as Lego). Attach some kind of flywheel (a cardboard disc or a large Lego wheel) to the motor spindle, connect the power and let it rev up. Now disconnect the battery and it will slowly spin down. If, having disconnected the battery you now short-circuit the connections of the motor (drawing lots of electrical power) you will find the motor and flywheel stop very quickly.
Experiments like this make it very intuitive.
In fact, with one of the small grey (fast) Lego motors you can feel that it's harder just to flick/spin the motor spindle between your fingers when the electrical connections are shorted compared to when open-circuit (totally disconnected). (Be aware that the shaft rotation is "lumpy" anyway owing to the permanent magnets and small number of internal windings.)
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"