0
$\begingroup$

I have this doubt in my mind for a long time, see if we rotate a fan in space: it would keep on rotating forever and would never stop. Due to no air friction present in space. I could never really accept this fact. So i thought of the following situation: I take a fan and rotate it, and this fan is on the top of a turbine. So wouldn't the turbine generate electricity forever? Or if you cant except a turbine, lets say i attach and axle to the fan which in turn has magnets attached to it, There are other magnets surrounding it and remember the thing is rotating, so it would make energy...? or if i make a ball go around an axle. I hope i have made myself clear, that according to me the ball/fan would not stop due to absence of friction. Would it? Why?

Sorry if this sounds stupid. I am just a 15 year old young student, enthusiastic about physics so i have a habit of challenging all the theories before accepting them...

Update From the question i mean to say that will this cause the law of conservation of energy to fail?(ofcourse it won't and i know it) but I want someone who could prove it to me without saying that ....as energy is conserved.... Got it, what i am trying to ask?

$\endgroup$
  • 1
    $\begingroup$ There's friction in the fan, no? $\endgroup$ – Kyle Kanos Feb 24 '14 at 17:09
  • 1
    $\begingroup$ The fan would never stop if if didn't interact with other systems. If its motion is used to "produce" energy via the interaction with other systems (electrical, magnetic, even mechanical or thermal) then that energy will be deducted from the initial energy of the fan. This will result in the slowing down of the fan. Oh, and by the way: there is no such thing as a stupid question. $\endgroup$ – Peltio Feb 24 '14 at 17:13
  • $\begingroup$ @KyleKanos What do you mean by friction in fan? in which form do you think it is??? $\endgroup$ – Daksh Shah Feb 24 '14 at 17:17
  • $\begingroup$ @DakshShah Friction inside the internal gears and parts of the fan. :) $\endgroup$ – mikhailcazi Feb 24 '14 at 17:19
  • $\begingroup$ @Peltio Why? Can this be explained further? why would the magnets cause its speed to decrease? $\endgroup$ – Daksh Shah Feb 24 '14 at 17:19
2
$\begingroup$

Lenz's rule is related to the principle of conservation of energy. It states that:
The induced current produced in a circuit always flows in such a direction that it opposes the change or the cause that produces it.

So, induced current is going to stop the fan in your case.

I hope the fan is connected to the coil which rotates in between the magnets. As the induced current opposes the magnetic flux, there is a need to do work. Every time you do work, current will be produced by the expense of mechanical work done.

$\endgroup$
2
$\begingroup$

The problem with that type of schemes is that, because of Newton's third law of motion, when the fan applies a force to some external system to perform work on it, the external system also applies a force on the fan in a direction that will tend to stop it. This performs exactly the same amount of work against the fan's initial energy as it is performing on the system, because the forces are equal and opposite, and the distance travelled is the same. The net result is that the kinetic energy of the fan does eventually wind down to zero, if it is connected to an external system like that.

$\endgroup$
  • $\begingroup$ But why the work done from the fan is not 0? W=Fs and as s=0 W should be 0 :p $\endgroup$ – Daksh Shah Feb 24 '14 at 17:40
  • $\begingroup$ I'm not sure why you think that $s=0$. It will depend on the set-up, but in general if $s=0$ then there is no movement and essentially nothing happens, so the turbine (or magnets or whatever) can't "see" that the fan is applying a force, and therefore can't get any energy from it. The point is that whatever formula controls how much energy you actually get from the fan winds up also controlling how much energy the fan loses - and vice versa. $\endgroup$ – Emilio Pisanty Feb 24 '14 at 17:46
  • $\begingroup$ @DakshShah The total displacement of the fan as a whole surely is zero in your case, but the blades are continuously moving, aren't they? $\endgroup$ – mikhailcazi Feb 24 '14 at 17:48
  • $\begingroup$ @mikhailcazi yes blades are moving continuously why? what? $\endgroup$ – Daksh Shah Feb 24 '14 at 17:49
  • $\begingroup$ @DakshShah well, didn't someone take a fan into outer-space and start rotating it? :/ $\endgroup$ – mikhailcazi Feb 24 '14 at 17:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.