Electric Car - Perpetual Motion?

Discussion in 'Community Discussion' started by cosmichobo, Jul 26, 2014.

  1. cosmichobo, Jul 26, 2014
    Last edited: Jul 26, 2014

    cosmichobo macrumors 6502


    May 4, 2006

    I never was very good at physics... It was in fact the first class that I ever failed (ok - hang on - to clarify - I received a 7/20, which was a "technical fail", but did not require re-sitting)... So, I could use some help with this little idea that smacked me in the head just now.


    Did you know some chap/s are working on an Electric Delorean? I know electric cars aren't new... And really the Delorean part was just the carrot that made me click on the link... but as I was looking at the boot full of 13 batteries, my brain said something along the lines of this:

    As noted... 7/30 for physics (and 11/20 for chemistry)... Be kind.


  2. Intell macrumors P6


    Jan 24, 2010
    You'd loose energy when converting between electricity to mechanical and back to electricity. The energy would be lost to friction/heat.
  3. Don't panic macrumors 603

    Don't panic

    Jan 30, 2004
    having a drink at Milliways
    certainly we will eventually get to more efficient cars (combustion engines are very inefficient), but there is always some loss.
    perpetual motion machines are physically impossible.
  4. noodlemanc macrumors regular


    Mar 25, 2010
    I believe some electric cars and trains use that to brake. Obviously when you're not braking there wouldn't be any point though, as it would be like using an electric motor to spin a wind turbine to generate electricity.
  5. Huntn macrumors P6


    May 5, 2008
    The Misty Mountains
    I'm not a scientist, but even I can see that battery power goes to running the electric motors on the wheels, and if they also ran alternators, they could produce some charging back to the batteries, but it would be a sum net lose of charge. In other words the alternator(s) could not overcome the energy being spent to move the car, until magic alternators come along that produce more electricity than the electricity it would take to turn them. :)
  6. localoid macrumors 68020


    Feb 20, 2007
    America's Third World
    I thought surely I would be able to find a YouTube video that would explain why this wouldn't work, but none seem to exist.

    So... I'll refer you to the Help Session on Standard MNRG.2.b webpage. The tl;dr version: You can't violate the first law of thermodynamics and the second law of thermodynamics.


    You can never accomplish anything with a scheme as show above
  7. mobilehaathi macrumors G3


    Aug 19, 2008
    The Anthropocene
    You should have just left it at this, mate.
  8. maflynn Moderator


    Staff Member

    May 3, 2009
    There's no such thing as perpetual motion, if there were, we'd not having any sort of energy crisis and not have to deal with high gas prices.

    No, energy is lost (transformed into heat) when being converted and you cannot create more energy from machine, i.e., a machine cannot consume 100 watts and produce 120 watts.
  9. sixrom macrumors 6502a

    Nov 13, 2013
  10. kingalexthe1st macrumors 6502


    Apr 13, 2013

    There are plenty of explanations offered by others, so here's my take to add to the list.
    Energy transfer in a normal car usually goes like this:

    1) Energy is stored in petrol/gas in the chemical bonds
    2) Your electric alternator helps kick-start the reactions necessary to release that energy and transfer it to the motion of wheels
    3) Some of the energy is used in a feedback loop to kick-start the next reaction, which powers the next reaction and so on, keeping the car moving
    4) On top of that, some more of that energy is pumped back in to the alternator to keep it charged ready for the next startup.

    So the energy is primarily coming from the petrol/gas. In your case you are thinking of using electric batteries used to move the wheels, but also to power themselves up again. As others have said, you will always lose energy during transfer unless we existed in a perfectly cold, frictionless universe which, of course, we don't. So in transferring energy out of the battery and then back again, you'll lose energy and not get back what you used.

    Hope that explains rather than mystifies.

  11. cosmichobo thread starter macrumors 6502


    May 4, 2006
    Hi guys, and thanks for all the fish replies.

    I do realise that a perpetual motion machine is impossible... I may have not entirely passed physics ("Anti-Man", my teacher used to call me, as I often provided the opposite answer to what he was after... Mind you - he called himself God!), but I am also a Doctor Who fan. In that regard, the threat title was a bit of a red herring.

    My thoughts were simply that, with a trad car having the alternator to charge the battery... then an electric car could also use an alternator to provide some charge... Realistically, more to simply extend the drive-life of the batteries, rather than actually lasting forever...

    I presumed that an alternator would however essentially slow the car down, adding "drag", so to speak, which may well balance out any benefit from converting the drive energy into electrical...

    Maybe what we need is electric cars with a tank of water, being pumped over a turbine... and... I mean, water and electricity - can't go wrong, right?


    At any rate... I can't wait for the real non-fossil-fuel car race to really heat up.
  12. Phil A. Moderator

    Phil A.

    Staff Member

    Apr 2, 2006
    Shropshire, UK
    The best you could hope for from the alternator if it were 100% efficient (which it couldn't / wouldn't be) would be that it provided the same power in output that it took to spin it round. In other words, it would take more power to spin the alternator than it would give back in charging so you'd have a net negative effect and flatten the battery faster!
  13. ideal.dreams macrumors 68020


    Jul 19, 2010
    I'm going to beat the horse a little more here and add that energy can not be created or destroyed. It's one of the major fundamental laws of physics. It can only be converted to other types of energy. In the case of an electric car, the battery provides electric energy which is then changed to mechanical energy and the car moves. That energy is not reclaimable. If the battery was also powering an alternator, and therefore the alternator charging the battery, the energy that was lost as mechanical energy is still gone and there's still going to be an overall net decrease in energy until the battery is dead.

    Much the same way attaching buttered toast (always lands butter side down) to a cat (which always lands on its feet) can not be used to create perpetual motion.
  14. jav6454 macrumors P6


    Nov 14, 2007
    1 Geostationary Tower Plaza
    As many others have stated, you always loose energy when converting from one form to another. In this case, the conversion of electric to mechanical and back to electric.

    There is friction which turns energy to heat, magnetic permeability which heats up coils in a motor by turning electric energy into heat. Not to mention air drag/resistance which will slow down a moving object.

    Perpetual motion is nothing new and everyone has come to agree, there is no such thing as perpetual motion. Law of conservation of mass and energy will make it happen as to there is never one.

    The only thing we can ideally do, is create a system which recharges its energy by using outside energy that isn't linked to gas/petrol, such as the sun. Furthermore, you could increase efficiency of all systems making them loose less energy in the electric to mechanical transformation; hence, more mpg can be had.
  15. Sydde macrumors 68020


    Aug 17, 2009
    Take a look at the design of Toyota's "Hybrid Synergy Drive": it uses a planetary gearset with a generator on the "backload". Hence, at lower ratios, when the motor is spinning faster and outputting less to the wheels, the generator is capturing the excess spin and feeding it back into the motor or the battery – the generator actually regulates the backload, serving as a CVT controller. It allows the gas engine to run at a fairly optimal speed all the time, but there is also an electric input motor that can push the input. The plug-in Prius has a range of something like 11 miles on battery only, and this system is almost as efficient as you could get (more direct power on each of the wheels themselves instead of through a differential would be somewhat more efficient, at least on electric only). So that is what you are talking about, right there in the real world. You not only have mechanical losses to deal with, but the wires are not superconducting, and the best battery loses energy to heat from the chemical reactions. Enough that generator/alternator drag is going to cost you more power than you can spare to move the car itself.

    Get a bicycle.

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