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Energy distribution in wheel

  1. Aug 14, 2013 #1
    Recently I heard about using exercise bikes to generate electricity. It is more or less like a bi-cycle fixed in a training stand. They convert the rotation of the wheel into electrical energy.

    The scenario is that they take some W watt of power from that model. In their case a disc like arrangement is used to transfer the rotation to a dynamo or a generator (I am not sure about, what is the difference between using a dynamo and DC generator).

    My question is if I use a shaft kind on arrangement on either side of the same wheel and connect it to two different dynamos, will I get 2W watt of power or will the two dynamos separately contribute (W/2) watt of power and thus contributing the same W watt of power.

    If I make an arrangement such that both the wheels get rotated when you peddle the bi-cycle (Assuming both the wheels are not in contact with the ground i.e both the wheels lie above the ground as a training stand is used). Having one shaft on either side of the two wheels, Therefore, getting output from 4 shafts and using four different generators, will I get 4W watt of power or will the power from each dynamo be (W/4) watt of power, thus contributing the same W watt of power.
  2. jcsd
  3. Aug 14, 2013 #2


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    Staff: Mentor

    You only get out what the human puts in. At best (with 100% efficient generator), if the human puts in 100W of power by pedaling, then the generator can put out 100W of electrical energy. You cannot get out more power than is put in by the human.
  4. Aug 14, 2013 #3
    I still have a doubt. Assume a bus is moving at some speed. A small generator is connected on one side of a wheel. W watt of power is obtained. If another generator is connected on the other side of the same wheel would the output be the same W watt.

    Assume that the mechanical input is x when only one generator is connected, the power produced is W watt. I doubt whether two 2x input of mechanical energy would be needed to produce 2W watt of electrical power. I believe when two generator is connected, the mechanical input required to produce 2W of power would require a mechanical input less than 2x.
  5. Aug 14, 2013 #4


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    Staff: Mentor

    If that were true, it would violate conservation of energy, which is a fundamental law of the universe.

    Moreover, every engineer would already be powering their house from a stationary bike.
  6. Aug 14, 2013 #5
    I suppose if you believe that then you will also have trouble answering this question "If one man can dig a hole in 10 minutes, how long does it take 7 men to dig 7 holes?"

    Basically, it is Power in = Power out, and energy in = Energy out.
    If the second generator is more efficient in converting mechanical energy to electrical energy, then disconnect the first generator and use the second.
  7. Aug 15, 2013 #6


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    Staff: Mentor

    Over-unity energy generation is a forbidden topic here on the PF. Your thread is closed. Please do not post like this again here.

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