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About energy in general

  1. Feb 17, 2009 #1
    I have a question that relates to energy, and then a follow up question if my first premise is true. As I understand the Conservation of Energy theory, it takes more energy to produce a certain amount of energy. Ok that is a rough explanation by an unlearned student of physics, but here is my premise. Please tell me if it is accurate:

    It takes more than 9 volts of energy to produce a 9 volt battery, and will always take more than 9 volts.

    True or false? Thanks...
     
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  3. Feb 17, 2009 #2

    russ_watters

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    Welcome to PF.
    No, that's not conservation of energy (1st law of thermodynamics). Conservation of energy says that the inputs to a system must exactly equal the outputs. What you describe is closer to the 2nd law of thermodynamics: entropy. Entropy is "lost" energy and is why you need more energy input to achieve a certain output (as opposed to an equal amount input). Put another way:

    1st law: you can't win.
    2nd law: you can't break even.
    No, volts are an electrical equivalent of force, not energy. There is no conservation of force.
     
  4. Feb 17, 2009 #3
    Ok, well perhaps my use of a 9 volt battery was a bad one, but it sounds like the 2nd law of thermodynamics is closest to the point I'm trying to make.

    It will always take more than 9 'units' of energy to store 9 'units' of energy.

    Would that be a true statement?

    More to my point I'm wishing to make is, that there really is no such thing as a truly 'renewable' source of energy, in the strictest sense the laws are applied.

    True?
     
  5. Feb 17, 2009 #4

    Mapes

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    "Renewable" doesn't mean that a type of energy in nature must be converted with 100% efficiency to a type of energy that we can use (e.g., wind energy to a battery's chemical energy). It specifies a type of energy in nature that essentially cannot be depleted.
     
  6. Feb 17, 2009 #5

    Non depletable perhaps but reliable and abundant enough to power America, with an electrical car in 2 out of every 3 garages let's say?

    Sure wind/solar/geothermal/tidal could supplement, and through conservation & recycling we can do better but my principal argument is this:

    The laws of physics are such that energy will always cost more energy than you can store/generate/produce. Thus the pipe-dream green environmentalists have, that we can ultimately forgo any consumption of fossil fuels is just that, a pipe-dream.

    Not saying we should not think outside the box by any means, but I would argue its a far higher hurdle than one imagines. Due in large part to the unavoidable physics of the world around us.
     
  7. Feb 17, 2009 #6

    Danger

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    The laws of thermodynamics are thus:
    1) The best that you can do is break even.
    2) You can only break even at absolute zero.
    3) Absolute zero is impossible to attain.
    4) No matter how many times you shake it, the last drop always goes down your pants.

    It seems to me that you are inquiring about over-unity (perpetual motion) machines. In that case, you are absolutely correct—such a thing can't exist. It is possible to get a completely equal energy output, but nothing can be done with that energy without losses.
     
  8. Feb 17, 2009 #7
    Thanks for the definitions. That helps (as well as the others that have posted)

    Really what I'm trying to say/do/theorize is debunking the idea that somehow a renewable energy source exists. Given the laws of physics, I don't really think there is one.
     
  9. Feb 17, 2009 #8

    Danger

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    That really depends upon what you mean by 'renewable' energy. Coal, petroleum and natural gas are not renewable. Timber is. Solar, wind, and wave energy are 'self renewing' in that they are all driven by the sun, which is immortal on a human time scale.
     
  10. Feb 18, 2009 #9

    Mapes

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    What an odd statement. You're saying that because energy can't be transferred with perfect efficiency, we must rely on energy sources that can be depleted rather than on energy sources that can't be depleted. There must be more to this argument; how does the second statement possibly follow from the first?
     
  11. Feb 18, 2009 #10
    No, I'm not suggesting its about a choice of one over the other. We'd want to use all sources we have available to us.

    I'm just saying it will always 'cost' energy to produce energy. And there's no way around that. Whether its solar, wind or oil.

    Electric cars seem like a great idea. But what keeps the chargers running? What happens if everyone or most everyone plugs in their electric car at night to charge? Surely the infrastructure has to follow to meet that demand. Will solar and wind farms be enough?
     
  12. Feb 18, 2009 #11

    Mapes

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    OK, I think I see what you're saying. But it should be clear that neither the laws of thermodynamics nor any physical laws imply that a civilization must use fossil fuels or that restricting energy sources to renewable sources is impossible. Economic constraints, yes, but not physical laws. The connection you made earlier between energy transfer efficiency and the need for fossil fuels doesn't make any sense.
     
  13. Feb 18, 2009 #12
    You seem to be asking many different questions.

    First off the unit of energy you were looking for would be the joule, or watt hour. You are correct that it will always take more joules to create, or charge a battery than you will be able to get back.

    Then about renewables not really being renewable. There really is no absolute definition of renewable, but in general usage it means things like solar and wind. Regardless you are correct that all forms of energy will eventually run out. The sun will die in a few billion years. The entire universe has a finite amount of energy in it, eventually all energy will be consumed and converted to waste heat, maximum entropy will be archived, this is called the heat death of the universe, and is one possible ultimate fate for our universe. However, generally speaking renewables will be renewable for at least a billion years, this is seen as sufficient for energy planning on a decade or century scale.

    Lastly about if renewables could power the US, especially if we phase out fossil fuel usage. This is a point of debate, and I personally feel the choice for the backbone of power generation is between coal or nuclear, and that coal is unacceptable. However, you'll find no shortage of opinions on this point.
     
  14. Feb 18, 2009 #13

    russ_watters

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    This is true, but I really think you're barking up the wrong tree with a COE argument for or against certain fuels. Here's why:

    Coal plants are only about 35% effiicent, but their fuel is the cheapest around and thus they are the electricty source of choice today.

    Solar plants are only 20% (typical) efficiency, but they require no fuel, so they could theoretically provide "free" electricity, indefinitely. The stumbling block is the capital cost of the plant. If that drops substantially, it doesn't matter that it is less thermodynamically efficient than coal - they will be the energy source of choice.
    You're right about that argument, but it doesn't have anything to do with conservation of energy or thermodynamic efficiency...

    I don't like arguing the wrong point even if it is for the right reason.
     
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