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Doubt Of Energy Conversion

  1. Nov 23, 2012 #1
    Hi there people, I have a doubt, if you convert all energy has an expense which makes it is always converted less than 100 percent for example kinetic energy turning electrical energy turning 90 percent of electrical energy, there was a loss of the energy that I'd like to know and if for example it is possible to reinforce what has been converted to 100 percent again, to create more electric power to strengthen until the 100 percent?? thanks for the attention.
  2. jcsd
  3. Nov 23, 2012 #2
    Your post was a bit hard to follow, but I think what you are asking is along the lines of :

    If energy is neither created nor destroyed, why isn't all the energy put into conversion system such as a motor or an engine converted? Where does the lost energy go?

    Energy conservation is a real thing, so it's safe to say that the lost energy isn't destroyed. It all still exists, It has just, in most cases, been dissipated through thermal interactions between particles within the system. This "thermal dissipation" is also the reason kinetic energy is not conserved in inelastic collision.
  4. Nov 23, 2012 #3
    sorry for the hard topic ^^ so i try to write more clear. one exemple who not exist just for try to explain a car with kinetic energy and we transform this on eletric energy when you change the energy have a dissipated turning the car just 90 per cent of energy this 10 per cent is lacking on the car, its possible to use something to make the car 100 of energy or more than that. thanks for attention again.
  5. Nov 23, 2012 #4
    The 10 per cent is being converted into heat. This is energy that the car is unable to use. There is nothing in physics theory that forbids getting very close to 100 per cent efficiency, But, In most cases, it is difficult to do achieve in practice.
  6. Nov 23, 2012 #5
    thanks again the part who become heat is useless, but if upgrade with more eletricity the 10 per cent lost can come back?? one more example to make more clear before the conversion 1000 kinect energy after the conversion 900 of eletric energy if someone put something to upgrade inside the car like tuning the eletric system to became 1100 the energy its possible?
    Last edited: Nov 23, 2012
  7. Nov 23, 2012 #6


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    If you have a machine that produces electrical energy when you put kinetic energy into it, that machine will always obey two laws:

    H > 0

    where K is the kinetic energy that goes in, E is the electric energy that comes out, and H is the energy of the waste heat that escapes into the environment around your machine as it operates.

    The first law say that energy is conserved, and the second law says that the amount of electrical energy you get out of the machine will be less than the amount of energy you put into the machine.
  8. Nov 23, 2012 #7

    There is something lacking in your understanding of energy. I think it is the second law of thermodynamics. Energy is a quantity, governed by the first law. It also has a quality, governed by the second law. Translational, rotational, and electrical energy examples of high quality energy, but they are transitory. Heat is the lowest form of energy. That means that you can convert high quality energy into an equal amount of low quality energy, but you will need more low quality energy like heat to get a lessor amount of high quality energy. The second law also says that high quality energy will eventually degrade into low quality energy, but low quality energy will never turn into high quality energy on its own. A hot plate will heat a pot of hot beverage, but putting a pot of hot beverage on top of a hot plate will never cause an electrical current to exist. Those are examples of the second law of thermodynamics. No one knows why it works that way, but it has never been proven to fail with respect to those principles.

  9. Nov 24, 2012 #8
    You may want to read about a thought experiment called Maxwell's Demon. Don't read about it on Wikipedia though. Might be confusing.

    In the case of a car, chemical energy is converted many types of energy. Let's say that you only measured the resulting kinetic energy and this amount of energy is lower (less Joules) than the chemical energy you started with (the energy within the bonds of the hydrocarbons in the fuel in the car.)

    Therefore we can assume that 'lost energy' ended up as sound from the friction of the moving parts or as warmth (trying not to use the word 'heat'!) on the tires or in the engine somewhere.

    If someone refers to energy as 'burned' or 'used' they mean that it is no longer able to do useful work in the machine.

    A machine cannot be 100 percent efficient due to the 2nd Law of Thermodynamics and this has to do with entropy as well. Einstein said this is the only scientific law which will stand the test of time and that it is the most important law in science.
  10. Nov 24, 2012 #9
    thanks people, so if you lose part of energy like your total became 92 per cent for example and in the case of eletric car you can upgrade the energy of the car to like 95 or 96 per cent but you never reach the 100 per cent right??
  11. Nov 24, 2012 #10


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    Never 100% - correct. You can always do 'a bit better' but it gets progressively harder and harder (like approaching absolute Zero or the speed of light). This problem can be addressed just theoretically or, in the real world, where it's practice that counts. It's analogous to keeping your home warm cheaply. First shut all the doors and windows, then draft-proof them, then lag the rood space, double glazing, cavity wall insulation etc. etc..... progressively more expensive measures
    You reach a stage when the pay-back time, for a particular improvement, is greater than the cost of borrowing money for the job.
    Ultra-efficient machines tend to be very expensive - and then there's always the energy of manufacturing to take into account. Already, a motor car takes about as much energy to produce as it actually consumes as fuel in its lifetime.
  12. Nov 24, 2012 #11


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    Right. It is just an engineering issue to get closer to 100%, but you cannot reach (or even exceed) that.
  13. Nov 25, 2012 #12
    Important life lesson which I think is relevant here - anyone selling perpetual motion or an ever-lasting engine (which is what the "100% car" would be), is a fraud (or a crackpot who is just nuts, rather than malicious)

    If it were possible you could use the energy of the wheels to recharge the battery and at 100% efficiency your electric car would never run out of power, but it's not possible.
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