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B The Play of Gravity

  1. Feb 13, 2018 #1
    Hi everyone! I'm here for a little conveyance towards a question that is disturbing me for days.

    Just suppose, you and I design a hydroelectric generator that provokes energy from the downflow of precipitating rain. These falling drops of rain surely have originated from the vapours of the ocean and no doubt it is the Sun who has provided the Potential energy to the vapours to go up. These vapours then formed clouds which thereafter changed into rain droplets. These rain droplets then made their way down and of course, it is the invisible Gravity which brought them down by providing them Kinetic energy.

    So, the condition arises is if the accomplished energy of our generator is the outcome of the Kinetic energy provided by Gravity, where has the energy of the Sun gone; and in contrast, if the potential energy of the Sun rays transformed into kinetic energy then why is Gravity required?

    My teacher answered, "Gravity is just a force, not a donor of Kinetic energy." Then the question that can be further asked is — "A heavy ball placed on a thin plastic sheet conclusively breaks the sheet after some while. For that to happen there must have been a constant supplier of Kinetic Energy for all the plastic bonds to break. If Gravity is not the perfect provider then what it is?"
  2. jcsd
  3. Feb 13, 2018 #2


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    Into whatever the generator powers.

    It isn't in general. See solar cells.

    It is a conservative field.
  4. Feb 13, 2018 #3
    I would really appreciate if you can provide me with the equations for what you are replying!!!
  5. Feb 13, 2018 #4


    Staff: Mentor

    You can start at the sun:
    1) The photons from the sun's nuclear processes travels to Earth as they exit the sun's gravity well they lose some small amount of energy.
    2) The Sun's photons travel to Earth and agitate the water molecules by colliding with them causing them to move faster (ie heat the water)
    3) The water molecules escape from the water surface rising into the air and continue to collide with photons and other molecules
    4) As the molecules rise, they gain potential energy and they begin to coalesce into water droplets and then fall to Earth
    5) As they fall, they convert their potential energy back into kinetic energy.
    6) As they fall on your generator they cause it to rotate and to thus generate electricity
    7) During this conversion the generator dissipates some of the energy into heat and the rest into electrical energy to power devices.

    You have to remember the potential energy sources are like stretched springs, gravity converts kinetic energy of movement into potential energy of no movement. Same goes for a capacitor, storing electric charge up until released by making a circuit for the electric charge to move.
  6. Feb 13, 2018 #5


    Staff: Mentor

    Okay, but you didn't ask with equations. You asked with concepts and so that's how we answered.
  7. Feb 13, 2018 #6
    Firstly, they are reluctant to lose energy. Photons have no mass, thus are unaffected by Gravity

    Concluding this, I asked Why is Gravity required? Do you want to show me Gravity as an Energy transformer?
  8. Feb 13, 2018 #7


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    This is not correct. Despite not having mass, photons are affected by gravity in two ways:

    1. A photon moving outward from a gravitating object is red-shifted and similarly, a photon moving inward toward a gravitating object is blue-shifted.
    2. A photon passing a gravitating object is deflected due to the curvature of space-time.

    A car rolling down a hill and crashing into a building is an example of gravity as an energy transformer.

    If you mean to ask: "why is gravity required to convert solar energy into electricity" then that has already been answered by @A.T. It is not required.
  9. Feb 13, 2018 #8


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    Some equations...

    The energy needed to raise the water vapour is mgh where m is the mass of water, g the acceleration due to gravity an h Is the height. This energy is provided by the sun.

    Once the water vapour is up there it has potential energy also equal to mgh.

    As the water droplet falls the potential energy is converted to kinetic energy. For the moment we will assume that all of the potential energy is converted to kinetic energy so the kinetic energy the water gains as it falls is also equal to mgh.

    So in summary the energy in the falling water is exactly the same as the energy provided by the sun. In practice it's less because some energy is lost to friction with the air as the water falls etc.

    So gravity was only a temporary store for the sun's energy, not a source of energy.
  10. Feb 13, 2018 #9


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    You might also care to Google conservative forces. Gravity is a conservative force so if you move something around a closed path the net work done is zero. Eg the energy needed to raise something up is exactly the same as the energy you get back on the way down. So the net sum is zero.
  11. Feb 13, 2018 #10
    That's what I needed. I can now draw a whole conclusion.

    Gravity is a conservative field. The sun was the donor of energy and its rays transformed water into vapour. If I see this whole along with the concept of Latent Heat, everything becomes clear. The change in the height of vapour and the transportation of energy was just The Play of Gravity.

    Thanks CWatters, jbriggs444, jedishrfu, and AT
    Last edited: Feb 13, 2018
  12. Feb 13, 2018 #11


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    This link seems to be pretty comprehensive, if you want a load of equations.
  13. Feb 13, 2018 #12


    Staff: Mentor

    You started with:
    Gravity is therefore required because you decided to discuss the operation of a device which is specifically designed to extract energy from a fluid due to a change in the gravitational PE of the fluid.

    Nature didn’t force us to use a hydroelectric generator, you chose it.
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