4 ms·

The sun's fusion is powered by the gravitational attraction of ~10e30 Kg of matter. That is not even remotely an option on Earth, so there is no evidence of fe

by Retra 8y ago

The sun's fusion is powered by the gravitational attraction of ~10e30 Kg of matter. That is not even remotely an option on Earth, so there is no evidence of feasibility about the energy produced by the sun, aside from the fact that it demonstrates that fusion as a concept makes sense.

dekken_ 8y ago
I was asked recently if "I believed in fusion" my response - "I don't think it will cost effective" and what you have posted is basically that reason
dogma1138 8y ago
Does it tho? Star fusion produces the exact amount of pressure needed to counter the gravitational attraction which is why they don’t collapse or blow up while the fusion reaction is stable, when it’s not the result isn’t pretty. So in essence stars aren’t energy possitvie (or if so only slightly relative to the energy of the reaction) they just convert one type of energy into another.
- pfdietz 8y ago
  This argument makes no sense at all. "Pressure" and "power" don't even have the same units. They are entirely different things. Let's try to rephrase things in a way that makes physical sense. There is energy embodied in the Sun. This is the kinetic energy of the electrons and ions that make it up, the energy of the thermal radiation mixed with this plasma, and the (negative) gravitational potential energy. As the sun formed, energy was liberated by gravitational collapse. This heated up the Sun's material and got things going. We know (by the wonderful Virial Theorem) that the negative gravitational binding energy of the Sun is ~2 times the internal energy of its particles and radiation. We can then ask: how does this energy compare to the rate that the Sun is radiating energy? This ratio would give a timescale over which the Sun would cool off, if there was no energy input. This ratio is on the order of ten million years. But the sun is hundreds of times older than this. So, we can conclude the energy yield of the sun, the ratio of fusion energy output to the initial gravitation energy input that heated it up, is pretty darned good, on the order of hundreds.
  - dogma1138 8y ago
    The pressure caused by the sun’s ongoing fusion reaction is directly opposite to the force of gravity which constantly wants to contract it, these are exactly equal and opposite this isn’t an argument this is the basis for how stellar fusion works. ∆P · A = −GM(r)m This is the formula. The sun isn’t powered by the initial gravitational force that heated it up but by the continuous gravitational attraction it experiences all the time. A star is constantly in the balance between the forces of gravity and the outbound pressure from the fusion reaction this balance is what keeps it going. http://large.stanford.edu/courses/2011/ph241/olson1/ http://large.stanford.edu/courses/2011/ph241/olson1/ http://www.ucolick.org/~woosley/lectures_fall2012/lecture12.12.pdf http://www.ucolick.org/~woosley/lectures_fall2012/lecture12.... So unless you somehow found a way to turn off gravity the sun and every other star in the universe experiences a constant “input” of energy due to gravity. So the only thing that doesn’t makes sense here is your attempt at physics, hydrostatic equilibrium is what keeps stars going and it’s between the pressure of fusion (or degenerate pressure once fusion stops) and gravity.
    - pfdietz 8y ago
      "The sun isn’t powered by the initial gravitational force that heated it up but by the continuous gravitational attraction it experiences all the time." This is nonsensical bullshit. Pressure in a static situation does not work, and produces no power. Maybe you should take a course in freshman physics?
      - dogma1138 8y ago
        The fusion reaction of a main sequence star is a balancing act between the outward pressure of the fusion reaction (radiation and gas pressure primarily) and the inward force of gravity. Like this isn't a conjecture we have a very good understanding of how stars work, stars achieve a hydrostatic equilibrium, if they wouldn't they would either explode, collapse or oscillate, the reason why the fusion reaction doesn't cool down is because gravity is constantly pushes the material inwards if it wouldn't the star would expand until the fusion reaction stops and would cool down. The constant force of gravity is what constantly keeps the a star in a condition where fusion can happen, basically gravity is what is used to overcome the nuclear forces allowing protons to fuse. "Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity.[1] Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter will collapse to form pockets of higher density, typically creating a hierarchy of condensed structures such as clusters of galaxies, stellar groups, stars and planets. A star is born through the gradual gravitational collapse of a cloud of interstellar matter. The compression caused by the collapse raises the temperature until thermonuclear fusion occurs at the center of the star, at which point the collapse gradually comes to a halt as the outward thermal pressure balances the gravitational forces. The star then exists in a state of dynamic equilibrium. Once all its energy sources are exhausted, a star will again collapse until it reaches a new equilibrium state." https://en.wikipedia.org/wiki/Gravitational_collapse https://en.wikipedia.org/wiki/Gravitational_collapse https://en.wikipedia.org/wiki/Stellar_nucleosynthesis https://en.wikipedia.org/wiki/Stellar_nucleosynthesis This is what won Bethe the Nobel prize. Now in fusion experiments we supplement gravity with magnetic fields and pressure from high temperature plasma, but the same thing stands we need to apply constant pressure to allow for protons to fuse into neutrons in order to have hydrogen > helium fusion, not only that but we need to apply even more pressure when the reaction actually happens to keep it because now you have radiation pressure that wants to expand your plasma which would stop all fusion unless countered. This is exactly why I asked if we have any models which are actually energy positive. >Maybe you should take a course in freshman physics? Maybe you should?
mac01021 8y ago
Are you saying that calculations based on the established theory of nuclear physics are not admissible as evidence?
- Retra 8y ago
  Obviously not.
pmarreck 8y ago
> there is no evidence of feasibility about the energy produced by the sun Don't the most recent fusion experiments have almost the same energy return as input? That is "no evidence"?
- Retra 8y ago
  What I said was that there is no evidence that fusion on Earth will work that can be derived from simply looking up at the sun. Context matters.

by Retra 8y ago

dekken_ 8y ago

I was asked recently if "I believed in fusion" my response - "I don't think it will cost effective" and what you have posted is basically that reason

dogma1138 8y ago

Does it tho? Star fusion produces the exact amount of pressure needed to counter the gravitational attraction which is why they don’t collapse or blow up while the fusion reaction is stable, when it’s not the result isn’t pretty. So in essence stars aren’t energy possitvie (or if so only slightly relative to the energy of the reaction) they just convert one type of energy into another.

pfdietz 8y ago
This argument makes no sense at all. "Pressure" and "power" don't even have the same units. They are entirely different things. Let's try to rephrase things in a way that makes physical sense. There is energy embodied in the Sun. This is the kinetic energy of the electrons and ions that make it up, the energy of the thermal radiation mixed with this plasma, and the (negative) gravitational potential energy. As the sun formed, energy was liberated by gravitational collapse. This heated up the Sun's material and got things going. We know (by the wonderful Virial Theorem) that the negative gravitational binding energy of the Sun is ~2 times the internal energy of its particles and radiation. We can then ask: how does this energy compare to the rate that the Sun is radiating energy? This ratio would give a timescale over which the Sun would cool off, if there was no energy input. This ratio is on the order of ten million years. But the sun is hundreds of times older than this. So, we can conclude the energy yield of the sun, the ratio of fusion energy output to the initial gravitation energy input that heated it up, is pretty darned good, on the order of hundreds.
- dogma1138 8y ago
  The pressure caused by the sun’s ongoing fusion reaction is directly opposite to the force of gravity which constantly wants to contract it, these are exactly equal and opposite this isn’t an argument this is the basis for how stellar fusion works. ∆P · A = −GM(r)m This is the formula. The sun isn’t powered by the initial gravitational force that heated it up but by the continuous gravitational attraction it experiences all the time. A star is constantly in the balance between the forces of gravity and the outbound pressure from the fusion reaction this balance is what keeps it going. http://large.stanford.edu/courses/2011/ph241/olson1/ http://large.stanford.edu/courses/2011/ph241/olson1/ http://www.ucolick.org/~woosley/lectures_fall2012/lecture12.12.pdf http://www.ucolick.org/~woosley/lectures_fall2012/lecture12.... So unless you somehow found a way to turn off gravity the sun and every other star in the universe experiences a constant “input” of energy due to gravity. So the only thing that doesn’t makes sense here is your attempt at physics, hydrostatic equilibrium is what keeps stars going and it’s between the pressure of fusion (or degenerate pressure once fusion stops) and gravity.
  - pfdietz 8y ago
    "The sun isn’t powered by the initial gravitational force that heated it up but by the continuous gravitational attraction it experiences all the time." This is nonsensical bullshit. Pressure in a static situation does not work, and produces no power. Maybe you should take a course in freshman physics?
    - dogma1138 8y ago
      The fusion reaction of a main sequence star is a balancing act between the outward pressure of the fusion reaction (radiation and gas pressure primarily) and the inward force of gravity. Like this isn't a conjecture we have a very good understanding of how stars work, stars achieve a hydrostatic equilibrium, if they wouldn't they would either explode, collapse or oscillate, the reason why the fusion reaction doesn't cool down is because gravity is constantly pushes the material inwards if it wouldn't the star would expand until the fusion reaction stops and would cool down. The constant force of gravity is what constantly keeps the a star in a condition where fusion can happen, basically gravity is what is used to overcome the nuclear forces allowing protons to fuse. "Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity.[1] Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter will collapse to form pockets of higher density, typically creating a hierarchy of condensed structures such as clusters of galaxies, stellar groups, stars and planets. A star is born through the gradual gravitational collapse of a cloud of interstellar matter. The compression caused by the collapse raises the temperature until thermonuclear fusion occurs at the center of the star, at which point the collapse gradually comes to a halt as the outward thermal pressure balances the gravitational forces. The star then exists in a state of dynamic equilibrium. Once all its energy sources are exhausted, a star will again collapse until it reaches a new equilibrium state." https://en.wikipedia.org/wiki/Gravitational_collapse https://en.wikipedia.org/wiki/Gravitational_collapse https://en.wikipedia.org/wiki/Stellar_nucleosynthesis https://en.wikipedia.org/wiki/Stellar_nucleosynthesis This is what won Bethe the Nobel prize. Now in fusion experiments we supplement gravity with magnetic fields and pressure from high temperature plasma, but the same thing stands we need to apply constant pressure to allow for protons to fuse into neutrons in order to have hydrogen > helium fusion, not only that but we need to apply even more pressure when the reaction actually happens to keep it because now you have radiation pressure that wants to expand your plasma which would stop all fusion unless countered. This is exactly why I asked if we have any models which are actually energy positive. >Maybe you should take a course in freshman physics? Maybe you should?

mac01021 8y ago

Are you saying that calculations based on the established theory of nuclear physics are not admissible as evidence?

Retra 8y ago
Obviously not.

pmarreck 8y ago

> there is no evidence of feasibility about the energy produced by the sun Don't the most recent fusion experiments have almost the same energy return as input? That is "no evidence"?

Retra 8y ago
What I said was that there is no evidence that fusion on Earth will work that can be derived from simply looking up at the sun. Context matters.