Heat and pressure, is there really a difference?

[AluminumStudios]

I've been reading about astronomy and bit and watching a few documentaries and I heard two things that at first sounded like they may have been different reasons for fusion in stars, but after thinking I'm wondering if they aren't different views of the same thing ...

Stars form when large clouds of gas condense due to gravity. As the gas is condensed into a smaller and small volume it's temperature increases. Eventually the forces slamming protons into each other are greater than the electromagnetic repulsion force pushing them apart and the strong nuclear force is able to make them stick thus fusing hydrogen into helium and a star ignites.

My question is - what is fundamentally responsible for the force slamming the protons together hard enough to enable them to fuse? Is it that their heat is so great that their motion has enough energy to cause this? Or is it pressure of so much mass being squeezed by gravity?

At this level of protons slamming into each other, can heat and pressure be viewed as one and the same?

 

[silverpig]

Heat is the transport of thermal energy. Thermal energy is kinetic energy of molecules. Pressure if force per unit area.

 

[CycloWizard]

Temperature and pressure are related by equations of state (e.g. the ideal gas 'law', really the ideal gas equation of state, where PV=RT). 'Heat', as silverpig stated, is one mechanism for transferring energy from one place to another when a temperature gradient exists. So, in a roundabout way, heat is sometimes related to pressure via temperature, but they are not the same thing.

What I'll say below comes from the kinetic theory of gases and may or may not apply to the formation of stars, though the physicists on this forum will be able to tell you if it does or not. The reason temperature and pressure are related is because kinetic energy is heavily temperature-dependent, as the molecules move at a higher rate when they have more available energy. Increased pressure is a result of this kinetic energy increase, since the molecules impact their container more frequently. One example of how you might observe this experimentally is to fill a balloon with air at room temperature. Then, put the balloon in an oven. The oven transfers energy to the balloon via heat transfer because the oven is hotter than the balloon. This increases the energy in the balloon, which in turn increases the temperature within the balloon. This will also cause a rise in balloon pressure such that the balloon would expand.

 

[Circlenaut]

I think it's due to temperature. At those temps the vibrating atoms are more likely to overcome the electromagnetic force due to their velocity and thus come close enough to be captured by the strong force. So pressure leads to increase temperature which leads to fusion.

 

[msparish]

This is off the top of my head, so may not be totally accurate...you may want to take it with a grain of salt.

Anyway, I would say that both the pressure and heat are necessary to fuel the chain reaction. The heat is actually what allows the atoms to fuse, but the pressure from gravity is what causes them to be in the same vicinity to begin with. To fuse, the atoms must not only have sufficient energy, but effectively collide with one another.

As an interesting side note, the temperature (and as a result the kinetic energy) of an atom of hydrogen in the interior of the sun is not sufficient to overcome the Coulomb barrier (as you bring the protons closer and closer, they expercience an ever stronger repulsive force due to their charge). However, not every atom has the same speed (as given by the Maxwell-Boltzmann distribution). Only the atoms with higher than average energies are able to fuse...however, the released energy is absorbed by the surrounding atoms so as to keep the temperature constant. As a result, there is a constant supply of atoms that are capable of undergoing fusion.

 

[MrDudeMan]

gravity has very minimal effects at the level you are talking about. fusion is usually deuterium and tritium atoms slamming into each other, which, when you really get down to it, combine because the gluon field between the quarks will interact and overlap, which will cause a neutron to change to a proton and emit a W-boson, which will decay into an electron and its anti-neutrino. the electron anti-neutrino actually carries away most of the energy from the reaction even though it has (virtaually) no mass and no charge...this is what people are referring to when they say the reaction radiates energy.

to explain why this happens, well im not entirely sure, but here is my best guess: at high thermal energies, the atom is vibrating like crazy and ejecting more photons and electrons from interactions with other atoms. at a high enough energy, a subatomic (or fundamental) particle could strike the nucleus and eject a neutron, which will then decay as i mentioned earlier. for fusion, it is the two atoms i mentioned in the last paragraph that "fuse" into a beta particle (helium nucleus). if you add up all of the mass without taking into account the electron anti-neutrino (which is wrong but most people do it this way to do the calculations), the mass doesnt add up right. before the collision, you have 2 protons and 3 neutrons, and after the collision you hvae 2 protons, 2 neutrons, , 1 electron and 1 electron anti-neutrino (which carries away mass because of its momentum but it has almost zero mass itself).

as an aside, there are several types of neutrinos. you have a tau, electron, and muon, plus their superpartners. the mass of a neutrino is around 0.5 eV compared to 0.511 MeV for the electron. i know this didnt have a ton to do with your question but i thought you might be interested.

 

[silverpig]

IIRC the neutron doesn't decay into a W. One of the d quarks in a neutron turns into a u quark while kicking out a W boson. The W then goes on to decay into an electron and an electron's anti-neutrino.

 

[MrDudeMan]

maybe i phrased it wrong...one of the downs (-1/3) changes to an up (+2/3) so a W (-1) is carried away. the neutron is now a proton and the W is about to split into the electron and electron anti-neutrino. so yeah, you were right.

 

[DrZDO]

How much pressure would be required to overcome the EM force and fuse Deuterium atoms simply through compression?

 

[imported_Seer]

Originally posted by: DrZDO
How much pressure would be required to overcome the EM force and fuse Deuterium atoms simply through compression?

Research Tokamak reactors, I think you find have your answer.

 

[f95toli]

Originally posted by: Seer

Originally posted by: DrZDO
How much pressure would be required to overcome the EM force and fuse Deuterium atoms simply through compression?

Research Tokamak reactors, I think you find have your answer.

I don't think so. Fusion reactors (including Tokamaks) work by increasing the temperature of a deuterium gas, the pressure is sort of a "secondary" effect.

I am not sure it is even theoretically possible to fusion deuterium through pressure. Assming you could increse the pressure adiabatically you would first reach a point where you have a deuterium crystal (D2, which is a quantum molecular solid) which happens at around 100 GPa. However, in order to find out the pressure needed for fusion you would need a full quantum mechanical calculation which takes into account not only EM but also the strong force, it is VERY complicated and I can think of a few reasons why it might not even be possible. This is complettely different situation from what you would find in a reactor (where it is the kinetic energy, essentially the speed, of the ions which is important).

 

[DrZDO]

Originally posted by: f95toli

Originally posted by: Seer

Originally posted by: DrZDO
How much pressure would be required to overcome the EM force and fuse Deuterium atoms simply through compression?

Research Tokamak reactors, I think you find have your answer.

I don't think so. Fusion reactors (including Tokamaks) work by increasing the temperature of a deuterium gas, the pressure is sort of a "secondary" effect.

I am not sure it is even theoretically possible to fusion deuterium through pressure. Assming you could increse the pressure adiabatically you would first reach a point where you have a deuterium crystal (D2, which is a quantum molecular solid) which happens at around 100 GPa. However, in order to find out the pressure needed for fusion you would need a full quantum mechanical calculation which takes into account not only EM but also the strong force, it is VERY complicated and I can think of a few reasons why it might not even be possible. This is complettely different situation from what you would find in a reactor (where it is the kinetic energy, essentially the speed, of the ions which is important).

I was just reading an old Asimov science book, which stated that at about 750,000 tons per square inch (or 10342 GPa if my calculations are correct), electron fields break down and nuclei approach one another, becoming degenerate matter. Do the atoms remain separate though? And if so, what kind of catalyst could result in fusion of degenerate matter?

I should find easier topics to think about...

 

[nismotigerwvu]

+1 for T being a bigger factor....as stated before T is realtyed to Kinetic enegery,a nd you have to have the particles moving quite fast to overcome the repulsive forces of like charges