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Something basic missing

Something basic missing - Physics Forum

Something basic missing - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 09-23-2005, 04:44 AM
Zarkovic
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Default Something basic missing



Hi guys
This is something that's very simple, yet I don't get it because I guess I
am missing some basic info on nuclear fusion.
Ok. In our Sun, as in all the stars, 4 protons get fused together into the
helium nucleus, containing 2 protons and 2 neutrons.
Now if this is correct, and protons are a bit of less mass than neutrons,
how can that be?
How can you have 4 fused p+ = 2 p+ + 2n + radiation that we observe, if the
neutrons are higher in mass than protons. It just does not equate and
something is missing. Please help, this is bugging the hell out of me.
Thanks and I have not had time to consult any undergrad physics books yet on
this.


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  #2  
Old 09-23-2005, 04:54 AM
N:dlzc D:aol T:com \(dlzc\)
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Default Something basic missing

Dear Zarkovic:

"Zarkovic" <[Only registered users see links. ]> wrote in message
news:XKLYe.541790$s54.374143@pd7tw2no...

It actually 3 protons get fused to 3Li which is unstable, and
decays by "ingesting" an electron (or emitting a positron) into
3He. Additional protons may be involved, but neutrinos and
antineutrinos are certainly involved (which is what enables
positron emission/electron).

David A. Smith


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  #3  
Old 09-23-2005, 01:01 PM
tadchem
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Default Something basic missing


N:dlzc D:aol T:com (dlzc) wrote:

David and Zarkovic,

Check this page out.
[Only registered users see links. ]
Scroll down the *huge* index in the right side window and click on
"proton-proton fusion".

A lithium nucleus with *no* neutrons [Li-3] would require extremely
high temperatures and pressures to form. Due to electrostatic repulsion
and the instability of the helium-2 'isotope' which rapidly emits a
positron and a neutrino to become a deuteron, the formation of [Li-3]
would require a highly improbable three-body interaction - thus
*extremely* high temperatures and pressures, probably only available
during the early universe or on the surface of a neutron star.

Enough energy is put into the system of a proton-proton fusion by
overcoming coulombic repulsion to not only account for the mass
increase of a neutron over a proton but also to account for the mass
and energy of the positron and the neutrino.

Tom Davidson
Richmond, VA

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  #4  
Old 09-23-2005, 01:18 PM
N:dlzc D:aol T:com \(dlzc\)
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Default Something basic missing

Dear tadchem:

"tadchem" <[Only registered users see links. ]> wrote in message
news:1127480465.895828.111270@z14g2000cwz.googlegr oups.com...

Good! Somebody that knows something! ;>)

So it seems like two protons combine to form [He-2], which decays
via EC/PE (electron capture/positron emission) to deuterium, then
*later*, another proton can be added. (And another, and
another.)

David A. Smith


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  #5  
Old 09-23-2005, 02:26 PM
Clemens W
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Default Something basic missing

Zarkovic wrote:

Wikipedia is your friend.


Not directly. See proton-proton chain reaction:

[Only registered users see links. ]


You didn't take the mass defect of the nucleus into account. See
binding energy:

[Only registered users see links. ]


HTH. HAND. Good luck,

A. Friend

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  #6  
Old 09-23-2005, 11:22 PM
Steven Gray
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Default Something basic missing

"Zarkovic" <[Only registered users see links. ]> wrote in
news:XKLYe.541790$s54.374143@pd7tw2no:


What you're missing is that the masses of protons and neutrons bound into
nuclei is not the same as their masses when they are unbound. Note, for
example, that the atomic weight of helium is less than four times the
atomic weight of hydrogen. Some of the "extra mass" in the four protons
that got fused together accounts for the energy difference between protons
and neutrons, as you've pointed out. The rest is the energy that we get
out of the reaction.

--
Steve Gray
[Only registered users see links. ]
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  #7  
Old 09-24-2005, 07:07 AM
Zarkovic
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Default Something basic missing

Thank you, I think I got most if it
It was just that it didn't add up, as you all read my post before. Thank you
for replying, I checked the sites and so forth and read the useful comments.
"Steven Gray" <[Only registered users see links. ].comSPAM> wrote in message
news:Xns96DAC49E46DDCsgray2cflrrcom@65.32.5.121...


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