The Cosmic Classroom - Proton Proton Chain


Uploaded by vmargoniner on 09.10.2009

Transcript:
So, hi. Welcome back to
the Cosmic Classroom. Now we are going to talk
about the Proton-Proton Chain. Lots of confusion about
the Proton-Proton Chain. Let's see if I can
summarize it for you. Alright?
So the Proton-Proton Chain is one of the ways by which
we can fuse atoms of Hydrogen, which are simply Protons,
into one atom of Helium. So we'll start with four
atoms, four Protons right there, brought my Protons.
1-2-3-4 Right?
And they will combine to form, you know, at the end
of all of this a lot of stuff is going to happen in the,in
between, but they will at the end become an atom of
Helium, which has you know two Protons and two Neutrons.
Now for fusion to, to occur remember that
these particles need to be moving really fast
because the Protons are all, are both positively charged.
So if they get close to each other they will end up
repelling and move away. So they need to be
traveling super fast to really get close to each other.
That's why fusion only happens when you have
very high temperatures and very high density so
that the Protons are smashing into one another.
Now let's, let's consider first two Protons.
What happened to two Protons. Because what happens
is symmetric those two Protons will combine
and those will combine. So let's talk about
those first, let's start with these two Protons.
The first thing that happens to these two Protons is they
combine and then became, they become something
that's called a Deuterium. It's a heavy atom of Hydrogen.
it still is an atom of Hydrogen because it only has
one Proton, but it's a heavy Hydrogen atom.
So this is Deuterium and these are the two Protons.
Now something is missing from this equation.
Right? Because you can see
that in here we have two charges, two positive
charges, and here we only have one positive charge.
So something has happened. Actually more than you
can tell has happened. One thing that has
happened is that this charge must be somewhere.
The charge goes, becomes something
that's called a Positron. [oops]
A Positron is an Electron with a plus in
there, that's a Positron. A Positron is the anti-
particle of an Electron. It's an Electron, but instead
of being negative it's positive. Alright?
So that's one thing. What also comes out
from this reaction is a Neutrino comes
out of that, that's something else you haven't heard
about until, til to, til today. So that's the Electron
Neutrino there's, there's more than one kind of
Neutrino is an Electron Neutrino that it's created,
and there's also some energy Right?
Some energy that are going to be Gamma Rays
that will be created. So for a Proton to become
a Neutron it needs to get rid of some charge and it
also ejects a Neutrino and some energy.
To really go from four Protons to one atom
of Helium,there are different ways different
paths to be taken. What are, what I really
think it's important in, is to understand that
the two were combined the other two will
combine and then at some point they will
combine together and will become this atom of Helium.
So this happens twice here so because it happens
twice they'll actually have, every time one atom of
Helium is created they'll actually have two Positrons,
two Gamma Rays, and two Electron Neutrinos.
And what goes wrong right here in the middle
is really not that important because there're different
ways for this to happen anyway. So the book tells you one
way,which happens frequently, but there are many ways.
So for everything to happen, this is going
to repeat and then we are going to have
two of each particles. This is the Proton-Proton Chain
now there's something very interesting here, happening here
there're two things very interesting. Right?
First thing is you never knew that the Proton can
become a Neutron did you. A Proton can to become a
Neutron plus, Positron in fact a Neutron left alone decays
into a Proton plus an Electron. So this is something that's,
that's, that is strange. The other thing that's funny
is that let's say you get these four Protons here
and got this atom of Helium and let's put them in a
scale, there's a scale. It turns out that this
combination of four Protons is heavier than here.
Ok? So it has more mass,these
will be heavier than those. Now think about it, the
mass of the Neutron is approximately equal the
mass of the Proton, in fact if you go look in
your old chemistry book you'll find out that the
mass of the Neutron is slightly greater than the
mass of the Proton. How is it possible then
that the, the Helium is less massive than the
four Protons together. It is, is like saying that
you had four bananas and then you divided
two bananas in two bananas and then suddenly
the mass doesn't add up. You should add up,it doesn't
add up, what's going on here? Because you hear
everywhere everything, what's happening is that
the energy that's coming from that energy
emitted from the sun is generated by nuclear fusion,
because this is lighter than this, this has less mass
matter than that and the difference in mass, mass
of the Helium minus the mass of four Protons is
transformed in energy. So the energy is the difference
in mass is mass of the Helium, minus four times the mass
of the Protons, times C squared. But how is it possible?
Now you the need to start thinking about
what does mass mean? What is mass?
Mass is how hard it is to put something into motion.
If something, if it's hard to move something it has
a lot of mass, if it's easy to move something it
doesn't have much mass. It turns out that because
of the way this atom becomes bound together
it's easier to push one atom of Helium than it is to
push four atoms of Proton. Is the same material, but
once they became, they came together it became
easier to make it move. That's why we say that
it has a smaller mass. Is the same particles, but
those are bound together. Those are moving as one
or those are moving as four. So, I hope you find this as
interesting and I, as I do. I think this is fascinating.
Four things that should be exactly the same, they are
not and it then we're talking, we're talking about
Nuclear fusion and it, it all comes back to Newton's
second law F = M A. What is M?
Right? M is how hard it is
to push on something. I find it fascinating.
I hope you can share that with me.
And I'll see you next time.