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Author Topic: Why don't the protons in a nucleus fly apart?  (Read 4087 times)

chris

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Why don't the protons in a nucleus fly apart?
« on: 07/01/2010 04:24:09 »
Since like charges repel one another, what keeps the protons in a nucleus bound up together? Why don't they fly apart?

lightarrow

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Why don't the protons in a nucleus fly apart?
« Reply #1 on: 07/01/2010 18:45:57 »
Since like charges repel one another, what keeps the protons in a nucleus bound up together? Why don't they fly apart?
Glue.
Well, not exactly but...quite  :) Actually the call them "gluons"  :)

Infact it's for the reason you notice, that they called this kind of force "strong force": it have to counteract the extremely high (at those distances) coulombian repulsion.
Strong force acts between quarks (the nucleons' constituents) and is mediated by massless particles (class: bosons) called gluons.

(But you already knew it, come on...!  ;))
« Last Edit: 07/01/2010 18:58:44 by lightarrow »

chris

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Why don't the protons in a nucleus fly apart?
« Reply #2 on: 07/01/2010 19:33:28 »
I'm tempted to quote Dr McCoy from Star Trek here and say something like "I'm a doctor dammit, not a particle physicist..."

Actually, I must admit that I posted this thread because it was something asked by someone on the radio; whenever interesting questions come in on the radio I always check the forum to see if it's something we've discussed in detail. If we haven't then I post on it, reasoning that others probably are wondering the same thing as the original questioner. In the process I lso inevitably learn something.

In this instance, I am also not as "across" atomic structure and particle physics as I'd like to be and so a lesson on this subject would be appreciated.

Do we know how the strong force is created? And if there's a strong force, is there also a "weak" force?

Chris

JP

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Why don't the protons in a nucleus fly apart?
« Reply #3 on: 07/01/2010 20:00:14 »
Weak force: http://en.wikipedia.org/wiki/Weak_interaction

One of the key differences between the forces we see on a macroscopic scale (electromagnetism and gravity) and those which act on a much smaller scale (weak and strong forces) are that the macroscopic forces have long range and die off as 1/r^2.  The other forces have a much more complicated structure, but essentially are only strong over very small scales, so that you don't see them much outside of small-scale interactions.

lightarrow

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Why don't the protons in a nucleus fly apart?
« Reply #4 on: 08/01/2010 20:14:24 »
Yes. Infact it's exactly the fact strong force acts only at a very short range (~ the atomic nucleus) that, if you can break a (big) nucleus in two parts, the coulombian repulsion between the two liberates the enormous energy we see in an atomic bomb explosion (well, it's a little bit more complicated but the essence is this one).

The curios thing is that, on the converse, if you can make two (appropriate) little nuclei come together, winning the enormous coulombian repulsion, the energy liberated because of the strong force is even higher (thermonuclear bomb).

World is really strange! ;)
« Last Edit: 08/01/2010 20:16:09 by lightarrow »

Farsight

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Why don't the protons in a nucleus fly apart?
« Reply #5 on: 09/01/2010 18:16:32 »
Since like charges repel one another, what keeps the protons in a nucleus bound up together? Why don't they fly apart?
Sorry to nitpick with lightarrow, but the strong force keeps the quarks of a proton together. The residual strong force keeps the protons together in a nucleus.

The latter is also known as the nuclear force, see http://en.wikipedia.org/wiki/Nuclear_force. You can dig around and look up pion exchange or maybe some of the guys here can give you something succinct, but it's important to note that you need neutrons to keep the protons together. For myself when explaining it to the layman, I use something like this:

Think of protons like girls on a dancefloor with their hands on their hips, and neutrons as boys with their arms free to link the girls. The number of protons in a nucleus determines the element, and the number of neutrons determines the isotope. Hydrogen is usually like a girl on her own. But there's an isotope of hydrogen called deuterium that's like a girl and a boy. Helium is usually like two girls linked by one boy. But there's an isotope of helium called helium-4 that's like two girls linked by two boys.

 

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