Understanding the Repulsive Strong Force and its Role in Nucleons and Quarks

[JZR]

Hi,

I have been taught at school that the strong force is responsible for holding the nucleons together in the nucleus but becomes repulsive below about 1.5 femtometres. I now read that the strong force is also responsible for holding the quarks together inside the nucleons, although the diameter of a proton or neutron is less than 1.5fm so surely the strong force should be repulsive not attractive?

Can someone please explain what is really happening?

 

[granpa]

I see this is your first post.
Until you establish yourself around here don't be surprised if you don't get many responses.
You will get more responses once you establish that you are really here to learn and not just stir up trouble and controversy.
until then you would probably be better off asking more conventional questions first.

 

[mathman]

I have been taught at school that the strong force is responsible for holding the nucleons together in the nucleus but becomes repulsive below about 1.5 femtometres.

This doesn't sound right. It is weaker at very short ranges, but not repulsive.

 

[humanino]

The confusion is between the fundamental strong force between quarks and the effective strong force between hadrons.

 

[JZR]

What is the difference between the fundamental strong force and effective strong force?

 

[daschaich]

To vastly oversimplify, the fundamental strong force is the force gluing the quarks together into neutrons and protons. The effective strong force is a remnant of this interaction that can bind neutrons and protons together into nuclei. I've seen the effective strong force compared to the van der Waals forces between molecules, if that helps you visualize it. Both protons and neutrons are uncharged (neutral) under the fundamental strong force.

Wikipedia calls the "fundamental strong force" the "strong interaction" and the "effective strong force" the "nuclear force":
http://en.wikipedia.org/wiki/Strong_interaction
http://en.wikipedia.org/wiki/Nuclear_force

PS. bcrowell wrote a nice explanation of the force between nucleons here.

 

[JZR]

Thank you, I think i will have to wait until i can do a degree in physics to fully comprehend it all

Is there any truth in what I was taught at school about the strong force being repulsive below 1.5fm? I think the reason our teachers gave us was that it was to prevent the nucleus collapsing in on itself.

 

[daschaich]

I was too slow in adding to my previous post a link to a nice explanation of this issue by bcrowell. To summarize it very briefly, models of the "effective nuclear force" that include short-range repulsion tend to fit the data better than those that are purely attractive. However, the fact that nuclei don't collapse does not imply that there must be such a "repulsive core".

 

[Drakkith]

I was too slow in adding to my previous post a link to a nice explanation of this issue by bcrowell. To summarize it very briefly, models of the "effective nuclear force" that include short-range repulsion tend to fit the data better than those that are purely attractive. However, the fact that nuclei don't collapse does not imply that there must be such a "repulsive core".

Yes. Here's the last part of that post for those that don't want to read.

"Relatively sophisticated models of the nucleon-nucleon interaction do usually include repulsion under certain circumstances, e.g., there may be a "hard core" in the potential at short ranges. The fact that all such models seem to do a better job of reproducing certain data when the repulsive features are turned on suggests that this repulsive feature is model-independent. "

What this says to me, is that repulsive features in models SOMETIMES work better than those without. And vice versa. I guess it's still up in the air.