1. The Strong Nuclear Force
Understand the need for a strong force to bind nucleus
Know its approximate magnitude
Sketch the graph

2. Consider the following
A helium nucleus consists of 2 protons and 2 neutrons. The spacing between particles is not very great ~ 8 x m. At these separations, what forces are in play?

3. Electrostatic Repulsion
Consider the force between 2 protons at the separation given. F= q1q2/4ε0 pi r2 F = ________ N

4. Gravitational Attraction
Consider the force between 2 protons at the separation given. F= Gm1m2/r2 F = ________ N

5. Conclusions
By all rights the nucleus should ping apart due to the repulsive force being dominant. That is not what happens. The force we are interested in now is called....

6. The Strong Nuclear Force
And it has the following properties:
Acts between nucleons (protons and neutrons.)
Is very short ranged.
Is repulsive at very short distances to prevent the nucleus collapsing.
Is by far the strongest force relatively (~1039 times stronger than gravity!)

7. The bad news  The strong nuclear force is not that well understood.
Simple mathematical treatments like Newton’s law of gravitation won’t do and quantum mechanics governs the SNF.
The force is mediated by particles called gluons (!)

8. The strong force binds nucleons together.

10. The strong force is: Repulsive at short ranges (<0.5 fm)
Attractive at ranges up to about 3.0 fm.
Beyond this it becomes negligible.

11. Nuclei are generally very stable.
Inside the nucleus the resultant force on the nucleons is zero.
Nucleons near the edge of the nucleus are held firmly in place as a small increase in their distance produces a large resultant force on them.

12. Protons and neutrons
The equilibrium distance between two protons is very similar to the equilibrium distance between two neutrons.
The separation of nucleons within a nucleus is approximately independent of the size of the nucleus.

13. Role of neutrons
Help to ‘dilute’ the strong repulsive forces between the protons.
Without neutrons the strong force would have to be stronger.
Light elements tend to have n ≈ p
Heavier elements tend to have n > p

14. Nuclear density Stays approximately constant with size.
This is because separation is roughly constant so more mass added increases the volume correspondingly.

15. Estimation Helium contains two protons and 2 neutrons.
The diameter is of the order 100 pm.
By modelling as spherical, calculate the nuclear density.