The Strong Nuclear Force Revision Notes | A Level AQA Physics

1. The Problem at the Heart of Every Nucleus

Picture a helium nucleus: two protons packed into a space roughly $10^{-15}$ m across. Both protons carry positive charge, and like charges repel — so the electrostatic (Coulomb) force between them is enormous at that range. By all rights, the nucleus should explode in an instant.

Yet nuclei are stable. Something must be overpowering that repulsion. That something is the strong nuclear force — one of the four fundamental forces of nature, and the one responsible for holding every nucleus together.

Understanding this force means understanding why nuclei exist at all. It also sets the stage for everything that follows in particle physics: radioactive decay, nuclear reactions, and the limits of nuclear stability.

2. What the Strong Nuclear Force Actually Does

The strong nuclear force has two critical properties that make it unlike gravity or electrostatic forces. First, it acts between any two nucleons — proton–proton, neutron–neutron, or proton–neutron — regardless of charge. Second, its behaviour changes dramatically depending on the separation between those nucleons.

Think of it like a very specialised spring. Pull two nucleons slightly apart and the spring pulls them back together. Push them too close and the spring pushes back. But move them more than a few femtometres apart and the spring goes completely slack — the force vanishes.

3. The Force at Different Separations

This is where the spec gets precise, and where examiners love to set questions. The strong nuclear force does not behave the same way at all distances — its character changes in three distinct regimes, all measured in femtometres (fm), where $1 \text{ fm} = 10^{-15} \text{ m}$.

• Separation greater than approximately 3 fm: the strong nuclear force is negligible — effectively zero. The force has no reach beyond this distance.
• Separation between approximately 0.5 fm and 3 fm: the strong nuclear force is attractive. This is the range over which it holds nucleons together and overcomes electrostatic repulsion.
• Separation less than approximately 0.5 fm: the strong nuclear force becomes repulsive. This prevents nucleons from being crushed into each other and gives the nucleus a stable, finite size.

The boundary between the repulsive and attractive regimes — where the strong nuclear force passes through zero — sits at roughly 0.5 fm. Nucleons in stable nuclei typically sit at separations closer to approximately 1 fm, where the attractive strong force is near its maximum strength.

Notice in the chart how the force is strongly repulsive at very small separations (below 0.5 fm), passes through zero at around 0.5 fm, reaches its maximum attraction somewhere between 1 fm and 2 fm, and then falls to zero beyond about 3 fm. This short-range nature is what makes the strong nuclear force so different from gravity or electrostatic forces, which both follow inverse-square laws and act over infinite distances.

4. Comparing the Strong Force with Electrostatic Repulsion

To appreciate why the strong nuclear force is so remarkable, compare it directly with the electrostatic (Coulomb) force it must overcome. Both forces act between protons, but they behave very differently with distance.

The charge-independence of the strong nuclear force is a key A-level point. Neutrons experience the strong force just as protons do, even though neutrons carry no charge. This is why adding neutrons to a nucleus can increase stability — more neutrons means more strong-force attraction without adding to the electrostatic repulsion.

5. Why the Short Range Matters for Nuclear Stability

The short range of the strong nuclear force has a profound consequence: each nucleon only interacts strongly with its immediate neighbours. In a small nucleus, every nucleon is close to every other nucleon, so the strong force is very effective. In a large nucleus, nucleons on opposite sides are too far apart to attract each other via the strong force — but the electrostatic repulsion between all the protons still acts across the whole nucleus.

This is why very large nuclei become unstable. As the proton number increases, the total electrostatic repulsion grows faster than the strong-force binding can compensate. Beyond a certain size, the nucleus can no longer hold itself together — which is why all elements with atomic number above 82 (lead) have no stable isotopes.

The very-short-range repulsion below 0.5 fm is equally important. Without it, the attractive strong force would pull nucleons into an ever-smaller volume until the nucleus collapsed to a point. The repulsive core sets a minimum separation and gives the nucleus its characteristic, finite size.

6. Exam Technique and Common Mistakes

AQA questions on the strong nuclear force tend to target three things: the correct distance values, the direction of the force at each range, and the comparison with electrostatic repulsion. Getting all three right in a single 'explain' or 'describe' answer is what separates a 5 from a 6 on a 6-mark question.

For 'describe' questions, the examiner wants a clear sequence: state the range, state the direction, and state what happens at each boundary. For 'explain' questions, add the 'because' — link each feature of the force to its physical consequence for the nucleus.

7. Pulling It All Together: A Model Exam Answer

Here is how to structure a full exam response on the strong nuclear force. The question below is typical of AQA 6-mark 'explain' questions on this topic.

Worked example: Constructing a 6-Mark Explanation of Nuclear Stability

Explain how the strong nuclear force keeps a nucleus stable. In your answer, refer to the range and nature of the force at different nucleon separations. [6 marks]

1. Protons in the nucleus are all positively charged. The electrostatic (Coulomb) force between them is repulsive and acts over an infinite range. — [B1] — Establishing the electrostatic repulsion problem earns the first mark and frames the whole answer.
2. The strong nuclear force acts between any two nucleons — proton–proton, neutron–neutron, or proton–neutron. It is charge-independent. — [B1] — Stating charge-independence and that it acts between all nucleon pairs is a common mark point that students miss.
3. At separations between approximately 0.5 fm and 3 fm, the strong nuclear force is attractive and greater than the electrostatic repulsion between protons. This net attraction holds the nucleons together. — [B1] + [B1] — Two marks here: one for the correct distance range, one for stating the force exceeds electrostatic repulsion.
4. At separations smaller than approximately 0.5 fm, the strong nuclear force becomes repulsive. This prevents nucleons from being compressed into an arbitrarily small volume and gives the nucleus a stable, finite size. — [B1] — The very-short-range repulsion is a distinct mark point. Many students omit this entirely.
5. Beyond approximately 3 fm, the strong nuclear force is negligible — effectively zero. The force is therefore short-range, unlike the electrostatic force which follows an inverse-square law and acts over infinite distances. — [B1] — Contrasting the short-range nature with the infinite range of the electrostatic force shows higher-order understanding and secures the final mark.

Answer: A complete answer covers: (1) electrostatic repulsion between protons as the problem; (2) the strong force acting between all nucleon pairs regardless of charge; (3) attractive and dominant between ~0.5 fm and ~3 fm; (4) repulsive below ~0.5 fm, preventing collapse; (5) negligible beyond ~3 fm, contrasted with the infinite range of the electrostatic force.

Every spec requirement for this topic traces back to one central idea: the strong nuclear force is short-range, charge-independent, attractive at intermediate separations, and repulsive at very small separations — and these four properties together explain why stable nuclei exist at all.