# The Strong Nuclear Force(OCR A Level Physics)

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Katie M

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Physics

## The Strong Nuclear Force

• In the nucleus, there are electrostatic forces between the protons due to their electric charge and gravitational forces due to their mass
• Comparatively, gravity is a very weak force and the electrostatic repulsion between protons is therefore much stronger than their gravitational attraction
• If these were the only forces, the nucleus wouldn’t hold together
• Therefore, the force that does hold the nucleus together is called the strong nuclear force
• The strong nuclear force keeps the nucleus stable since it holds quarks together
• Since protons and neutrons are made up of quarks, the strong force keeps them bound within a nucleus

Whilst the electrostatic force is a repulsive force in the nucleus, the strong nuclear force holds the nucleus together

#### Range of the Strong Nuclear Force

• The strength of the strong nuclear force between two nucleons varies with the separation between them
• This can be plotted on a graph which shows how the force changes with separation

The strong nuclear force is repulsive before a separation of ~ 0.5 fm and attractive up till ~ 3.0 fm

• The key features of this graph are that the strong nuclear force is:
• Repulsive closer than around 0.5 fm
• Attractive up to around 3.0 fm
• Reaches a maximum attractive value at around 1.0 fm (the typical nuclear separation)
• Becomes zero after 3.0 fm

• In comparison to other fundamental forces, the strong force therefore has a very small range (only up to 3.0 fm)

#### Exam Tip

• You may see the strong nuclear force also referred to as the strong interaction
• Remember to write that after 3 fm, the strong force becomes 'zero' or 'has no effect' rather than it is ‘negligible’.
• Recall that 1 fm = 1 × 10–15 m

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