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Lesson · Grade 8 · Science

The Strong Nuclear Force: What Holds the Nucleus Together

  • Grade 8
  • Science
  • 1 min 44 s
  • British Columbia
  • BC8-SC-27

Starting from a nucleus of protons and neutrons, the video zooms in on two protons. It shows the electric push driving them apart, gravity being far too weak to hold them, and the strong nuclear force pulling them together, far more strongly but only over about the width of a small nucleus. A neutron then adds pull without adding push, and back in the full nucleus every proton pushes on every other while the pull comes mostly from neighbours, which is why big nuclei like uranium-238 need more neutrons than protons.

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In one sentence

The question this lesson answers

Here is the tiny nucleus at the centre of an atom, packed with protons and neutrons. What holds it together?

Each proton has a positive electric charge. Neutrons have no charge at all.

Transcript

What the lesson says

Here is the tiny nucleus at the centre of an atom, packed with protons and neutrons. What holds it together?

Each proton has a positive electric charge. Neutrons have no charge at all.

Let's zoom in on two protons. Like charges repel, which means they push each other apart. This push is the electric force.

Is it gravity? No. Gravity between two protons is about a trillion trillion trillion times weaker than the electric push.

What holds them is the strong nuclear force. It pulls protons and neutrons toward each other.

Up close, it is far stronger than the electric push, so the protons stay together.

But the strong force has a very short reach, only about the width of a small nucleus.

Move the protons a little further apart, and the strong pull fades to almost nothing, while the electric push is still there.

Now add a neutron. It has no charge, so it adds strong pull without adding any electric push.

In a big nucleus, each proton pushes on every other proton, but it feels the strong pull mostly from its nearest neighbours.

So big nuclei need more neutrons than protons. Uranium-238 has 92 protons and 146 neutrons.

The strong nuclear force holds the nucleus together: far stronger than the electric push, but only over tiny distances.

Curriculum

Where this fits

British Columbia, Grade 8

Science, Physics: protons, neutrons, and quarksBC8-SC-27

Previous: Inside the nucleus: protons, neutrons and quarksNext: Electrons: Tiny Leptons Held by Electric Attraction

The outcomes and strands follow the British Columbia curriculum.

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