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Rutherford scattering experiments

Changing the source changed the energy of the alphas

By changing the radioactive source Rutherford could change the energy of the alpha particles he used.

Slower alphas are deflected more and don’t make it as close to the nucleus.

Try Why Do Astronauts Float by Julian Hamm

Or another way of looking at it is to say that with high energy alphas you need to be much closer to the nucleus to get a big deflection.  Changing direction is a change in motion.  If you’re going quickly the change in motion is big.  A big change in motion needs a big force, which is why the faster alphas need to be closer to the nucleus before they’re deflected a lot.

Changing the scattering material changes the size of the nucleus

An alpha is deflected less by an aluminium nucleus and gets closer to it than a gold nucleus.

This is because the charge on the gold nucleus is about six times bigger than the charge on the aluminium nucleus.

It is important to realise that the size of the force depends on the distance to the centre of the nucleus not its edge.  So the alpha particle doesn’t ‘know’ about the size of the nucleus unless it happens to hit it.  And with a gold nucleus this never happened.

back to Lesson 9: Atoms 3: The Nucleus