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Lesson 33 notes - The Nuclear AtomName. Class.Plymstock School Physics DepartmentModule G485.3 Nuclear Physics

student booklet

Lesson 33 notes - The Nuclear AtomObjectives

(a) describe qualitatively the alpha-particle scattering experiment and the evidence this provides for the existence, charge and small

size of the nucleus (HSW 1, 4c);

(b) describe the basic atomic structure of the atom and the relative sizes of the atom and the nucleus;

Outcomes

Be able to describe what makes up an atom and recall the relative sizes of the atom and the nucleus.

Be able to describe qualitatively the alpha-particle scattering experiment.

Be able to describe the observations made from this experiment.

Be able to explain how these observations are evidence for the existence, charge and small.

Rutherford Scattering

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The experiment takes place in a vacuum to avoid problems of absorption by air.

Alpha particles are shot at the Gold Leaf. Some were deflected, some rebounded (1 in 10000). This led to the explanation that the gold leaf was made up of very small particles that had the majority of there charge and mass concentrated in a very small space at the centre of the particle: The nucleus.

The Atom

We now think of atoms in terms of a positively charged nucleus with negative electrons orbiting it. In the Bohr atom these electron shells (energy levels) are just the most likely place that an electron will be found and explained using Quantum Theory.

Atomic Sizes

Electron size ~ 10-18 m

Nucleus sizes ~ 1 fm

Atomic sizes ~ 1

Hydrogen molecule length ~ 1.28

Water Molecule size ~ 2.8

Lesson 33 questions The Atom

1. Describe briefly the two conflicting theories of the structure of the atom.

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2. Why was the nuclear model of Rutherford accepted as correct?

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3. What would have happened if neutrons had been used in Rutherfords experiment? Explain your answer.

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4. What would have happened if aluminium had been used instead of gold in the alpha scattering experiment? Explain your answer.

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5. What three properties of the nucleus can be deduced from the Rutherford scattering experiment? Explain your answer.

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6Describe briefly one scattering experiment to investigate the size of the nucleus of the atom.Include a description of the properties of the incident radiation which makes it suitable for this experiment.

In your answer, you should make clear how evidence for the size of the nucleus follows from your description.

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[Total 8 marks]

Lesson 34 The Strong Nuclear ForceObjectives

(c) select and use Coulombs law to determine the force of repulsion, and Newtons law of gravitation to determine the force of attraction, between two protons at nuclear separations and hence the need for a short range, attractive force between nucleons (HSW 1, 2, 4);

(d) describe how the strong nuclear force between nucleons is attractive and very short-ranged;

Outcomes

Be able to describe why there is a need for the short range attractive force between nucleons.

Be able to use Coulombs Law and Newtons Law of gravitation to be able to calculate forces within nuclei to explain why there is a need for the short range attractive force between nucleons.

Be able to describe the properties of th strong nuclear force.

Be able to interpret graphical representations of the strong nuclear force.

Gravitation and Electrostatic forces

Imagine 2 particles with a charge +e (e.g. 2 protons).

Gravity will pull them together (gravity is an attractive force).

Since they have the same charge Coulombs electrostatic force will push them apart (a repulsive force).

Calculate the Gravitational force and Electrostatic force between them:

Data

p = 1.602x10-19C

0 = 8.85x10-12Fm-1

mp = 1.673x10-27kg

G = 6.673x10-11Nm2

r = 1fm

You should find that the ratio between the forces Fe:Fg is 1.2x1036

In other words its not gravity thats holding nuclei together; its contribution is negligible there must be another force

A NUCLEAR FORCE.

The Strong Nuclear Force

Characteristics:

I.It is an attractive force between nucleons - We have discovered this from our calculations of the Coulomb force.

II.It is repulsive at very short range - If it were not then all nucleons without a charge would be able to pull each other into an infinitesimally small point. This does not happen.

III.It does not extend beyond distances of a few femtometres - Evidence from the Rutherford Scattering experiment shows that alpha particles describe paths explained by Coulomb repulsion until thy get very close to the nucleus whereby explanations need an extra force the Strong Force to describe their paths.

IV.It does not depend on the charge of the nucleons - both neutrons

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