# Homework Help: Alpha-particle scattering experiment

1. Sep 6, 2004

### MA103

In Rutherford's experiment, alpha-particles were directed at a thin gold foil. A small fraction of the alpha-particles were back-scattered. Explain how this result would be affected if each of the following changes was (separately) made.
a) A thicker foil was used
b) Higher-energy alpha-particles were used.
c)A silver foil was used - the atomic number is less than that of gold.

c) is fine, I can do that. But a) and b) I seriously need help.

For a) I can thick of two things ...

1) alpha particles will get absorbed. If 1) is right, why is this?

2) because alpha particles will encounter more nucleus of more atoms, there will be more deflected alpha-particles.

For b) I have no idea. I'm guessing there will be more deflection but can't explain.

2. Sep 6, 2004

a) more back scattering
b) more particles getting through, less deflection

3. Sep 6, 2004

### MA103

Hi, thanks but may I ask why for b)?

4. Sep 7, 2004

### jdstokes

The $\alpha$-particles undergo hyperbolic trajectories when they pass nearby the positively charged gold nuclei. Increasing the thickness of the foil increases the probability of an $\alpha$-particle experiencing a close encounter with an Au nucleus resulting in greater deflection angles. Higher energy particles possess a larger component of velocity perpendicular to the foil so I'd say the deflection angle is reduced. Silver has a lower molar mass than gold, the weaker electric field exerts a weaker coulombic force on the $\alpha$-particles resulting in decreased deflection angles.

5. Sep 7, 2004

### MA103

Thanks for good explanation.

6. Aug 21, 2007

### liaminho

I'm the other way round, a) and b) i could do but c) i need help with!
What difference would it make if a silver foil was used instead of a gold one?