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Least distance of approach and nucleus radius

Ok.. I understand how to use coulomb's law and attain the value for the least distance of approach and can answer the questions but I'm confused on two things:

1) surely the least distance of approach is determined by the initial kinetic energy you give the alpha particles? so different energies will give different radius's?

2) surely the least distance of approach is not the radius but is greater than the radius? how can we be sure that the two distances are similar??

thanks:smile:
Reply 1
Avatar for teachercol
teachercol
9
Agreed - least distance of approach just gives an upper limit on size of nucleus.
Reply 2
Avatar for SillyEddy
SillyEddy
18
Well if given the choice, you could explain that electron diffraction is a better way to analyse the radius of the atom.

The approach radius is just that - How far it gets from the centre. It doesn't find the exact radius of the nucleus, just how close it can get with whatever energy it has.


So it's just an estimate. But they just want an answer based on their numbers, so do just that. If you have a question asking 'why is this not accurate' then suggest a better method and that'll pick you up the marks later on.
Reply 3
Avatar for tdx
tdx
OP
6
Original post by teachercol
Agreed - least distance of approach just gives an upper limit on size of nucleus.



Original post by SillyEddy
Well if given the choice, you could explain that electron diffraction is a better way to analyse the radius of the atom.

The approach radius is just that - How far it gets from the centre. It doesn't find the exact radius of the nucleus, just how close it can get with whatever energy it has.


So it's just an estimate. But they just want an answer based on their numbers, so do just that. If you have a question asking 'why is this not accurate' then suggest a better method and that'll pick you up the marks later on.


cheers :smile: reckon you could explain why the electron diffraction is better for me?

My understanding is the beam of electrons has a de brouglie's wave length in the same order as the diameter of the nucleus of the atom so is diffracted by it? why does this make it better? and then the electrons are attracted towards the nucleus so diffract closer to the nucleus? again why is this better? thanks :smile:
Reply 4
Avatar for SillyEddy
SillyEddy
18
Original post by tdx
cheers :smile: reckon you could explain why the electron diffraction is better for me?

My understanding is the beam of electrons has a de brouglie's wave length in the same order as the diameter of the nucleus of the atom so is diffracted by it? why does this make it better? and then the electrons are attracted towards the nucleus so diffract closer to the nucleus? again why is this better? thanks :smile:


The electrons also aren't affected by the 'strong force' (as they're a flavour of lepton) so the electrons can't be absorbed into the nucleus.

So the electrons will diffract when they hit the film of atoms, and spread out onto a sheet behind the film... So you can measure the angle between the normal and the first minimum (key - minimums, not maximums) and you can derive the diameter of the nucleus from there.


To be honest, I'm still going over it myself. But these are the key things that I've picked up about it.

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