Rutherford alpha ray scattering experiment

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Rutherford alpha ray scattering experiment

Post by sherlock221b on Thu Aug 04, 2016 12:31 pm

Why are the alpha particles not getting affected by the many number of orbiting electrons present in the gold foil and why are they not trying to get stabilized?

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Re: Rutherford alpha ray scattering experiment

Post by Keep Guessing! on Thu Aug 04, 2016 12:54 pm

I think that is because the mass of an alpha particle is large compared to the mass of an electron (since a proton weighs 1837 times an electron), which supersedes the fact that there are so many electrons. Plus there is so much of empty space for an alpha particle to move through that it may barely interact with an electron.

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Re: Rutherford alpha ray scattering experiment

Post by The Confused Guy (O.o)? on Thu Aug 04, 2016 8:31 pm

I agree to u keep guessing
and also maybe because electrons revolves very fast and is tiny tat there will be a very low chance of a collision :c
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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Thu Aug 04, 2016 9:53 pm

I suppose there are mainly two reasons for it
1)Since the size of an atom is very large in comparison to that of an electron the probability of a collision is very low.
2)The mass of an electron is very low in comparison to that of an alpha particle so even if a collision takes place it won't make a significant change in the path of the alpha particle.

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Re: Rutherford alpha ray scattering experiment

Post by Arnab Mukherjee on Fri Aug 05, 2016 8:59 am

Sreejith Nair has put it in a nice way. But the first point is more important than you can probably imagine. Remember the example of IISER campus? Electron here is even smaller than a chalk and it can be anywhere in the IISER campus when a few alpha particle again as the size of the chalk comes. What are the odds?

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Re: Rutherford alpha ray scattering experiment

Post by Confusions allowed on Fri Aug 05, 2016 11:54 am

But what about the electromagnetic forces? Can't they disrupt the motion of the alpha particles? Also, if not, then what percent of the alpha particles are deflected? The distances are so small, the electromagnetic forces exerted by the electrons would be quite large.
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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Fri Aug 05, 2016 12:35 pm

Collision means a change in the trajectory of a particle by some form of interaction with another particle it includes electromagnetic interactions as well. If we consider the fact that an alpha particle is travelling at a speed of about 15 million m/s and that it has a mass 7 thousand times that of an electron which is actually travelling at just 2millon m/s in average. It is easy to guess that the momentum of an alpha particle is in incomparable with that of an alpha particle so by Neurons laws the same force acts on both and as a result the electron may deviate from its path rather than the alpha particle.

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Re: Rutherford alpha ray scattering experiment

Post by Confusions allowed on Fri Aug 05, 2016 12:56 pm

So does that mean that the alpha ray ionizes the gold atoms in the sheet while passing through? There's basically no other way possible for the electrons, and also, your argument explains why alpha-ray is highly ionizing. That gives another reason to make the sheet as thin as possible.
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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Fri Aug 05, 2016 1:10 pm

By fact alpha rays are kmown to ionize matter.

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Re: Rutherford alpha ray scattering experiment

Post by Confusions allowed on Fri Aug 05, 2016 1:17 pm

They are also known to be stopped with a single sheet of paper. Then, the rays will have to battle to get through the sheet, although just hundreds of atomic layers thick. That will definitely change the total count by an amount, albeit small. What can be the count change, then?
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Re: Rutherford alpha ray scattering experiment

Post by Confusions allowed on Fri Aug 05, 2016 1:54 pm

Or does that mean that after some time, one can get better results of the experiment?
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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Fri Aug 05, 2016 4:11 pm

I do believe in the idea of improvement. But we cannot compare the gold foil used in Rutherford's experiment and a sheet of paper. A sheet of paper has a thickness of about 0.1mm on the other hand the gold foil has a thickness of 0.4 micro meater. And also the carbon atom has a radius of about.170 pm at the same time a gold atom has a thickness of 166pm. SO the number of atoms in a layer of paper is about 3 lack at the same time the number of atoms per layer of gold foil is about 1 thousand. And as the number of atoms encountered increases the probability of the alpha particle getting through decreases . And it is not that all the alpha particles are stoped by a sheet of copy paper. A respectable amount of alpha particles does make it through. It is not like you can store a source of alpha particle in a paper cover and keep it in your home.

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Re: Rutherford alpha ray scattering experiment

Post by Keep Guessing! on Fri Aug 05, 2016 4:44 pm

Sreejith, you said that the momentum of an alpha particle is much larger than the momentum of an electron. What did u conclude from that? Newton's laws are applicable anyway, right?

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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Fri Aug 05, 2016 5:14 pm

Force means rate of change of momentum so if the momentum is low a force can reverse the direction of momentum in a shorter interval of time.When compared to a body with larger momentum. And so a particle with lower momentum can be pushed out of its trajectory more easily.

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Re: Rutherford alpha ray scattering experiment

Post by The Confused Guy (O.o)? on Fri Aug 05, 2016 5:45 pm

Now it makes me wonder can electrons be captured by the alpha particle and become a helium atom #confused
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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Fri Aug 05, 2016 6:04 pm

It is a commonly known fact that alpha particles ionise matter. But I am not shure if the electron are taken away by the alpha particle to form helium or does it just kick out electrons. Can someone tell me the correct answer?

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Re: Rutherford alpha ray scattering experiment

Post by Confusions allowed on Fri Aug 05, 2016 9:34 pm

I think that the electrons are only knocked off the atom, as the electron energies would be too high to even get into the first orbit.
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Re: Rutherford alpha ray scattering experiment

Post by Arnab Mukherjee on Fri Aug 05, 2016 11:48 pm

There is a stabilizing interaction between nucleus and electron up to a certain distance (called potential energy curve). So, alpha particle can take up electrons. However, given the velocity of the alpha particles, ionization would be more probable. The force due to change in momentum will outweigh the attractive electrostatic force. However, it is a guess.

About the paper: Yes thickness matters. Papers are 10000 times thicker than the gold foil used in the experiment. So, the probability of backscattering is much more than the thin gold foil. Alpha particles will not penetrate a gold sheet even half thick as that of a paper. Again, this is a logical guess. See if you can find out more information on that.

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Re: Rutherford alpha ray scattering experiment

Post by The Confused Guy (O.o)? on Sat Aug 06, 2016 8:19 am

The bond length in Au-Au is aprox 288pm and if a sheet of gold is 0.05mm then there would be 17,40,000 Au atoms  Shocked  in it. This might form a wall of 17,40,000 nuclei  Cool  with maybe tiny gaps  Suspect (not sure here) and stop most of the alpha particles. And some of the deflected Alpha particles might get deflected even more by other nuclei :/. Am i right? Suspect
#confused
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Re: Rutherford alpha ray scattering experiment

Post by Confusions allowed on Sat Aug 06, 2016 8:41 am

You're right, but there wasn't any dire need of numbers. Me, Sreejith and Arnab sir have already talked about this. A more interesting question is that how many of the alpha particles will be deflected while passing through Rutherford's Gold sheet, thus deviating us from the real empty space. The answer seems negligible, though.
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Re: Rutherford alpha ray scattering experiment

Post by The Confused Guy (O.o)? on Sat Aug 06, 2016 9:24 am

@lets see what happens, i didnt get u
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Re: Rutherford alpha ray scattering experiment

Post by Student1 on Sat Aug 06, 2016 3:51 pm

I have a doubt regarding this.
When you direct a beam of alpha particles, since all of them are positively charged, there will be repulsion among the alpha particles as well, causing a slight deviation from their original straight line path of the beam, although perhaps not a complete rebound. So how come Rutherford came to the conclusion about the positive charge all being concentrated in one area? Maybe he could've thought that the few alpha particles getting deflected was because of the repulsion from the other alpha particles itself.

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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Sat Aug 06, 2016 4:08 pm

The forces are perpendicular to the direction of motion and since the velocity of the alpha particles are very large SO the perpendicular displacement is negligible in comparison to the linear displacement SO they can be considered to be parallel.

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Re: Rutherford alpha ray scattering experiment

Post by Sreejith A Nair on Sat Aug 06, 2016 4:18 pm

I have a serious doubt. During the class Sir said that when Rutherford first did the experiment he observed that most of the particles passed through the gold foil. And it validated plum pudding model. And it was only after Geiger and Marsden s observation of rebounding of alpha particles that plum pudding model was questioned. He said Rutherford s observation was based on plum pudding model as alpha particles are much denser than gold atom SO it passed through it. But according to plum pudding model the density of alpha particle should be about 582kg/m3 and the density of gold atom should be about 17211kg/m3. SO clearly the density of alpha particle is much lesser than that of gold atom. So can someone clear my doubt?

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Re: Rutherford alpha ray scattering experiment

Post by Student1 on Sat Aug 06, 2016 4:38 pm

I suppose you should compare the density of an alpha particle (Helium nuclei) with that of the gold atom.
Roughly, without really going into the calculations and taking the units as amu and pm
1. For the helium nuclei:
Mass = 4 units. the radius of helium atom is roughly 31 pm, so the approx. nuclear radii will be 31 * 10^-5 pm.
This gives density as (4/((4/3)*3.14*(31* 10^-5)^3))

2. For the gold atom:
Mass = 197 units, the radius is 174pm and since this is plum pudding model we assume the mass is distributed along this whole sphere corresponding to the atomic radii, so we get density as ((197/(4/3)*3.14*(174)^3))

So the density of the alpha particle is actually much more than that of gold atom.

But again this is confusing and I'm not entirely confident about this because we are applying plum pudding model to the helium NUCLEUS and to the gold ATOM

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Re: Rutherford alpha ray scattering experiment

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