Q&Q - Have a problem with rutherford scattering experiment. (1 Viewer)

[Mathematician]

Why do alpha particles deflect by the thin gold foil??

Why not any material??

I thought the deflection was just because of repulsion of the alpha particle and the nucleus, so the repulsive power should be the same regardless of the material shouldnt it??

The book says its because of some high electric field associated with the foil. Is this because gold is a conductor??? well then how was this field set up? and why cant we use other conductors? would we need gold to guarantee more deflections that are greater than 90 degrees?And what is mica???

the book also says:The deflection is caused by the action of an electrostatic force which obeys an inverse square law. We know this because of the distribution of alpha particles and the fact that the alpha particles we're slowing down more and more as it approached the positive nucleus where the particles would be deflected at maximum. This is why they propose the electrostatic force because F(e)= kq(1)q(2)/d^2 so as d is very small (approaching nucleus)electrostatic force is maximum.

And why is the experiment performed in a vacuum, and why is the beam of alpha particles more wider in air??
doesnt air cause the alpha particles to slow down more, so wouldnt that be preferred as the alpha particles would have slowed down significantly when it approaches nucleus so more impulse or momentum of force by the F(e) to deflect the particles.

Sorry for all the q's but i cant help but have this stuff on my mind.
Im never completly clear lol.

 

[Constip8edSkunk]

gold has a heavy atom, so it will rebound the alpha particle rather be knocked around... Also wasnt rutherford's alpha scattering experiment just him trying to find something for geiger and marsden 2 do.... to investigate gold's lattice structure or something? im guessing its vaccum cuz they dun want paths of the alpha patrticles 2 b delected in flight as they wanted a accurated distribution of deflected particles(from the foil)?

 

[Mathematician]

....

gold has a heavy atom, so it will rebound the alpha particle rather be knocked around...

Did u mean gold's atoms are heavy?

rather than be knocked around? what do u mean?

with all atoms wouldnt the alpha particles deflect( is this what u mean by rebound?), but it may deflect off Gold more as more mass means more positively charged nucleues ----> greater repulsion.

"im guessing its vaccum cuz they dun want paths of the alpha patrticles 2 b delected in flight as they wanted a accurated distribution of deflected particles(from the foil)?"

delected or deflection ??? that would kinda make sense if u meant affected because air may alter the path of the particles affecting the accurate distribution. They only which to investigate the effect due only to the force. Vacuum is suited for this.

Is that what u meant?

 

[Constip8edSkunk]

yeah, the 2nd part is what i meant... as for the 1st part, first i dont think they deliberately set out to investigate the structure of the atom in the experiment, and they just happen to choose gold because rutherford was just finding something for geiger's student marsden to do, ie investigating either the structure of gold or penetrative properties of alpha particles... i cant exactly remember. what i meant was that with lighter elements, lighter nucleus, smaller charge, the deflection would be less obvious. i just recall something from chadwicks calculation of neutron bombardments of elements that the bombarded nuclei would recoil at magnitude inversely proportional to their mass, so i just guessed that it may b the same here. please correct me if i got anything wrong

 

[Mathematician]

...

This clears some stuff up for me and everyone

Try it yourself! In the following pictures, there is a target hidden by a black cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Click on "Look At Answers" button to compare your guess with the real answer

http://pdg.web.cern.ch/pdg/cpep/images/target1.gif Unkown Target 1

http://pdg.web.cern.ch/pdg/cpep/images/naked_target1.gif
Target 1 is a wedge

http://pdg.web.cern.ch/pdg/cpep/images/target2.gif Unkown target 2


http://pdg.web.cern.ch/pdg/cpep/images/naked_target2.gif
Target 2 is a circle.

Why didnt the pictures come up?

 

[Halo]

Probably since gold has high density (more positive nucleus per unit of space), the probability of interactions (deflections) between the nucleus and alpha particles is higher than other materials (remember, the size of the nucleus takes up very little of the atom and so most will past through undeflected). It would be more observable hence?

The experiment was performed in vacuum probably because alpha particles have low penetrating power, which will generally be stopped by a few cm of air. They probably wouldnt want ions in the air to affect the experiment either.

Just some wild guesses. What do you think?

 

[Halo]

Also, neutron bombardments wont apply here because neutrons are uncharged and thus wont be subject to electrostatic interactions as alpha particles would.

 

[Constip8edSkunk]

hmm true, havent thought bout that. thanx 4 the info

 

[ReaveR]

Some book like Jacaranda or Excel said something about Gold being easily manipulated to form a layer only one atom thick, which would've suited the experiment.

 

[Bannanafish]

actually gold was probably used because it can be beaten a lot thinner than other elements

 

[allyteaded]

I think (from what I can remember) ... Rutherford conducted this experiment to find out the structure of the atom. He hypothesized that all the alpha particles would bounce back but was surprised that most of them passed through and a small portion were deflected and bounced straight back. He concluded that the nucleus takes up a very small portion of the atom and that most of the atom is made up of space. For a guess, Halo, I think you're pretty accurate.

I think they used gold because it has a high density. As far as they were concerned, Gold was solid. (It looks pretty solid) which is why they expected the alpha particles to bounce back.

And yes, the reason for their deflection is because alpha particles have a positive charge and as they travel thorugh the sheet of gold, some collide or come close to the nucleus of the gold atoms thus an electrostatic force of repulsion.

I think they used a vacuum so the alpha particles would not collide with air particles. And as Halo said, Alpha particles have a low penetration power.

 

[deyveed]

Originally posted by Bannanafish
actually gold was probably used because it can be beaten a lot thinner than other elements

I remember reading about this in a textbook.