Unit 4 PHYA4 - Fields and Further Mechanics (7408)
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Unit 4 PHYA4 - Fields and Further Mechanics (7408)
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3.8 Nuclear physics
3.8.1.1.
Who proposed the plum pudding model?
- J.J. Thompson
Descr. the plum pudding model?
- the plum pudding model stated that…
…the atom was made up of a sphere of positive charge
…with small areas of negative charge evenly distributed throughout like
plums in a plum pudding
Who conducted the alpha scattering experiment?
- Rutherford
Descr. the alpha scattering experiment?
- apparatus used:
● Alpha source
● An evacuated (so there’s a vacuum inside) chamber covered with fluorescent coating
● Gold foil in the evacuated chamber
● Microscope
- process:
● Alpha particles fired at the golden foil
● due to fluorescent coating you could see where the alpha particles hit the inside of
the chamber + microscope can be moved around the chamber to observe these
Results of the alpha scattering experiment?
1) most alpha particles passed straight the foil with no deflection
⇒ atom is mostly empty space
⇒ NOT uniform density, disproving the plum pudding model
2) small amount were deflected by a large angle (below 90 though)
⇒ centre of the atom is positively charged
3) very few particles were deflected backwards by more than 90 degrees
,⇒ the centre of the atom is very dense and small compared to the rest of the
atom
Conclusions deduced from the alpha scattering experiment?
- nucleus is much smaller than the separation between nuclei
- the vol. of the nucleus is much smaller than the vol. of the atom
- most of the mass is concentrated in the nucleus
- nucleus has a positive charge
Meaning of alpha particles being monoenergetic?
- they possess the same energy
Main interaction when an alpha particle is scattered by a gold nucleus?
- as in: one of the 4 fundamental forces/interactions
- electrostatic
- the nucleus' proton number (atomic no.) stays the same
==> the repulsive force stays the same
==> the scattering distribution stays the same
In which direction will the number of α particles per second be a maximum?
- deflection of 0 degrees
How does the deflection of an alpha particle change if the nucleus’ proton no.
decreases?
- less deflection
(deflection of lower angle)
Why is it necessary to remove the air from the chamber in a Rutherford scattering
experiment?
- to prevent the alpha particles from (colliding and) being absorbed/scattered by the air
particles
Explain why the gold foil should be very thin?
- the α particles must not be absorbed by the foil
+ the α particles must only be scattered once
Explain why there are more alpha particles passing straight through than particles
being deflected at large angles (more than 90 degrees)?
- atoms consist of mainly open space
- the nucleus is much smaller than the atom
- most of the mass of the atom is concentrated in the nucleus
- the nucleus is positively charged
((basically the conclusions of the alpha-scattering experiment/describe the atomic model))
+ most alpha particles don't pass close enough to be deflected
Why must all the alpha particles have the same speed?
, Why must the source of the alpha particles (the thing that decays, emitting alpha
particles) have a long half life?
Why must the beam of alpha particles be narrow?
History of the atom?
https://docs.google.com/document/d/1Gvtp7oiXvLLPOEmy_1kddWUR7B2Sf8GxFeWKY9Y
NJ48/edit?usp=sharing
3.8.1.2.
What does ionising radiation do to atoms?
- makes them into ions
Types of radiation from most to least ionising?
- alpha > beta > gamma
Range in air of diff. types of radiation?
- alpha - 0.04m
- beta - 0.40m
- gamma - infinite range that follows the inverse square law
Alpha, beta gamma rad. properties?
Experimental identification of diff. types of radiation?
~absorption experiments~
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