Summary OCR A level Physics A 6.4 Nuclear and Particle physics

6.4 Nuclear and
Particle
Physics

, Alpha particle -




scattering experiment

narrow beam of alpha particles targeted thin piece of
•


at a


gold foil
•
alpha particles scattered by foil + detected on zinc sulfide
screen in front of a microscope .




•


microscope moved to count number of alpha particles through
different angles
Observations t conclusions :




•
Most alpha particles passed straight through the foil with
very little
scattering
o most of the atom is
empty space ,
mass concentrated in small
region (nucleus)
•


Very few alpha particles were deflected through angles 90° (1 in 10000 )
nucleus is
positively charged ( repels +ve alpha particles)

The distance d of closest approach between an alpha particle and the gold
nucleus be calculated the idea of At this
can
using conservation of energy .




distance ,
the alpha particle momentarily stops therefore ,
:




initial kinetic
energy of alpha particle electrical potential energy at distance d
-

_




F- n 1.2×10
"
J 1-2×10-12 QQ
_




: =

atomic
41T Eod gold number



=
79%2 ✗ (1-60×10-19) '



41T 8-85×10 "
Xd
_




✗
"
d
"
=
3. 0×10 m



this gives an upper limit for the radius of a
gold nucleus as higher energy
alpha particles may get closer


radius of nucleus 10 5m
'
-




a :




radius of an atom : to -10m



°o° nucleus is ~
105 × smaller than the atom