2.7 - Electrons and nuclei
Alpha particle scattering
● Rutherford’s atomic model experiment, observations:
1. Most alpha particles went straight through (the gold atoms) undeflected
2. Some alpha particles were deflected at small angles
3. Small proportion of alpha particles (with lower energy) were reflected >90˚
Conclusions:
1. An atom is mostly empty space
2. There is an area of (larger) +ve charge
3. The fact that very few alpha particles were deflected suggests the mass is
concentrated in a very small area (more deflection nearer the nucleus)
a. If mass of nucleus is small (compared to alpha particles), gold
nucleons/nuclei will be ejected upon collision (due to momentum)
Thermionic emission
● ***Definition: emission of electrons (from a metal surface) due to heating
○ ∵ Force on an electron in an electric field ∥ the field (F E = Eq)
Force on an electron moving in a magnetic field ⟂ the field (FB = B⟂qv)
∴ An electron will only be accelerated by an electric field
(A moving electron in a uniform magnetic field undergoes uniform circular
motion)
■ ∵ Electrons have a -ve charge, ∴ direction of force\
acceleration on electron is opposite the direction of electric field
Particle accelerators
p
● r=
Bq
● Charge & momentum
LINAC (linear motion)
● AC supply changes direction when electron is in MIDDLE of tube, ∴
electron attracted to next tube (which is +ve at that point of time)
1
Alpha particle scattering
● Rutherford’s atomic model experiment, observations:
1. Most alpha particles went straight through (the gold atoms) undeflected
2. Some alpha particles were deflected at small angles
3. Small proportion of alpha particles (with lower energy) were reflected >90˚
Conclusions:
1. An atom is mostly empty space
2. There is an area of (larger) +ve charge
3. The fact that very few alpha particles were deflected suggests the mass is
concentrated in a very small area (more deflection nearer the nucleus)
a. If mass of nucleus is small (compared to alpha particles), gold
nucleons/nuclei will be ejected upon collision (due to momentum)
Thermionic emission
● ***Definition: emission of electrons (from a metal surface) due to heating
○ ∵ Force on an electron in an electric field ∥ the field (F E = Eq)
Force on an electron moving in a magnetic field ⟂ the field (FB = B⟂qv)
∴ An electron will only be accelerated by an electric field
(A moving electron in a uniform magnetic field undergoes uniform circular
motion)
■ ∵ Electrons have a -ve charge, ∴ direction of force\
acceleration on electron is opposite the direction of electric field
Particle accelerators
p
● r=
Bq
● Charge & momentum
LINAC (linear motion)
● AC supply changes direction when electron is in MIDDLE of tube, ∴
electron attracted to next tube (which is +ve at that point of time)
1
