Franck Hertz experiment
6B
Tam Fei Ying(22)
Pang Sze Man(28)
• Glass tube contains mercury vapour at low
pressure
• Hot cathode (emitter) C emits electrons by
thermionic emission.
Grid G is at a +ve potential V relative to C
The plate (anode) P is at a small -ve potential V
relative to G.
Thermoelectrons accelerated by
+ve grid potential
Most of e- pass
through the grid
Travel towards to
plate
Micro-ammeter
measures the current
as the accelerating voltage V
IThe current I
is increased until V=4.9 where there is a
sudden drop.
• I
= a gain
• Another sudden drop at V=9.8V
Electron collide with mercury atoms
lose energy of incident electrons
1. In an elastic collision,
Total KE conserved.
Mercury atom massive >> electron
 Carries always negligible KE
 Almost no K.E. loss of electron
2. Inelastic collision
some KE lost converted into the energy
inside the mercury atom
recoil of the mercury atom is negligible
amount of KE lost by e- = gain in E inside the
mercury
atom.
 Almost all the mercury atoms are in the ground state.
When K.E. max of C < 4.9eV
 When an electron hits a mercury atom, there is no way for
it to excite the atom.
All collisions are elastic. (the energy of the electron is not lost to
the atom).
The electrons go through the grid with the original
energy.
The energy is enough to overcome
The
electrons are p.d.(Vr).
gain energy.
the retarding
e- collides with mercury atom
enough KE   the atom into 1st excited state.
After inelastic collision,this amount of E is not enough to
overcome the Vr. the current shows a sharp drop. (line a)
The p.d. V for every sharp drop marks an allowed value
of energy absorption for the atom.
The sharp drop at 6.7V corresponds to the transition
indicated by line b.
The values of the p.d. for the transitions
(I.e. 4.9V and 6.7V) - excitation potentials of this atom.
The corresponding energies (4.9eV and 6.7eV) are called
excitation energies.
A sharp drop in current at V=(6.7-4.9)V=1.8V
At this voltage , enough E to raise the atom
from 1st excited state  2nd excited state. (line c)
The drop is not observed because at ordinary T
extremely few mercury atoms in 1st excited state.
 After a mercury atom has been raised to an excited state
  back down in a relatively short time.
 The excess energy can be released by emitting EM
radiation.
THE END