Franck Hertz experiment

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Franck Hertz experiment

• 6B Tam Fei Ying(22)
• Pang Sze Man(28)

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• Glass tube contains mercury vapour at low
  pressure
• Hot cathode (emitter) C emits electrons by
  thermionic emission.

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• 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.

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Thermoelectrons accelerated by ve grid potential

• Most of e- pass through the grid
• Travel towards to plate
• Micro-ammeter measures the current I

• The current I as the accelerating voltage V
  is increased until V4.9 where there is a sudden
  drop.
• I a gain
• Another sudden drop at V9.8V

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Electron collide with mercury atomslose energy
of incident electrons

• In an elastic collision,
• Total KE conserved.
• Mercury atom massive gtgt electron
• Carries always negligible KE
• Almost no K.E. loss of electron
• 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.

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• Almost all the mercury atoms are in the ground
  state.

• When K.E. max of C lt 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 retarding p.d.(Vr).

• The electrons are gain energy.

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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.

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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.
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• A sharp drop in current at V(6.7-4.9)V1.8V
• 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.

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• 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.

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THE END