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Franck Hertz experiment

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

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Franck Hertz experiment

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  1. Franck Hertz experiment 6B Tam Fei Ying(22) Pang Sze Man(28)

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

  3. 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.

  4. 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 V=4.9 where there is a sudden drop. • I = a gain • Another sudden drop at V=9.8V

  5. Electron collide with mercury atomslose energy of incident electrons • In an elastic collision, Total KE conserved. Mercury atom massive >> 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.

  6. 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 retarding p.d.(Vr). • The electrons are gain energy.

  7. 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.

  8. 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.

  9. A sharp dropin 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.

  10. 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.

  11. THE END

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