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PHYWE Frank-Hertz Exerperiment with Hg |
Learn about:
- Energy Quantum
- Electron Collision
- Excitation Energy
Electrons are accelerated in a tube filled with mercury vapor. The excitation energy of mercury is determined from the distance between the equidistant minima of the electron current in a variable opposing electric field.
Niels Bohr introduced the planetary model of the atom in 1913: An isolated atom consists of a positively charged nucleus about which electrons are distributed in successive orbits. He also postulated that only those orbits occur for which the angular momentum of the electron is an integral multiple of h/2p, i.e. n*h/2p, where n is an integer and h is Planck’s constant. Bohr’s picture of electrons, in discrete states with transitions among those states producing radiation whose frequency is determined by the energy differences between states, can be derived from the quantum mechanics which replaced classical mechanics when dealing with structures as small as atoms. It seems reasonable from the Bohr model that just as electrons may make transitions down from allowed higher energy states to lower ones, they may be excited up into higher energy states by absorbing precisely the amount of energy representing difference between the lower and higher states. Franck and Hertz used a beam of accelerated electrons to measure the energy required to lift electrons in the ground state of a gas of mercury atoms to the first excited state.
The electrons emitted by a thermionic cathode are accelerated between cathode and anode in the tube filled with mercury vapor and are scattered by elastic collision with mercury atoms. From an anode voltage of 4.9 V, however, the kinetic energy of the electrons is sufficient to bring the valence electron of the mercury to the first excitation level by an inelastic collision. Because of the accompanying loss of energy, the electron can now no longer traverse the opposing field between the anode and counter electrode: the current is at a minimum. If we now increase the anode voltage further, the kinetic energy of the electron is again sufficient to surmount the opposing field: the current strength increases. When the voltage is 2 x 4.9 V ,the kinetic energy is so high that two atoms in succession can be excited by the same electron: we obtain a second minimum. These minima are not, however, very well-defined because of the initial thermal distribution of the electron velocities.
According to the classical theory the energy levels to which the mercury atoms are excited could be random. According to the quantum theory, however, a definite energy level must suddenly be assigned to the atom in an elementary process. For this experiment, we determine the voltage values of the minima. From the differences between these values we obtain the excitation energy E of the mercury atom by taking an average.
Includes a Franck-Hertz operating unit, Franck-Hertz Hg-tube on plate, Franck-Hertz oven, NiCr-Ni thermocouple, 5-pin connecting cable for Hg-tube, shielded BNC-cable, RS 232 data cable, Franck-Hertz software, and manual on CD-ROM.
System requirements:
PC, Windows® 95 or higher
For your convenience this experiment is available to you as a complete set.
Everything you require to perform the activities are included in the total price.
For assistance in using your new equipment we also offer professional development (WLS1808-24) on all PHYWE line products.
For customization options and pricing please contact your local Cenco representative.
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