J. Franck and G. Hertz in the years 1913-1914 made a series of experiments which demonstrate the existence of electron orbitals quantized energy levels. Their results confirm the correctness of the formulated shortly before Bohr's model of the hydrogen atom nespektroskopickou method.
Educational phase: fixation, diagnostic
- FH device
- connecting wires.
The experiment used a tube filled with gas at low pressure with 3 electrodes:
heated cathode that emits electrons, accelerating grid and anode. The anode is at a slightly negative
electric potential due to the grid (but positive due to the cathode), so
electrons need a certain amount of kinetic energy to the grid retarding potential and
anode overcome. The device was designed to measure the electric current flowing through
change between the electrodes and potential difference (voltage) between the cathode and the accelerator grid.
Plug in the device as described above and switch scheme. Gradually increase the voltage.
The emergence of the first ring - the first decline in current.
The emergence of the second ring - the second decline in current.
The emergence of the third ring - the third decline in current.
The emergence of the fourth ring - the fourth decline in current.
The emergence of the fifth ring - the fifth decline in current.
The result of experiment is at first sight in classical physics is incomprehensible. According to her, we expect rather monotonous dependence of current on the grid voltage. Significant local maxima equidistantly distributed on the experimental curve are understandable only if we accept Bohr's idea of quantization of the internal energy of atoms.
When you make an attempt with neon can see visible evidence of the process of absorption of energy in collisions of electrons with atoms. The accelerated electrons cause electron excitation of neon atoms, these electrons then quickly transferred to a more stable lower energy state while radiating energy in such a way that at the excitation
follow the visible radiation.
Franck and Hertz explained the shape of the volt-ampere characteristics based on the elastic and inelastic
precipitation of accelerated electrons with atoms of mercury. For the accelerating voltage is less than 4.9V
electron energy is so small that the atoms of mercury only condenses flexibly, because the lack
energy for the excitation of mercury atom. In elastic collisions lose only a small part of their energy
and can therefore pass through the anode grid. With increasing electron energy increases the number of incident
the anode and thus the current in the plate circuit. After reaching the energy 4.9 eV, which is the lowest
energy that can excite atomic packing of mercury, is the first inelastic electron collision
with mercury atoms. Electron energy is absorbed by the atomic packing of mercury, which is excited and
a sharp decrease in the number of electrons hitting the anode. With increasing energy
electron can pass more electron inelastic collisions and more excited atom of mercury.