Frank-Hertz experiment

Question

Prince Budha · Accepted Answer

Franck-Hertz Experiment was used to describe the quantum nature of atoms clearly. The existence of discrete energy levels in atoms was demonstrated directly by James Franck and Crustav Hertz in 1914. A scheme of the experimental set up shown in figure (1) Figure 1: Franck-Hertz Experiment. The tube contains three electrodes: a filament (F), a plate (P), and a grid (G). A grid is a charged screen that can attract or repel electrons. But due to the open space, the majority of the electron pass through it. A variable accelerating potential Vo is applied between the filament and the grid and small retarding voltage Vr(≈1V) is applied between the plate and the grid. The electrons will reach the grid with a kinetic energy Ek = eVo. After reaching the grid majority,  these electrons will go through the open space and contribute to the plate current I. If Vr>Vo, the electrons will be turned back before they can reach the plate and they will not contribute to the current measured by Ammeter. But even if Vr<Vo, the electron will not able to reach the plate if they lose enough kinetic energy through collision with atoms in the tube as they travel between the filament and the grid. Explanation of Graph Figure 2: Dependence of plate current with Vo Consider the case of beam of slow electrons travelling through the mercury vapour at low pressure. If the electron suffers no energy loss due to elastic collision with the atoms of the gas. They will reach the collector plate, increasing the accelerating potential, collector current increases. As 'V' reaches the value of critical potential 4.9V, an electron acquire 4.9V of energy on reaching 'G'. The electron losses all its energy in an in-elastic collision with the mercury atom, so the electrons unable to reach plate 'p' and current decreases. On increasing the accelerating potential beyond 4.9V again the electrons reach to the plate at V=9.8V, the current again dip to a second minimum. This can be explained if an electron of 9.8V energy suffers to consecutive in-elastic collision with the different mercury atom before reaching the plate. Such an electron excited both the mercury atoms to their first excited state losing 4.9eV energy in each collision. This explains the second minimum. Each time there is an in-elastic collision, the mercury atoms will be excited and return to the ground state by emission of photons. By using spectro-scopic technique, the wave length of emitted radiation is found to be 2536Ao. Thus, this experiment shows that the energy loss by electron in in-elastic collision with mercury atom appears quantum of energy of wavelength λ = h c/E. Franck-Hertz experiment supports the bohr atomic model because it also explain the quantum nature of the atom same as the bohr atomic model theory.

Describe Frank-Hertz experiment. Interpret how the results of this experiment advocate atomic model proposed by Bohr?

Leave your Answer:

Hits Counter

Question

Describe Frank-Hertz experiment. Interpret how the results of this experiment advocate atomic model proposed by Bohr?

Leave your Answer: