Question

1. Compare the Franck-Hertz experiment with the photoelectric effect experiment in terms of the following:

a. Experimental setup
b. The physics involved

c. The relationship to quantum mechanics

d. The chemical materials involved

Answer 1

On April 24, 1914 Franck - Hertz experimented was made by James Franck and Gustav Hertz. Franck- Hertz experiment shows the quantum nature of atoms.

a. Experimental setup:

In order to show this experiment Franck and Hertz designed a vacuum tube in which electrons flew through a thin vapour of mercury atoms at a temperature of 115 °C, at which the vapor pressure of mercury is about 100 pascals . It was found that when an electron is collided with a mercury atom, it would lose only a specific quantity (4.9 volts) of its kinetic energy.It is fitted with three electrodes: an electron-emitting, hot cathode; a metal mesh grid; and an anode.The grid's voltage is positive relative to the cathode, so that electrons emitted from the hot cathode are drawn to it. The electric current measured in the experiment is due to electrons that pass through the grid and reach the anode.Thus the Franck-Hertz experiment combines the physics of collision with the results of optical spectroscopy. Its result indicates that atoms can only absorb a discrete amount of energy, regardless the way the energy is transferred to the atom.

b. The physics involved:

This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.

c. The relationship to quantum mechanics:

in 1913 Niels Bohr had published a model for atoms that was very successful in accounting for the optical properties of atomic hydrogen. These were usually observed in gas discharges, which emitted light at a series of wavelengths. Ordinary light sources like incandescent light bulbs emit light at all wavelengths. Bohr had calculated the wavelengths emitted by hydrogen very accurately.Since an electron is attracted to the positive charge of the atomic nucleus by a similar force, so-called "classical" calculations suggest that any binding energy should also be possible for electrons. However, Bohr assumed that only a specific series of binding energies occur, which correspond to the "quantum energy levels" for the electron. An electron is normally found in the lowest energy level, with the largest binding energy. Additional levels lie higher, with smaller binding energies. Intermediate binding energies lying between these levels are not permitted. This was a revolutionary assumption.

In 1915 Bohr published a paper noting that the measurements of Franck and Hertz were more consistent with the assumption of quantum levels in his own model for atoms. In 1918, Franck and Hertz had largely adopted the Bohr perspective for interpreting their experiment, which has become one of the experimental pillars of quantum mechanics.Thus Franck and Hertz gave the proof of Bohrs relation.

d. The chemical materials involved:

The materials involved is mercury vapour, anode, cathode, ammemeter.