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Franck Hertz Experiment was first carried out in 1914 by James Franck and Gustav Hertz and they presented it to the German Physical Society. It was the first electrical measurement which is to show the quantum nature of atoms. The Franck Hertz experiment consisted of a vacuum tube designed to study energy from an electron which flew through a thin vapour of mercury atoms. It was discovered that only a specific amount of kinetic energy would be lost as the electrons collide with the mercury atom.
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Keyterms: Franck Hertz Experiment, Quantum nature, Atoms, Kinetic Energy, Electrons, Collide, Mercury, Energy, Electrical measurement, Bohr’s Model of an atom
What is the Franck Hertz Experiment?
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Aim: The aim is to demonstrate the concept of quantization of the energy levels which is according to Bohr’s Model of an atom.
Apparatus Required: The material which is required for the experiment is as follows:
- An Oven
- A control unit which will be for the power supply
- A DC Amplifier
- A Franck-Hertz tube filled with mercury
- A Franck-Hertz tube to be filled with neon
Franck Hertz Experiment Theory
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In the original experiment, they used a heated vacuum tube of temperature 115ºC with a drop of mercury of vapour pressure of 100 PAns. The three electrodes, an electron-emitting hot cathode, one metal mesh grid and also one anode which is attached to the cube.
Franck Hertz Experiment Theory
For drawing the emitted electron, the grid’s voltage is made positive with the respect to the cathode. The electric current which is measured in the experiment results from the movement of electrons from the grid to the anode. The electrode potential at the anode is slightly negative than the grid so the electrons will be having the kinetic energy the same as the kinetic energy in the grid.
Franck Hertz Experiment Theory
Franck Hertz experiment was explained in terms of the elastic and inelastic collision which takes place between the mercury atoms and the electrons.
Certain observations were derived from the dependence of the electric current flowing out of the anode and the electric potential placed between the cathode and the grid:
- The current in the tube increases steadily as the steady current in the potential difference increases.
- The current at 4.9 volts drops almost to zero.
- There is an increase in the current as the voltage increases to 9.8 volts.
- And again, a similar drop is observed at 9.8 volts.
Energy absorption from the electron collisions can be seen in the case of neon gas. When the accelerated electrons excite the electron in neon to the upper states, the accelerated electrons get de-excited in such a way as to provide a visible glow in the gas region in which the excitation is taking place. They got de-excited by dropping to the lower states at 16.57 eV and 16.79 eV. This energy difference is responsible for giving light in the visible range.
Franck Hertz Experiment: Conclusion
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Franck Hertz experiment data for mercury. The electron will be losing 4.9 eV with every collision that it will make with the mercury atoms. An individual can observe ten sequential bumps after one interval of 4.9 volts.
Franck Hertz experiment data for mercury
Franck Hertz experiment data for neon. For neon gas, the powered electrons will be exciting the neon electrons to the higher states and further, they will decelerate in a way to provide a glow in the gas region. About ten excited levels can be acquired within the range of 18.3 to 19.5 eV, and they will decelerate at a range of 16.57 eV to 16.79 eV. This difference in energy is responsible for the light produced.
Franck Hertz experiment data for neon
Elastic Collision
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An elastic collision can be defined as an encounter between any two bodies such that the total kinetic energies of the two bodies will remain the same. At the time of the collision, the kinetic energy is first converted to the potential energy which is related to the repulsive force that is between the particles that are converted back to the kinetic energy. The Rutherford backscattering is an example of an elastic collision.
Elastic Collision
Inelastic Collision
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An inelastic collision is defined for the two bodies whose kinetic energy is not conserved due to the initial friction. The macroscopic collision results in the effects, vibrations of the atom and deformation of their bodies.
Inelastic Collision
Things to Remember
- Franck Hertz Experiment was first carried out in 1914 by James Franck and Gustav Hertz.
- The experiment was the first electrical measurement to prove the quantum nature of atoms.
- Franck Hertz Experiment was carried out with the aim of demonstrating the concept of quantization of energy levels based on Bohr’s atomic model.
- The electrons in the test were boosted by mercury vapor causing a loss of energy due to inelastic scattering.
Sample Questions
Ques. Who was the first to conduct the Franck Hertz experiment? (1 Mark)
Ans. The Franck Hertz experiment was first conducted by two scientists named James Franck and Gustav Hertz in the year 1914.
Ques. Which experiment can be used to explain the elastic and inelastic collision between the electrons and the mercury atoms? (1 Mark)
Ans. The Franck Hertz experiment can be used to explain the elastic and inelastic collision between the electrons and the mercury atoms.
Ques. What is the dissimilarity between Ionization and Excitation? (2 Marks)
Ans. The main difference between the ionization and the excitation potential is that the former describes an electron removal from an energy level. The latter describes the propagation of an electron which is from low energy to a higher energy level.
Ques. Franck Hertz experiment supports which model of the atom? (1 Mark)
Ans. The Franck Hertz experiment supports Bohr’s Model of the atom.
Ques. Rutherford backscattering is an example of which type of collision? (1 Mark)
Ans. The Rutherford backscattering is an example of an elastic collision.
Ques. Under which conditions does a body undergo inelastic collision? (1 Mark)
Ans. The inelastic collision will occur when the two bodies whose kinetic energy is not conserved due to the internal friction undergo inelastic collision.
Ques. What is the amount of energy which is required to excite an electron? (2 Marks)
Ans. When considering the electron to be in the ground state, which will be having an energy of -13.6 eV. The next level of energy will be -3.4 eV. Thus, the amount of energy that is required to excite the same can be calculated using the following relation:
E2 - E1 = ( -3.14 eV) – ( -13.6 eV) = 10.2 eV
Ques. What causes the excitation of an electron? (2 Marks)
Ans. The excitation of an electron takes place under electrical influence. In this an original electron will take up the energy of the other electron and the fastest way to do the same is when we provide them with a high amount of temperature, as a result of which the collisions can occur among the atoms, which will eventually be exciting the electrons.
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