Last updated on Friday, March 31st, 2023

Atom is neutral, consists electrons revolving around nucleus at its center. This experiment was performed in 1914 by German scientists James Franck and Gustav Hertz.

Atom is Objective of the Franck-Hertz Experiment

The purpose of this experiment is to determine the discrete nature of the atomic energy or quantized energy of atoms. Yes! you can use any gas in the tube whether that is mercury or neon atoms.  As I said earlier its aim was to prove and show the discrete nature of the quantum energy of atoms, according to the Bohr’s atomic model in atomic physics.

In the experiment you can use either neon or mercury or any other available gas filled tube. The original experiment was performed with the mercury. There are some sections in this experiment;

1. Filament
2. Cathode
3. Grid
4. Plate

My viva questions are based on the experiment components.

Viva Questions

1. What is the role of filament in this tube?
2. Why do we put cathode plate on the filament or you can say why do we heat up the cathode?
3. What is thermionic process of atoms?
4. Does electrons emit from the filament or cathode?
5. Which terminal is connected with cathode, positive or negative?
6. Can you specify the physical shape of the grid?
7. Is the grid a net type or like a stick in the tube?
8. What type of potential is given to the Grid?
9. Plate P or Anode is for collection of electrons or emission of electrons?
10. If this is for collection the electrons, then why it is connected with negative terminal of the battery ?

The above image is for the Franck-Hertz experiment. There are three batteries one is connected with the filament, second with the Grid and third one with the plate. Please check the range of the voltage difference and terminals carefully.

1. In this picture grid voltage can vary upto 80 Volts while plate voltage upto the 10 Volts. Why is this too much difference?
2. There are two grids; first one near to the cathode known as “control grid”, what is the name of second one grid that is near to the anode?

READ ALSO: Geiger-Mueller Experiment

Current with applied voltage

Now the next segment of the questions that is related to the output in terms of the current with applied voltage.

On the X-axis applied voltage (Volts) and on Y-axis beam current (mA) are shown. Please check the behaviour of graph.

1. In the beginning when voltage increases current also increases. But when voltage reaches at a particular value, the beam current starts to decrease. Again after falling a minimum value of the current it starts to increase with voltage. Why it so happening?
2. The peak value of the current are in integral multiple of the first voltage that is 4.9 volts. What it indicates?

Now some questions are related to this picture that depicts luminescence between the control and plate grid.

Photoluminescence Related Viva Questions

1. 1. In this section first question is about the kinetic energy of the electrons emitted by thermionic process. Can you explain about kinetic energy of the electron how they can reach upto the plate?
   2. Okay I ask this question in this way; As you are looking that both the grids are connected with the positive terminal of the battery. The plate grid is connected with the variable battery and you can increase its potential relative to the plate itself by eight times. So when plate grid is at high potential and plate itself at lower potential what it signifies and what will be the behaviour of the electrons in this situation?
   3. Does kinetic energy of the electron depends on the potential difference between the grid and cathode? If your answer is yes!! then how?

2. Atoms Related Viva Questions

   1. An electron is accelerated in an electric discharge tube by the potential difference in between cathode and anode plate, which formula will be correct for the kinetic energy of the electron?   (i) eV  (ii) 1/2 mv^(2) ;this is one by two m v square
   2. What do you mean by elastic and inelastic Collison of atoms?
   3. In an electric discharge tube or a vacuum tube, vapour of mercury is filled. When an accelerated electron passes through the vapour it collides with the mercury atoms. As a result mercury atoms goes into the excited state and emits radiation after de-excitation. Why do we observe different levels of light (radiation) in the tube?
   4. Okay this question will clear the concept. You know the electrons emitted  from the cathode and reaches at the plate. If there is no loss of electrons energy in between these we will observe actual current accordingly. But if somehow an electron loses the energy in in-elastic Collison the energy get reduced and you will notice a shortfall in the beam current. Does the peak and valley in the above picture shows electrons reach at the plate without collision and losing energy after collision (an inelastic collision)?

Kinetic Energy Related Viva Questions

1. Can you define the decrease in velocity by the maths equation?
2. If V is the potential difference and eV is that energy which is required to accelerate the electron in a tube. Then what will be the meaning of this => eV – E ; where E is the amount of energy transferred to the mercury atom to excite.
3. In the above case, if the term (eV- E) value is relatively less than the value of the plate, does it will attract the electrons?