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# The Franck-Hertz Experiment

by | Oct 20, 2022 | Physics, Experiments

Contents

## Introduction

In the Franck-Hertz Experiment experiment, we will be testing the hypothesis that the energy absorbed by the excited atom is quantised.

## Aim

To determine whether the energy absorbed by the excited state of an atom is quantised.

## Basic building concept

1. The packages of energy are called photons. They correspond to different types of electromagnetic radiation, for instance, radio waves, X- rays, microwaves etc. and to different colours in the spectrum.

2. The fact that the energy occurs as discrete packets known as photons at the subatomic level. And this fact is called the quantisation of energy.

3. A unique amount of energy is contained in each photon. For example, the specific energy value of blue photons is different from the specific energy value of red photons.

4. E=hf is the formula, where E denotes the energy, h denotes the Planck’s constant, whose value is 6.62 X 10^(-36) m^2kg/sec, and f denotes the frequency.

5. The energy of the universe is quantised and regulated by Planck’s constant.

## Theory

1. The first electrical measurement that showed the quantum nature of an atom and transform our understanding of the universe is done by the Franck-Hertz experiment

2. This experiment was performed by James Franck and Gustav Hertz.

## Requirements

1. Thyratron tube, filled with a small amount of gas to be excited.

2. Cathode, to heat and ‘boil electrons off’ of the cathode,

3. A filament,

4. An anode that has a slight retarding voltage,

5. A positively charged control grid,

6. Graph paper,

7. Oscilloscope.

## Procedure

Step 1: Set up your instruments to perform this experiment.

Step 2: Measure the current on the anode by increasing the grid potential.

Step 3: On a graph, draw two axes- the X-axis and Y-axis.

Step 4: On the Y-axis, plot the measured current. And on the X-axis, plot the corresponding voltage.

Step 5: If you want, you could also do this using an oscilloscope.

## Observation

1. We observed that the graph of anode current and grid potential is obtained as an up-down profile of a peak, a dip, a higher peak, a dip, and so forth.

2. The limitations on the design of the thyratron tube inherit the maximum grid potential that can be applied for a sustained period.

3. While using the oscilloscope, a sudden increase in grid potential yields nine dips before the ionisation of the gas. This yields an unchanging and sharp increase in the anode current.

## Result

1. In our result, we find that our hypothesis is correct.

2. Linear and steady increase in the anode current would be seen if we excite the gaseous atoms in non-quantised amounts.

3. The dips and peaks in the graph occur because, at first, the current increases on increasing the potential grid. Then it decreases because the colliding electrons have enough energy to excite other electrons but do not have enough energy to overcome the retarding potential of the anode and reach the anode.

## Precaution

1. All the devices should be working properly.

2. Carefully plot the graph.

## Conclusion

In this way, we have determined whether the gaseous atoms excite in quantised amounts.

Q.1 What was the hypothesis of your experiment?

ANS. To determine whether the energy absorbed by the excited state of an atom is quantised.

Q.2 Describe the graph you obtained during this experiment?

ANS. The graph of anode current and grid potential is obtained as an up-down profile of a peak, a dip, a higher peak, a dip, and so on and so forth.

Q.3 Why are there peaks and dips in your graph?

ANS. The dips and peaks in the graph occur because, at first, the current increases on increasing the potential grid, then it decreases because the colliding electrons have enough energy to excite other electrons but do not have enough energy to overcome the retarding potential of the anode and reach the anode.

Q.4 What do you understand about the quantisation of energy?

ANS. The fact that the energy occurs as discrete packets known as photons at the subatomic level. And this fact is called the quantisation of energy.

ANS. Yes, our hypothesis is correct. And a linear and steady increase in the anode current would be seen if we excite the gaseous atoms in non-quantised amounts.

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