Frank-Hertz Experiment
Introduction
Why did Franck and Hertz use mercury vapor instead of hydrogen gas?
James Franck
Gustav Ludwig Hertz
- Gustav Ludwig Hertz (22 July 1887 – 30 October 1975) was a German experimental physicist and Nobel Prize winner.
- James Franck (26 August 1882 – 21 May 1964) was a German physicist.
- In 1925, Franck received the Nobel Prize in Physics.
Hydrogen atoms combine in pairs to form hydrogen molecules. Therefore, some of the energy lost in inelastic collisions of the electrons with a hydrogen gas would result from separating the hydrogen molecules into atoms and this would complicate the analysis of the measured collector current. A mercury gas consists of single atoms.
The Experiment
- In 1914, the German physicists James Franck and Gustav Ludwig Hertz sought to experimentally probe the energy levels of the atom.
- They performed an experiment which demonstrated the existence of excited states in mercury atoms.
- The Franck–Hertz experiment confirmed Niels Bohr's quantized model of the atom by demonstrating that atoms could indeed only absorb (and be excited by) specific amounts of energy (quanta).
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The Experiment - Apparatus
The Experiment - Working
Observations and Findings
Instruments were fitted to measure the current passing between the electrodes, and to adjust the potential difference (voltage) between the cathode (negative electrode) and the accelerating grid (anode).
At 4.9 volts the current drops sharply, almost back to zero.
The current increases steadily once again if the voltage is increased further, until 9.8 volts is reached (exactly 4.9+4.9 volts)
At 9.8 volts a similar sharp drop is observed.
This series of dips in current at approximately 4.9 volt increments will visibly continue to potentials of at least 100 volts.
At low potential differences, up to 4.9 volts when the tube contained mercury vapour—the current through the tube increased steadily with increasing potential difference. The higher voltage increased the electric field in the tube and electrons were drawn more forcefully towards and through the accelerating grid.
The classic experiment involved a tube containing low pressure gas(Mercury Vapour) fitted with three electrodes:
1- An electron-emitting cathode
2- A mesh grid(anode)
3- A Collecting Plate
- When the accelerating potential reached 4.9 volts, each free electron possessed exactly 4.9 eV of kinetic energy when it reached the grid.
- Consequently, a collision between a mercury atom and a free electron at that point could be inelastic: that is, a free electron's kinetic energy could be converted into potential energy by raising the energy level of an electron bound to a mercury atom: this is called exciting the mercury atom.
- With the loss of all its acquired kinetic energy in this way, the free electron can no longer overcome the slight negative potential at the ground electrode, and the measured current drops sharply.
1- Electrons are emitted from the hot cathode (Thermionic Emission)
- As the voltage is increased, electrons will participate in one inelastic collision, lose their 4.9 eV, but then continue to be accelerated.
- In this manner, the current rises again after the accelerating potential exceeds 4.9 V.
- At 9.8 V, the situation changes again. Each electron now has just enough energy to participate in two inelastic collisions, excite two mercury atoms, and then be left with no kinetic energy.
- Once again, the observed current drops. At intervals of 4.9 volts this process will repeat; each time the electrons will undergo one additional inelastic collision.
The collecting plate was held at a slightly negative electrical potential relative to the grid (although positive compared to the cathode), so that electrons had to have at least a corresponding amount of kinetic energy to reach it after passing the grid.
Electrons are accelerated in the Franck-Hertz apparatus and the collected current rises with accelerated voltage.
2- Electrons emitted by the cathode are accelerated by voltage Va as they move toward the anode.
3- At low potentials, the accelerated electrons acquired only a modest amount of kinetic energy. When they encountered mercury atoms in the tube, they participated in purely elastic collisions.
4- The accelerated electrons will pass through the perforated anode and will be measured at the collector.
The Frank-Hertz Setup
-Aditya Shailaj