bentzy said:My predecessor's reply isn't clear. Is it a question, or an emphasis of some sort ?
Parlyne said:If we run electrons through the Frank-Hertz tube without the Hg gas, we'll find the electrons' kinetic energy is pretty much a linear function of how far they've traveled down the tube from the cathode. This means that the electrons must travel a certain distance to gain a given amount of kinetic energy. If the driving voltage is V, and the tube length is L, the electron's energy when it's at point x should be [itex]E = e V \frac{x}{L}[/itex].
If the energy difference between the first and second excited states of Hg is [itex]E_{12}[/itex], the electron will need to travel a distance [itex]d = L \frac{E_{12}}{e V}[/itex] down the tube after it already has enough energy to excite an atom to the first excited state to be able to excite one to the second excited state. However, if this electron collides with an atom before it travels this extra distance, it is likely to excite that atom to the first excited state and, thus, never have enough energy to excite and atom to the second excited state.
This means that, in order to cause a large enough number of transitions to the second excited state to detect, it must not be overwhelmingly likely that the electron encounters and atom before it travels a distance d. The typical distance an electron can travel in the gas before a collision is called the "mean free path." So, if the mean free path is much smaller than d, it should be rare that any atom gets excited to the second excited state by a single electron.
The Frank-Hertz tube is generally designed to have a rather short mean free path, unlike the low pressure tubes used for spectroscopy.
sah-sah said:in frank hertz expriment with Hg vapor,in our digram I(current) in terms of accelerated potential, we have some sharp drops,that each drop corresponds with integral multiple of excitation energy, means V=n.(4.9ev).where n is integer.
how to explain it?
MartinV05 said:I don't think you understood his question correctly. This has been troubling me too. Let's say an electron with 5.4 eV collides with an Hg atom and the energy needed to move an electron from the ground level to the second level is 5.4 eV. Why will the electron continue to move with speed reduced by 4.86 eV (the one that the atom of Hg absorbs) and not absorb it all to move the ground level electron to the second excited level.
No, not if the electron collides inelastically with a mercury atom before getting to that energy. The electron loses 4.9 eV of kinetic energy when such a collision happens, that is why the electron doesn't (or very rarely) gets to a high enough energy to excite the next excited state.MartinV05 said:... in one moment it WILL have the same amount of energy as the one needed to move it to the second level.
The Frank-Hertz experiment is a classic physics experiment that aims to demonstrate the quantization of energy levels in atoms. It involves passing an electric current through a tube filled with mercury vapor and measuring the energy of the electrons that are emitted from the vapor.
In the Frank-Hertz experiment, a high voltage is applied to the tube filled with mercury vapor, causing the electrons in the vapor to gain energy and move towards a positively charged electrode. As they move, they collide with the mercury atoms, transferring energy and causing the atoms to emit light. By measuring the energy of the emitted light, scientists can determine the energy levels of the mercury atoms.
Energy levels in atoms refer to the specific energies that electrons can have within an atom. These energies are quantized, meaning they can only exist at certain discrete values. The lowest energy level is known as the ground state, and higher energy levels are known as excited states. When an electron moves from a higher energy level to a lower one, it emits a photon of light.
The energy level of mercury vapor is crucial in the Frank-Hertz experiment because it determines the energy of the emitted electrons. If the energy level is too low, the electrons will not have enough energy to overcome the potential barrier and reach the positive electrode. On the other hand, if the energy level is too high, the electrons will collide with multiple atoms, making it difficult to measure their energy accurately.
The Frank-Hertz experiment is significant because it provided strong evidence for the quantization of energy levels in atoms, which is a fundamental principle of quantum mechanics. It also helped to confirm the existence of discrete energy levels in atoms and paved the way for further research in the field of atomic physics.