Ponderable: Activity - Let’s be Franck (Hertz)
WID 1132810 (15 minutes)
Franck_Hertz.py
How do atoms get excited in the first place? We can try shooting electrons at the atoms.
Show them the program 07_franckhertz.py, which is a simplified form of the FranckHertz experiment of 1914, a crucial proof that energy is quantized in atomic systems.
The positively and negatively charged plates apply to an electron a nearly constant
electric force (if the plates are large compared to distance between them; the display is
not to scale). We see the work done going into raising the kinetic energy. The kinetic
energy is plotted as a function of distance. The slider on the right adjusts the amount of
electric force applied, and thus how fast electron is moving when it hits a mercury atom.
Your job: “run” the experiment for various values of the electric force. What do you see?
Pay attention to observations vs. hypotheses (inferences you can make about the system
based on your observations). Discuss within group, then between groups. Finally, pick
one recorder from table to write the observations and hypotheses on wallboard.
We observe: At lower KE, nothing happens, electron goes straight through. At a critical
KE (about 5 eV), electron drops to zero KE when it collides. At higher KE, the difference
before and after collision is still 5 eV.
We conclude: The first excited energy level for Hg must be 5 eV above its ground state.
NOTE: If below critical, not enough KE to cause it to jump to higher level. If above
critical, atom can absorb some KE from electron—enough to make “quantum jump”, no
more—and electron has some KE left over.
If the electron has less than 4.9 eV of kinetic energy at the time it hits the mercury atom,
nothing happens. But if the kinetic energy is greater than 4.9 eV when the electron hits
the mercury atom, the electron can be observed to lose 4.9 eV. There are only certain
possible discrete bound-state energies possible for a mercury atom. The first such state
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above the normal lowest-energy “ground state” evidently lies 4.9 eV above the ground
state energy. If the electron has less kinetic energy than 4.9 eV, it cannot change the
bound-state energy of the mercury atom. (The actual experiment involved firing large
numbers of electrons at large numbers of mercury atoms in a low-density gas of mercury
and observing the energy-loss effects shown in the program.)
This is very different from what we observe with macroscopic systems such as planetary
orbits, where K+U for a bound state can have any value whatever – a continuous range of
possible bound-state energies. Discrete “energy levels” are found in all kinds of atomicsized systems. Point out that we have been talking about “electronic” energy levels, but
this isn’t the only kind of energy levels in atomic systems, as we'll see later.
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