Electrically excited mercury atoms have particularly strong emission of 4.9 eV photons, corresponding to a transition of one of the atom’s electrons from a higher energy state to a lower. Mercury vapor absorbs light of this wavelength as well; the energy of the photon moves an electron from the lower state to the higher.
It’s also possible to produce an excitation through other means. If an electron with a kinetic energy of 4.9 eV strikes a mercury atom, it can transfer energy, moving an electron in the mercury atom from the lower level to the higher. The electron loses kinetic energy in the process; this is an inelastic collision. Electrons with kinetic energies lower than this transition energy undergo elastic collisions, leaving their kinetic energy unchanged.
This was the idea behind the Franck-Hertz experiment, a classic experiment of early-20th-century physics. The basic setup is illustrated in Figure P28.88a. A tube is filled with mercury vapor. A heated electrode emits slow-moving electrons, and a variable power supply provides a voltage to accelerate the electrons toward a second electrode.                                  The current varies as the voltage is changed, as shown in Figure P28.88b. As the voltage is increased, this initially leads to an increased current. At a certain point, the electrons have enough energy to excite the mercury atoms, and the collisions lead to a loss in the energy of the moving electrons and a reduction in current, a clear demonstration of the existence of quantized energy levels in the mercury atom.
In modern versions of this experiment performed in student laboratories, the onset of inelastic collisions that excite the mercury atoms leads to a visible glow in the tube. This is illustrated in Figure P28.88a. The position of the glowing gas shows the location in the tube where the accelerating electrons reach the proper energy.

If the electrons that lose energy to a mercury atom continue to accelerate, they can acquire enough kinetic energy to excite another atom, leading to a drop in current. At what voltage does the drop in current begin?
A. 4.9 V        B. 7.4 V
C. 9.8 V        D. 14.7 V

Electrically excited mercury atoms have particularly strong emission of 4.9 eV photons, corresponding to a transition of one of the atom’s electrons from a higher energy state to a lower. Mercury vapor absorbs light of this wavelength as well; the energy of the photon moves an electron from the lower state to the higher.
It’s also possible to produce an excitation through other means. If an electron with a kinetic energy of 4.9 eV strikes a mercury atom, it can transfer energy, moving an electron in the mercury atom from the lower level to the higher. The electron loses kinetic energy in the process; this is an inelastic collision. Electrons with kinetic energies lower than this transition energy undergo elastic collisions, leaving their kinetic energy unchanged.
This was the idea behind the Franck-Hertz experiment, a classic experiment of early-20th-century physics. The basic setup is illustrated in Figure P28.88a. A tube is filled with mercury vapor. A heated electrode emits slow-moving electrons, and a variable power supply provides a voltage to accelerate the electrons toward a second electrode.                                  The current varies as the voltage is changed, as shown in Figure P28.88b. As the voltage is increased, this initially leads to an increased current. At a certain point, the electrons have enough energy to excite the mercury atoms, and the collisions lead to a loss in the energy of the moving electrons and a reduction in current, a clear demonstration of the existence of quantized energy levels in the mercury atom.
In modern versions of this experiment performed in student laboratories, the onset of inelastic collisions that excite the mercury atoms leads to a visible glow in the tube. This is illustrated in Figure P28.88a. The position of the glowing gas shows the location in the tube where the accelerating electrons reach the proper energy.

Approximately how fast must an electron move to excite a mercury atom in a collision?
A. 9.3 x 10⁵ m/s          B. 1.3 x 10⁶ m/s
C. 1.9 x 10⁶ m/s          D. 2.6 x 10⁶ m/s