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Title: GR8677.047
Post by: conantee on January 03, 2010, 03:57:38 PM
47. The Franck-Hertz experiment and related scattering experiments show that
(A) electrons are always scattered elastically from atoms
(B) electrons are never scattered elastically from atoms
(C) electrons of a certain energy range can be scattered inelastically, and the energy lost by electron is discrete
(D) electrons always lose the same energy when they are scattered inelastically
(E) there is no energy range in which the energy lost by electrons varies continuously.


Title: Re: GR8677.047
Post by: conantee on January 03, 2010, 04:07:03 PM
Even though you don't know the Franck-Hertz experiment, you can guess from choices that it is about quantum mechanics of atoms, possiibly Bohr's model of atoms. Electrons in atoms can be excited in discrete way (i.e., the energy used to excite atoms is at least equal to the energy difference between energy levels of atom).

In this problem, then, the elastic collision when atoms do not hold energy inside. In other words, there is no excitation so that all energy is still in the form of kinetic energy. Hence, (B) and (E) are wrong since elastic collision can happen where incoming electrons have energy less than minimum requirement for excitation.

Then, in case of inelastic collision, where atoms are excited, (B) and (D) are wrong since atoms can be excited, and can be excited in different ways (for example, from 1st to 2nd level, or from 2nd level to 3rd level).

The answer is then (C). It also fits Bohr's model of atoms.

The correct answer is (C). 46 of 100 got the correct answer.