**mastering physics 13**

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## Question “mastering physics 13”

An atom of massis initially at rest, in its ground state. Amoving(nonrelativistic) electron of mass collides with theatom. The atom+electron system canexist in an excited state in which the electron is absorbed intothe atom. The excited state has an extra, “internal,”energyrelative to the atom’s ground state.(Intro1figure)

A)Find the kinetic energy that the electron must have inorder to excite theatom.Express your answer in terms of,, and .

We can use the result from Part A to study a process of interestin atomic physics: a collision of two atoms that causes one of theatoms to ionize (lose anelectron). In this case, is the energy needed to ionize one ofthe atoms,called the *ionization energy*. The most efficient way toionize an atom in a collision with another atom is for thecollision to be completelyinelastic (atoms stick together afterthe collision). If the collision were perfectly elastic, thentranslational kinetic energy would be conserved, and therewould beno energy left over for exciting the atom. If the collision werepartially elastic, then some of the initial kinetic energy would beconverted intointernal energy, but not as much as in a perfectlyinelastic collision. In practice, interatomic collisions are neverperfectly inelastic, but analyzing thiscase can give a lower boundon the amount of kinetic energy needed for ionization.

Is it possible to ionize an atom of , initially at rest, by a collision with an atom of that has kinetic energy of 4.0 electron volts? The ionization energy of thecesium atom is 3.9 electronvolts.

You can take the mass of the oxygen atom to be 16 atomic massunits and that of the cesium atom to be 133 atomic mass units. Itdoesn’t matter what mass unitsyou choose, as long as you areconsistent. For this question, it is most convenient to use atomicmass units, since these are the numbers you are provided with.

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Part CWhat is the least possible initial kineticenergy theoxygen atom could have and still excite the cesiumatom?Express your answer in electronvolts, to one decimal place.

## Answer

### Answer 1

### Answer 2

### Answer 3

To be in an excited state, KE= 1/mV2 must be greater than E

### Answer 4

## Conclusion

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