# Homework Help: Kinetic Enegry and Hydrogen Atoms

1. Oct 30, 2014

### ljucf

1. The problem statement, all variables and given/known data

(a) What is the minimum kinetic energy in electron volts that an electron must have to be able to ionize a hydrogen atom (that is, remove the electron from being bound to the proton)? Answer: 13.6 eV

(b) If electrons of energy 12.8 eV are incident on a gas of hydrogen atoms in their ground state, what are the energies of the photons that are emitted by the excited gas?
Energy of highest-energy photon: 12.8 eV
Energy of next highest-energy photon: 12.089 eV
Energy of next highest-energy photon: 10.2 eV
Energy of next highest-energy photon: 2.6 eV
Energy of next highest-energy photon: 1.9 eV
Energy of lowest-energy photon: ?? eV

(c) If instead of electrons, photons of all energies between 0 and 12.8 eV are incident on a gas of hydrogen atoms in their ground state, what are the energies at which the photons are absorbed?
Energy of highest-energy dark line: 12.8 eV
Energy of next highest-energy dark line: 12.089 eV
Energy of lowest-energy dark line: 10.2 eV

2. Relevant equations

-13.6/n2

3. The attempt at a solution

I thought you would take 12.8-12.089 = .711 eV, however it states this is wrong. I have tried answers 0, .7, .711, and .6. Not sure why it's wrong.

2. Oct 30, 2014

### Simon Bridge

OK - so how was the highest energy photon generated?

3. Oct 30, 2014

### ljucf

Due to the fact that the questions says 12.8eV is the ground state, wouldn't that result in 12.8 being the highest energy photon?

4. Oct 30, 2014

### Simon Bridge

The question does not say that 12.8eV is the ground state.
What is the ground state energy of a hydrogen atom?

But you didn;t answer the question: what is the mechanism that generates the photons?

BTW: are those numbers given to you are did you work them out?

5. Oct 30, 2014

### ljucf

En=K+Ue=-13.6eV/N2

-13.6/No2+13.6=12.8

No2= -13.6/0.8= -17
√(17) ≈ 4

-13.6/42+13.6=12.75

6. Oct 30, 2014

### ljucf

I worked them out.

7. Oct 30, 2014

### Simon Bridge

That's just a bunch of equations - please use English to explain what you are doing.
What is the physical process you are thinking of ... an electron flies through the gas, it encounters a H atom, and then what does it do?

8. Oct 30, 2014

### ljucf

I have know idea...I am using the equations that were given in class.

9. Oct 30, 2014

### ljucf

I solved it.

-13.6/42+13.6/32=.66

10. Oct 31, 2014

### Simon Bridge

Well great - but I have no idea what you did because you have not used an physics.
It sounds like you don't know either. This does not bode well for the future - when there will be no equations given in class.