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### Help on Self-Test 1.8A

Posted: **Thu Oct 12, 2017 5:28 pm**

by **Amanda Hagen 1L**

I am having trouble with the Self-Test 1.8A problem on page 24 of Chapter 1. It states: "Use the same model for helium (referring to the example above) but suppose that the box is 100.pm long, because the atom is smaller. Estimate the wavelength of the same transition." In the example above the electron fell from N=2 to N=1.

I know that I need to use the equation: E2- E1= (2n +1) h^2/8meL^2 but I am not getting the correct answer. Can anyone help?

### Re: Help on Self-Test 1.8A

Posted: **Thu Oct 12, 2017 7:38 pm**

by **EveEmm3D**

I just did it and got the right answer. What I did was solve for the wavelength first and plug in the numbers at the end, just like they do in the example. I found that in problems like this you can end up with the wrong final answer if you plug in numbers earlier to calculate energy, frequency etc. Not keeping the correct number of s.f. or rounding errors can lead to a wrong final answer.

### Re: Help on Self-Test 1.8A

Posted: **Thu Oct 12, 2017 7:53 pm**

by **Amanda Hagen 1L**

So I solved for wavelength to equal (8*Me*C*L^2)/3h but I am still getting the answer wrong. What values did you use?

### Re: Help on Self-Test 1.8A

Posted: **Thu Oct 12, 2017 8:06 pm**

by **EveEmm3D**

For c, me and h the same as the example above. L^2 comes out to 10^-20 m^2. Is that what you have for L^2?

### Re: Help on Self-Test 1.8A

Posted: **Thu Oct 12, 2017 8:21 pm**

by **Amanda Hagen 1L**

I think I just forgot to square L! Thank you so much!