question on homework #7


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Kaylee Nezwek_2G
Posts: 30
Joined: Wed Sep 30, 2020 10:02 pm

question on homework #7

Postby Kaylee Nezwek_2G » Sun Oct 25, 2020 6:09 pm

On weeks 2-4 homework, question 7 asks to calculate the max wavelength of radiation that will eject e- from a metal that has a work function of 4.78*10^-19 J. I used E=hc/wavelength, isolating wavelength = (6.626*10^-34 J/s)(3.00*10^8 m/s) / (4.78*10^-19). I got 4.16 * 10^-7 m, which we are asked to convert to angstroms. I got 4.16 * 10^-17 angstroms, but its still saying my answer is incorrect. Could someone help me out with whatever I'm doing wrong?

Kaylee Messick 1F
Posts: 42
Joined: Wed Sep 30, 2020 9:54 pm

Re: question on homework #7

Postby Kaylee Messick 1F » Sun Oct 25, 2020 6:16 pm

The unit angstrom is 10^-10 m, so from 4.16 x 10^-7,which is the answer that you calculated, move the decimal over until it reaches 10^-10. Hope this helps!

Nathan Tong 1B
Posts: 40
Joined: Wed Sep 30, 2020 9:49 pm

Re: question on homework #7

Postby Nathan Tong 1B » Sun Oct 25, 2020 6:44 pm

It might be helpful to write out the dimensional analysis for the conversion. (4.16x10-7m)(1 angstrom/10-10m). Also remember to double check that the answer makes logical sense. If you convert from meters to angstroms, you would expect to get more angstroms than meters, since they are much smaller.

Arya Adibi 1L
Posts: 46
Joined: Wed Sep 30, 2020 9:41 pm

Re: question on homework #7

Postby Arya Adibi 1L » Sun Oct 25, 2020 6:46 pm

The value you got in meters is correct. To convert to angstroms, you can just divide by 10^-10 m, which would result in a multiplication of 10^10 m. Basically you will have 4.16 * 10^3 A

Kaylee Nezwek_2G
Posts: 30
Joined: Wed Sep 30, 2020 10:02 pm

Re: question on homework #7

Postby Kaylee Nezwek_2G » Sun Oct 25, 2020 9:01 pm

Thank you everyone!!

Ansh Patel 3I
Posts: 42
Joined: Wed Sep 30, 2020 9:42 pm

Re: question on homework #7

Postby Ansh Patel 3I » Sun Oct 25, 2020 9:13 pm

You almost got it! 1 angstrom is equivalent to 10^-10 meters, so simply divide your value of 4.16*10^7 meters by 10^-10 meters to get your answer (4.16*10^3 A).

Ayesha Aslam-Mir 2E
Posts: 52
Joined: Wed Sep 30, 2020 9:43 pm

Re: question on homework #7

Postby Ayesha Aslam-Mir 2E » Sun Oct 25, 2020 9:29 pm

I'm pretty good at making small mistakes in unit conversions; I find it helps if I just set up the conversion with a dimensional analysis-type setup so the units cancel and I plug into the calculator so I don't make an error with moving the decimal space. Sounds lowkey convoluted or lazy of me but that's how I'm able to avoid those types of errors!


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