Explain an example in Lecture

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Helen Mejia 1I
Posts: 67
Joined: Fri Sep 28, 2018 12:24 am

Explain an example in Lecture

Postby Helen Mejia 1I » Fri Oct 12, 2018 2:07 pm

Hi confused why a number (4.09x10^-19J) that is negative was used as positive in a further step. Why did it change from negative to positive?

The professor gave an example that said, "Calculate the frequency of light emitted by a hydrogen atom when an electron makes a transition from the fourth to the second principal quantum level"

E_4=(-1/16)(6.626x10^-34Js)(3.29x 10^15s^-1)= -1.36x 10^-19J
E_2=(-1/4)(hR)= -5.45x10^-19J
Change in energy: (-5.45x10^-19J)-(-1.36x 10^-19J)= -4.09x10^-19J

Electron's energy is negative because it has decreased.

To calculate the frequency:
Frequency= (4.09x10^-19J) / (6.626x10^-34Js) = 6.17x 10^14s^-1 or 6.17 x 10^14 Hz

Why did the 4.09x10^-19J (energy of the electron) become positive?

Cecilia Jardon 1I
Posts: 74
Joined: Fri Sep 28, 2018 12:16 am

Re: Explain an example in Lecture

Postby Cecilia Jardon 1I » Fri Oct 12, 2018 2:24 pm

The reason why the energy becomes positive is that when an electron releases energy it releases a photon of electromagnetic energy which is positive. So when something is absorbed it is positive. That is because due to the law of conservation of energy, energy can not be destroyed. Hopefully, that makes sense. :)

MaggieMatern_Dis1H
Posts: 30
Joined: Fri Sep 28, 2018 12:19 am

Re: Explain an example in Lecture

Postby MaggieMatern_Dis1H » Fri Oct 12, 2018 4:14 pm

This is because the change in energy is negative, as the electron went from a higher energy level (n=4) to a lower energy level (n=2), and therefore the difference is negative. Echange=Efinal-Einitial.

The energy becomes positive in the next step however, because photons cannot have negative energy. The previous step referred to change in energy or delta E, but in this step it is energy of the photon.

Hope this helps!

MaggieMatern_Dis1H
Posts: 30
Joined: Fri Sep 28, 2018 12:19 am

Re: Explain an example in Lecture

Postby MaggieMatern_Dis1H » Fri Oct 12, 2018 4:14 pm

This is because the change in energy is negative, as the electron went from a higher energy level (n=4) to a lower energy level (n=2), and therefore the difference is negative. Echange=Efinal-Einitial.

The energy becomes positive in the next step however, because photons cannot have negative energy. The previous step referred to change in energy or delta E, but in this step it is energy of the photon.

Hope this helps!


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