## Lecture 7 Question [ENDORSED]

H-Atom ($E_{n}=-\frac{hR}{n^{2}}$)

clairehathaway 2J
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Joined: Wed Sep 30, 2020 9:38 pm

### Lecture 7 Question

In today's lecture when working through the example asking for the frequency of light emitted by a hydrogen atom going from n=4 to n=2, we use the equation $\nu = \frac{E}{h}$ and when we input the energy value (E) we change it from negative to positive (the negative value was equal to delta E). Dr. Lavelle says that the sign change because the change in energy for the electron and for the photon it's equal and opposite.
I still don't completely understand that idea, why were we able to change the negative sign to a positive one? Also why do we need to change it?

Lucy Wang 2J
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### Re: Lecture 7 Question

The negative sign simply is to show that this is the amount of energy LOST by the electron. We changed it to a positive sign to show the amount of energy being ABSORBED by the photon or turned into light. This works because according to the conservation of energy, energy cannot be created or destroyed. So basically, while the electron is losing this energy, it is passing it on to the photon which allows the energy lost from the electron to be shown as light.

To give an analogy, its like making a payment to someone. In your account (electron) it may show -$2.00, but in their account (photon) it would be +$2.00. lol not sure if that makes sense but I hope it was helpful

Nayra Gharpetian 3F
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### Re: Lecture 7 Question  [ENDORSED]

yes the analogy is a perfect example. the negative sign is to show that the electron is losing energy, but we change it to a positive to calculate for wavelength and frequency, etc. basically to prove the conservation of energy.

Sana Nagori 2H
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### Re: Lecture 7 Question

Lucy Wang 1A wrote:To give an analogy, its like making a payment to someone. In your account (electron) it may show -$2.00, but in their account (photon) it would be +$2.00. lol not sure if that makes sense but I hope it was helpful

This analogy is very helpful, Thanks Lucy!

clairehathaway 2J
Posts: 101
Joined: Wed Sep 30, 2020 9:38 pm

### Re: Lecture 7 Question

Lucy Wang 1A wrote:The negative sign simply is to show that this is the amount of energy LOST by the electron. We changed it to a positive sign to show the amount of energy being ABSORBED by the photon or turned into light. This works because according to the conservation of energy, energy cannot be created or destroyed. So basically, while the electron is losing this energy, it is passing it on to the photon which allows the energy lost from the electron to be shown as light.

To give an analogy, its like making a payment to someone. In your account (electron) it may show -$2.00, but in their account (photon) it would be +$2.00. lol not sure if that makes sense but I hope it was helpful

That makes so much sense, thank you so much!