Change in E +/-


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Athena L 1B
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Joined: Fri Sep 28, 2018 12:18 am

Change in E +/-

Postby Athena L 1B » Fri Oct 12, 2018 8:30 pm

Today in lecture we went through a problem where the change in E was negative because an electron went from a higher energy level to a lower energy level. I understand why that is negative, but then when we went on to find the frequency (using E=hv), we made E positive. I also understand that frequency can't be negative, so it makes sense that it should end up positive, but do you just have to know that when finding frequency E can't be negative? Is there other reasoning for that I'm missing?

Lumbini Chandrasekera 1B
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Joined: Fri Sep 28, 2018 12:18 am

Re: Change in E +/-

Postby Lumbini Chandrasekera 1B » Fri Oct 12, 2018 8:52 pm

For that problem in the lecture, it was to find the frequency of the light being released and since the energy it contained must be positive, you solve for frequency by plugging in a positive energy value. You just have to know that you make the energy positive before solving for frequency because although change in energy is negative because energy is lost by the system, the energy of the light is positive.

megansardina2G
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Re: Change in E +/-

Postby megansardina2G » Fri Oct 12, 2018 9:02 pm

I don't think you're missing a specific reason other than frequency, by definition, cannot be negative, since frequency is the number of waves per second. The purpose of the negative sign in this example isn't really about numerical values, but to distinguish between gaining energy (+) and losing energy (-) and doesn't affect the calculations. Therefore, you can ignore the negative sign when finding the frequency when using a negative change in energy.

Sean Reyes 1J
Posts: 67
Joined: Fri Sep 28, 2018 12:24 am

Re: Change in E +/-

Postby Sean Reyes 1J » Sat Oct 13, 2018 11:22 am

When an electron moves from a higher energy level to a lower energy level, the change in energy is negative, which you understood.
The reason we used a positive value when using the equation, E=hv, is because we were using the energy of the photon emitted. When a photon is released, the photon will always have a value that is equal but opposite to the energy of the electron.


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