Using a rounded speed of light in EM problems


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005199750
Posts: 32
Joined: Fri Sep 28, 2018 12:27 am

Using a rounded speed of light in EM problems

Postby 005199750 » Sun Oct 21, 2018 8:58 pm

Hey y'all! As we all know, we have to use the equation "wavelength x frequency = speed of light" quite a lot in this topic. When I was solving problem #1A.9, I was questioning whether I was using the speed of light properly because I rounded 2.998 x 10^8 m/s to 3 x 10^8 m/s. Because of this, I'm not sure if this means that I have to limit my final answer to 1 sig fig vs. a different number of sig figs. For instance, if I was solving for the frequency of the EM radiation with a wavelength of 2.55 nm, would I limit my final answer to what the freuqncy is to 3 sig figs (because of 2.55 nm), or would I limit it to one sig fig (because of 3 x 10^8 m/s)? Or, as a third option, should I write 3 x 10^8 m/s as something like 3.0 x 10^8 m/s to give it additional sig figs? Thanks so much!

Maya_Peterson1C
Posts: 62
Joined: Fri Sep 28, 2018 12:28 am

Re: Using a rounded speed of light in EM problems

Postby Maya_Peterson1C » Sun Oct 21, 2018 9:19 pm

If the speed of light was given at 2.998 x 10^8 and you want to round it, I would use 3.000 x 10^8 because it has the same amount of sig figs but would require less digits to input in the calculator. Using that value with 4 sig figs and given a value 2.55 nm with 3 sig figs, your answer should have 3 sig figs in the end. There may be other ways of doing this, but that would be how I would approach the problem.

inlovewithchemistry
Posts: 104
Joined: Fri Sep 28, 2018 12:19 am

Re: Using a rounded speed of light in EM problems

Postby inlovewithchemistry » Wed Oct 24, 2018 11:01 pm

I always have literally typed 3E8 in my calculator and have never had a problem with it. Also, if you have a TI 36 calculator, there is a constants section that has the full number in there, so I use that a lot too.

KatelinTanjuaquio 1L
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Joined: Fri Sep 28, 2018 12:16 am
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Re: Using a rounded speed of light in EM problems

Postby KatelinTanjuaquio 1L » Thu Oct 25, 2018 12:41 am

Hello! On the formula/constant sheet, speed of light is given as 2.99792*10^8, so I would use that. I don't think it makes that much of a difference but it ensures your answer is precise as possible before rounding off using sig figs.

Sana_Mian_3G
Posts: 30
Joined: Fri Sep 28, 2018 12:23 am

Re: Using a rounded speed of light in EM problems

Postby Sana_Mian_3G » Fri Nov 02, 2018 10:41 am

I also have the same problem because in class we used 3.00, but on the constant sheet another value is given for the speed of light. On the test I used 3.00 and got the right answer so I think this value would be easier to use when using a calculator.


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