## E=hv vs. E=1/2mv^2

Libby Dillon - 1A
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Joined: Fri Oct 02, 2020 12:17 am
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### E=hv vs. E=1/2mv^2

When are we supposed to use E=hv, and when are we supposed to use E=1/2mv^2? Is E=1/2mv^2 only used for electrons, or can either be used? I am having some trouble for certain problems because I think I am using the wrong equation.Thanks!

Claire_Kim_2F
Posts: 67
Joined: Wed Sep 30, 2020 10:02 pm

### Re: E=hv vs. E=1/2mv^2

You are suppose to use E=hv with protons because they do not have mass thus the m is missing from the equation. While for E=1/2mv^2 is used for electrons or anything else that has mass and most of the time in the problem they either give you the velocity or the mass which is a huge giveaway to which equation to use. Hope this was helpful!

Trevor_Ramsey_3H
Posts: 70
Joined: Wed Sep 30, 2020 9:58 pm

### Re: E=hv vs. E=1/2mv^2

Hi,
I just saw this on another post so I will reiterate what they said. "E=hv and c=frequency x wavelength are equations that only work for massless matter(i.e. electromagnetic radiation/photons). Anything with mass, so electrons, protons, everyday objects, abide by classical mechanics, 1/2mv^2 for kinetic energy, and de broglie's for wavelength." I figured I'd just share this to you in case you didn't see it.

Tikva Cohen 1K
Posts: 69
Joined: Wed Sep 30, 2020 9:39 pm

### Re: E=hv vs. E=1/2mv^2

Claire_Kim_2F wrote:You are suppose to use E=hv with protons because they do not have mass thus the m is missing from the equation. While for E=1/2mv^2 is used for electrons or anything else that has mass and most of the time in the problem they either give you the velocity or the mass which is a huge giveaway to which equation to use. Hope this was helpful!

This is extremely helpful, thank you!

darchen3G
Posts: 80
Joined: Wed Sep 30, 2020 9:39 pm

### Re: E=hv vs. E=1/2mv^2

Claire_Kim_2F wrote:You are suppose to use E=hv with protons because they do not have mass thus the m is missing from the equation. While for E=1/2mv^2 is used for electrons or anything else that has mass and most of the time in the problem they either give you the velocity or the mass which is a huge giveaway to which equation to use. Hope this was helpful!

I think most of us know, but just to clarify in case it confuses anyone: you use it with photons and not protons; protons have mass but photons do not.

Kiara Phillips 3L
Posts: 59
Joined: Wed Sep 30, 2020 10:04 pm

### Re: E=hv vs. E=1/2mv^2

darchen3G wrote:
Claire_Kim_2F wrote:You are suppose to use E=hv with protons because they do not have mass thus the m is missing from the equation. While for E=1/2mv^2 is used for electrons or anything else that has mass and most of the time in the problem they either give you the velocity or the mass which is a huge giveaway to which equation to use. Hope this was helpful!

I think most of us know, but just to clarify in case it confuses anyone: you use it with photons and not protons; protons have mass but photons do not.

Thank you this was very helpful. With all these new terms it is helpful to simplify things in this way!

Mauricio Maravilla 3C
Posts: 50
Joined: Wed Sep 30, 2020 9:33 pm

### Re: E=hv vs. E=1/2mv^2

E=hv is used when you are trying to find wavelength, frequency, and energy of a photon. 1/2 mv^2 is going to be used when were dealing with electrons that have ejected from an atom as a result of a photon. I think you can do this with any object as well. Maybe not idk.