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

[Libby Dillon - 1A]

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]

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]

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]

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]

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]

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]

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.