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.

[Rainy Liu Ruoshui_2K]

Although light acts like a particle (photon), it doesn't have any mass, so E=1/2mv^2 doesn't apply to this condition. E=1/2mv^2 can only be used for some thing with mass, such as electrons.

[Kaira Shibata 1E]

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!

Hi! You should use E=hv when you are dealing with things that do not have mass like photons. On the other hand, electrons, protons and other things with mass should use the second equation listed E=1/2mv^2

[Noa Popko 3I]

E=hv is used to calculate energy of photons. E=1/2mv^2 has a mass component so it should be used with things with mass. Photons do not have mass so we cannot use that equation for them. However, we can use that equation for electrons which do have mass.

[Arjan G 2H]

Hi! I believe the E=hv equation is used to show the total amount of energy in a situation, that being when adding the work function with the kinetic energy, which is found using the 1/2mv^2 equation. 1/2mv^2 is used to find the kinetic energy of a system, which is essentially the "leftover" energy after the necessary amount of energy is used for a work function. SO, if a problem is asking you to find the kinetic energy, you would use E=1/2mv^2, but if they were asking you to find the net energy, you would use E=hv.

[Tianle Zheng]

I think what equation to use should be determined by what data is given. If the question provides the frequency of the wave or another information that can be used to determine its frequency, then we can use E= hv, vice versa. I believe both equations can all be used to get the energy of photon, using which equation is determined by what information given.

[Sarthika Chimmula 3H]

You are supposed to use E = hv when you are given the wavelength, frequency, or energy of a photon (form of electromagnetic radiation). You are supposed to use E = 1/2mv^2 when you are asked to find the kinetic energy of an electron given the mass or velocity of the electron. This energy can also be found by subtracting the threshold energy, or the energy required to remove an electron, from the energy of a photon (E=hv).