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

$E=hv$

Libby Dillon - 1J
Posts: 40
Joined: Fri Oct 02, 2020 12:17 am

### 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: 50
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_2J
Posts: 41
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 2I
Posts: 44
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: 48
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 1L
Posts: 42
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!