E=pc

[Carly Yoon]

When do we use the equation E=pc? Is it used for particles or photons?

[105605391]

Since p in this question is in reference to momentum which is equal to mass times velocity, I think this formula can only be used for particles that have mass. I think photons are considered massless particles, thus excluding them here.

[Carmen Kim 3I]

Hi,
I believe we use E=pc when the momentum of the photon is given in the question. In E=pc, E is the energy of a photon, p is the momentum of the photon, and c is the speed of light. As someone said above, I agree with using the formula for particles that have mass.

[Nicole 3G]

Hi,

We would use E=pc for things that have a mass. Since 'p' is momentum (mass times velocity) we can't use that formula for things that do not have a mass.

[Shreya Thyagarajan Dis1A]

It can be used for particles because it uses momentum, which is mass times velocity. This equation can help you find the energy of an electron, but not a photon since they do not have a defined mass.

[ThomasL1L]

This would only be for small particles or anything really that has mass. Since momentum requires for something to have mass. Thus why we would use another equation such as E = hv for photons which have almost no mass.

[Terrence Chi]

Hi! We would use the equation (E=pc) when the question mentions particles mass and velocity. However, we use another equation (E=hv) for questions that gives you the photon’s frequency or wavelength.

[Chiara Frank]

Hi! We can use this equation for particles, such as electrons, protons, neutrons, etc., therefore something that has a mass and velocity (particle qualities). However, we can use this for photons as well to calculate their momentum! We cannot use the mass and velocity of a photon to solve, as a photon does not have a mass, so we would first calculate the energy per photon using E=hv. After we have this we can divide this by the speed of light, so plugging the value for E into the E=pc equation to find the momentum of a photon. Hope this was helpful!

[Arden Napoli 1E]

Hi,

This equation only applies to particles with mass because p=momentum=mass*velocity. This equation would not be used to calculate the energy of a photon, however, because photons are massless particles. That is why for photons we use E=hv.

[AndreyCastellanos 3H]

use E=pc when the momentum of the photon is given

[Sofia Potter 1I]

I think the formula e = pc is only relevant when a mass is involved, because momentum is represented by p in the equation, and momentum is calculated as mass times velocity. This would mean that photons could not be applied to this formula.

[Tyler Olson 1E]

The equation E= pc is only used for particles with mass. Thus it wouldn't be used for photons because they have a negligible mass. You would use the equation E=hv using the frequency of the photon. So any particle with mass can use the E = pc equation!

[Michelle Jeong 1B]

We use that one for particles with mass, unlike photons.

[Rishab_Haldar_3B]

E = pc is used for particles as it includes momentum which equates to the particles mass times its velocity. Since photons are massless particles, you would instead use the equation E = hv to calculate the energy of a photon.

[sidneypalacios]

p is momentum, and you use this equation for things that have mass!

[Arshaun Faraji 2H]

I believe we use this when we are given mass and velocity and asked for the energy. Or any combination of two of those and missing one variable.

[605537776]

E=pc is only applicable to things with mass.