HW Achieve #13

[audreywang3C]

For question 13, I understand how to calculate the energy of the photon, but I'm confused on the energy of the electron.

I started out using the E=pc equation and De Broglie wavelength = h/p to get energy = hc/wavelength, but it said the answer was incorrect. The explanation said to use the KE equation, E= 1/2mv^2.

Can anyone explain why we can't use the E=pc equation for electrons? In which situations can we use the equation, and in which situations can we not? Thanks!

[Andrew Rizk]

From what I understand in class, we have never used the E = pc equation for any problems so far. I believe this equation can only be used for light as well since it uses the speed of light in the equation, though I am not completely sure if that is right or not. I looked it up previously and a website said it was used for finding the momentum of a photon, ignoring mass since photons do not have mass, but I wouldn't take that as the correct answer until Dr. Lavelle says what we will use that equation for.

As for the achieve #13 question, you should the right answer can be acquired using the de broglie equation in combination with the equation for kinetic energy. You would find the velocity using the de broglie equation (since you have the wavelength and mass already), then plug it into the kinetic energy equation to find the total energy.

[Joanna_Sung_3F]

To further elaborate and expand on the previous answer, anytime we are dealing with electrons, we should use the de broglie equation or E=pv. When we are dealing with photons, we should use the E=pc equation. This is because the variable c gives the speed of light, making the equation only applicable to photons. Electrons would not have the speed of light, so energy for electrons can use the equation E=pv. The de broglie equation only applies to electrons and not photons because photons do not have a rest mass.