- #71
Ivan Nikiforov
- 124
- 23
Thank you for the detailed explanation. To summarize, do I understand the process correctly now? For the reference frame of a photocell, a moving flashlight has more energy than at rest. The increase in the observed energy is related to the movement of the flashlight. This leads to the fact that for the reference frame of the photocell, the light of the flashlight has a higher total energy, that is, a greater amount of electric and magnetic energy. When the light of the flashlight hits the photocell (passes into another frame of reference), the total energy of the light decreases for the frame of reference of the photocell. Thus, the illumination of the photocell for a period of 2 times longer is due to the energy associated with the movement of the flashlight.Dale said:In this case 180 is the energy of the flashlight in the primed frame and 156 is the momentum of the flashlight in the primed frame after emitting the light.
the total energy of the light is not the density of the energy flow. Energy flow describes how much energy is moving from one location to another. This is the actual amount of energy in the fields. The relationship between the energy in the field, the energy flow, and the work done on matter is given by Poynting's theorem. I like this textbook: https://web.mit.edu/6.013_book/www/chapter11/11.2.html
In particular, see chapter 11 equation 10 for the energy in the fields. They use ##W## for the energy density, so if you integrate ##W## over all space then you get the total energy with $$W=\frac{\epsilon_0}{2}\vec E \cdot \vec E + \frac{\mu_0}{2} \vec H \cdot \vec H$$in free space.