Quantum physics perspective on radiation entropy

[jlefevre76]

Hi, I've been curious about his subject for quite a while now, and I figure I need to ask here in these forums to get started in the right direction in understanding this.

In classical thermodynamics, we define entropy:

$$ds = \delta q / T$$

However, when it comes to radiation in quantum physics, matching the bandgap is more important. For instance, say I have a PV material tuned to 500 nm (approximately), and I have a couple of lasers, one at 490 nm, one at 525 nm, and one at 500 nm. If I shine the lasers on the PV, I may get some energy from the 525 nm, and maybe a little more from the 490 nm, but the most from the 500 nm laser, since the PV is tuned to that wavelength (this assumes all laser outputs are equal as well).

So, how does quantum physics define entropy in terms of electromagnetic radiation? Is there a way to redefine entropy for radiation that takes into account that a laser, for instance, is easier to recover the energy than for the same amount of energy from a black body?

In addition, Wien's displacement law gives us a trend (maximum wavelength of a black body):

$$C_W = 2898 \mu m \cdot K = \lambda T$$

This, to me, suggests that since higher temperatures have shorter wavelengths (and therefore characteristically the average photon energies are higher), that perhaps higher energy photons (shorter wavelengths) are more useful generally in terms of work and/or potential. Could this be true, or is it unrelated?

 

[eljose79]

Hi there,

Thank you for your question regarding entropy in quantum physics and its relation to electromagnetic radiation. I can understand your curiosity and interest in this subject and I am happy to provide some insights.

Firstly, let's define entropy in the context of quantum physics. In the classical thermodynamic definition, entropy is a measure of the disorder or randomness in a system. In quantum physics, entropy is defined as the number of possible states that a system can have and is related to the uncertainty in the system. This means that a system with higher entropy has more possible states and therefore more uncertainty.

Now, let's talk about the entropy of electromagnetic radiation. In quantum physics, electromagnetic radiation is described as particles called photons. These photons carry energy and their behavior is governed by the laws of quantum mechanics. When a photon is absorbed by a material, it increases the energy of the material and can lead to the emission of electrons, as in the case of a photovoltaic (PV) material. This process is called the photoelectric effect.

In the scenario you described, the PV material is tuned to a specific wavelength, which means it has a bandgap that corresponds to that wavelength. This bandgap is the minimum energy required for an electron in the material to be excited and become mobile. When a photon with energy equal to or greater than the bandgap is absorbed, it can excite an electron and contribute to the flow of electricity in the PV material. This is why the 500 nm laser, which matches the bandgap, is most effective in producing energy in the PV material.

Now, to address your question about how quantum physics defines entropy in terms of electromagnetic radiation, we can say that the entropy of electromagnetic radiation is related to the number of available photons at a given energy level. This means that the entropy of radiation increases as the energy level of the photons increases. This is because as the energy level increases, there are more possible states that the photons can occupy.

Regarding your second question about the potential usefulness of higher energy photons, it is true that higher energy photons have more potential to do work. This is because they carry more energy and can therefore excite more electrons in a material, leading to a higher flow of electricity. However, this does not necessarily mean that they are more useful in all situations. The usefulness of a photon depends on its energy level and the specific material it interacts with.

In conclusion, the concept of entropy in quantum physics is related to