2 upvotes, 1 direct replies (showing 1)
View submission: Two questions about light waves
At the time I'm posting this, your post has -1 point, but no explanation of why it was downvoted, so I'm not certain whether to take it seriously. If there is anything wrong with **Irrasible Engineering**'s reply, please explain what that is. For now, I have to take it seriously. Possibly it was only downvoted because it uses math beyond my apparent level of comprehension.
The paper linked earlier by **agaminon 22** was way above my head. What you say here is just a little above my head. I am not competent at calculus or its language or terminology. But it looks completely logical.
I can't forget photons because photons are what I'm trying to understand.
However, you tell me that photons do not have a magnetic or electric field. That makes sense if my understanding of what a field is is flawed or incomplete, and it is certainly very incomplete.
Can you explain in simple terms what the difference is between a field strength and a potential? That is a question I hoped to ask at some future time, but it looks like I need to ask now. I can imagine that a fully accurate explanation would be way beyond what I can understand, and that these are probably relativistic concepts, which complicates them terribly. But you did say "classical".
Please let me start out by asking the most basic questions: What is the function of the leading dots on the **E** and **H** ? And what does **A** represent? I do not know how to look those up in a search.
I will return to this after I've watched the video linked by **James_James_85**
Thank you!
Comment by Irrasible at 27/12/2023 at 15:44 UTC
1 upvotes, 2 direct replies
I don't know why it was down voted either.
The leading dots are just bullet points. It is one of the formatting options that reddit gives us for lists. I can see that it might look like a math operator. I just use it to set off the equations to make them easier to pick out. I can edit it to change those into a numbered list.
1. Item A
2. Item B
3. Item C
If you will let me know your education level with respect to physics, then I will try to answer appropriately.
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I had suggested that you forget photons, because it they are not important to answer your top-level question, which can be answered entirely in terms of classical field theory. That is what I attempted to do. I will address photons in a separate reply.
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There is a philosophical divide about what the electric and magnetic fields *are*. For some, like Griffiths, the fields *are physical things* that are physically there. For others, like Feynman, the fields a just a calculation means. They are *nothing but numbers* attached to points in space. Purcel says it doesn't matter. Paradoxically perhaps, everybody agrees that the fields are real. No matter what you believe, you use the same math and calculations. If you do it correctly, you get the same results.
I follow Feynman, mainly because I am not tempted to bring in extraneous intuitive concepts that can become misleading. For me, electromagnetic effects are physical.
The electric kettle gets hot. The electric fan motor turns the blades. Fields are just numbers used to calculate the effects. There is only one electric field, hence, it is properly referred to as *the electric field*. The field exists and fills all of space the instant that I imagine it. Electromagnetic effects propagate; the field does not. Charge particles do not *have a field*; they influence the numbers that make up *the field* at points in their vicinity.
Sorry to be long winded.
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The potentials are just a different set of (four) numbers that give us the same information as the six numbers of the electromagnetic field (three electric and three magnetic).
At each point in space, there can be a charge density that is a scalar. And there can be a current density which is a three-dimensional vector. These are collectively called the sources of electromagnetic effects. The calculation chain goes as follows.
1. From the (four) sources, calculate the (four) potentials.
2. From the potentials, calculate the E&M fields.
3. From the E&M fields, calculate the effects.
In physics education, step 2 is ignored initially, and you are taught to calculate the E&M fields directly from the source terms.
But notice, there are only four numbers to describe the sources, but it takes six to describe the E&M fields. Clearly, there is some redundancy in the E&M fields. The upshot is that all six of the numbers that describe the E&M fields cannot be set arbitrarily for all space and time. This is the reason that E and H out in free space must be perpendicular. You could reach that result based only on the expressions that allow you to directly calculate E&M from the sources, but it is difficult. It is a lot easier if you include the intermediate step of calculating the potentials.
If you are interested, I will be glad to write more about the potentials.