Electromagnetic Radiation is - What Actually is it? Watch

PhysicsStudent17
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Hello everyone,

I am a high school student, who is looking to study Physics at university.

I would like to please kindly ask for your help, with a question about electromagnetic radiation

I can't visualize what it actually is. Perhaps this is more of a philosophy question?

Here's a list of what I understand:

1) Oscillating charged particles produce changing electric fields
2) Changing electric fields produce magnetic fields (I understand what a displacement current is)
3) Changing magnetic fields produce changing magnetic fields (i.e Faraday's law)
4) Therefore a changing electric field yields a changing magnetic field, and vice versa

Then I'm stuck.

I just can't tangibly visualize what an electromagnetic wave is. I've trawled through the Khan academy videos (usually a great resource, but I still can't put my finger on this)

So you have changing electric and magnetic fields. Is that what the wave is? Why is the wave moving away from the body? How far does it travel? Why does the wave move away from the body? How do changing fields create this thing?

Thank you for any help
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AyyJeh
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If I’m being perfectly honest, it’s not something you can just visualise. It’s more something you have to consider is around and just know about its effects. At the end of the day, the visualisation is your imagination as it’s something which is a mystery to us all
Posted on the TSR App. Download from Apple or Google Play
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Ljg2015
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(Original post by PhysicsStudent17)
Hello everyone,

I am a high school student, who is looking to study Physics at university.

I would like to please kindly ask for your help, with a question about electromagnetic radiation

I can't visualize what it actually is. Perhaps this is more of a philosophy question?

Here's a list of what I understand:

1) Oscillating charged particles produce changing electric fields
2) Changing electric fields produce magnetic fields (I understand what a displacement current is)
3) Changing magnetic fields produce changing magnetic fields (i.e Faraday's law)
4) Therefore a changing electric field yields a changing magnetic field, and vice versa

Then I'm stuck.

I just can't tangibly visualize what an electromagnetic wave is. I've trawled through the Khan academy videos (usually a great resource, but I still can't put my finger on this)

So you have changing electric and magnetic fields. Is that what the wave is? Why is the wave moving away from the body? How far does it travel? Why does the wave move away from the body? How do changing fields create this thing?

Thank you for any help
Waves are one way to visualise electromagnetic radiation which comes from how Maxwell derived his equations in the 1800s. So according to that picture, yes, a wave is comprised of oscillating transverse perpendicular electric and magnetic fields. I'm not sure what you mean by 'why' is the wave moving away from the body - that's simply what is defined as radiation. For example a radiator for warming your house is named as such because it radiates (gives out) heat. EM radiation travels as far as it likes before it's path is impeded by something else. If the universe were empty then the photon would travel the length of the universe without being stopped. But since it isn't empty then EM waves interact with things it comes into contact with, for example solar radiation interacts with our atmosphere which is what produces aurorae. 'How do changing fields create this thing?' Again, I'm a bit confused by this question too. What is very important to realise (which for me I only fully appreciated once at university) is that science is a model. It's a way of looking at the universe in a way that we can understand it. Radiation can be thought of as waves, photons and less commonly now, as rays. Our picture of the universe evolves over time as will continue to be evolved. You may have come across the wave-particle duality in quantum physics, that came about in the early 1900s with Planck, Bohr etc, where they considered how our view of the universe might be viewed if radiation was viewed as photons, and hence was born quantum mechanics (There was alot more that produced quantum mechanics but that's it simplified down as much as possible). For the past half century or so string theory has been investigated as the most fundamental building block of the universe and who knows, we might be investigating something else in 100 years. Sorry for waffling on, but just think of science as a process to produce models which describe the world around us as accurate as possible. If I've completely missed the point then please say and don't hesitate to ask again or any other questions you may have
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Farhan.Hanif93
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In order to answer these question in depth, you will have to wait until university. An understanding of electric and magnetic fields as vector fields, Maxwell's equations and the wave equation are vital to answering most of these questions. Loosely though, and it's very loose indeed, you can think of it as the following:

Picture an infinite sea of charged particles initially arranged 'flatly' like the surface of a trampoline, say. Suppose you impart an acceleration on one of these particles (akin to plucking a point on the trampoline), you can imagine the accelerating particle exerts a force on a neighbouring charged particles as it now oscillates up and down vertically about the point that it was plucked. This results in a ripple across the surface of the trampoline in the vertical plane perpendicular to the surface, namely the transverse oscillations of the electric field, carrying energy away from the source.

At this point, we must consider the duality of electric fields and magnetic fields - given that changes in the former induces changes in the latter and vice versa through Maxwell's equations, we expect a magnetic field to oscillate transversely in the plane of the trampoline and perpendicularly to the oscillations of the electric field. Given that the future behaviours of the electric and magnetic field are dependent on one another, it then follows that these oscillations become self-sustaining and eventually independent of their initial 'pluck'.

Now recall that a wave represents the transportation of energy. By the definition of transverse, these oscillations must be propagating energy in a direction perpendicular to both the oscillating electric and magnetic fields. By the right hand rule, this implies that the wave is travelling radially outwards from the point in space that it was produced. This is electromagnetic radiation.
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PhysicsStudent17
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Thank you all so much for the responses

Yeah, Ljg2015, I I agree. When things get below the sense level, they really start to fall over. I have to appreciate that these are just models and we may not ever know the reality. Science may not ever fully explain reality in a way which is comprehensible to the human brain, but it allows us to build rockets, high-rise buildings and cure cancer, so maybe I have to learn to accept grey areas more

Farhan.Hanif93, I like the trampoline analogy. Helps me visualise it a lot easier. This has helped me visualise the 'self-sustaining' nature. I was struggling with that earlier

If it's taking energy away from the source, does that mean the fields will oscillate less and less over time? I just get the temptation to draw EM waves as having a decreasing amplitude over time, each time I see them

Random additional story: My day of watching videos on EM radiation reminds me of a time in IT class two years ago. We had to design a flash game. The game had to have a 'start' button. Long story short: I spent so many hours staring at the word 'Start' that I thought I was spelling it wrong, so I started spelling it "Strat" and thinking it was correct
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Eimmanuel
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(Original post by PhysicsStudent17)
Hello everyone,

I am a high school student, who is looking to study Physics at university.

I would like to please kindly ask for your help, with a question about electromagnetic radiation

I can't visualize what it actually is. Perhaps this is more of a philosophy question?

Here's a list of what I understand:

1) Oscillating charged particles produce changing electric fields
2) Changing electric fields produce magnetic fields (I understand what a displacement current is)
3) Changing magnetic fields produce changing magnetic fields (i.e Faraday's law)
4) Therefore a changing electric field yields a changing magnetic field, and vice versa

Then I'm stuck.

I just can't tangibly visualize what an electromagnetic wave is. I've trawled through the Khan academy videos (usually a great resource, but I still can't put my finger on this)

So you have changing electric and magnetic fields. Is that what the wave is? Why is the wave moving away from the body? How far does it travel? Why does the wave move away from the body? How do changing fields create this thing?

Thank you for any help

I would usually tell the A level students to forget what they have studied of electromagnetic radiation (or EM wave) in A level. Because what they are usually told by the standard A level textbook or me is usually “wrong” or incomplete. I usually have to oversimplify the picture to convey the main ideas or make use of misleading words to teach the stuff. This is why I tell them to forget what I told them for this topics after their A level exam.


When we talked about the propagation of EM wave in A level, it is the far electric field and magnetic field that we are talking about. This far away electric field and magnetic field which are oscillating in phase is usually described as plane EM wave.
https://en.wikipedia.org/wiki/Electromagnetic_radiation

Note that it is NOT the changing electric field that produces (changing) magnetic field or changing magnetic field that produces (changing) electric field. This is NOT a cause and effect phenomenon of changing electric field or changing magnetic field. See the below link.
https://en.wikipedia.org/wiki/Electr...%99s_equations

I suspect you have not really understood what you stated.

(Original post by PhysicsStudent17)
Here's a list of what I understand:
1) Oscillating charged particles produce changing electric fields
2) Changing electric fields produce magnetic fields (I understand what a displacement current is)
3) Changing magnetic fields produce changing magnetic (I think you meant electric) fields (i.e Faraday's law)
4) Therefore a changing electric field yields a changing magnetic field, and vice versa
Which lead to you to ask “How do changing fields create this thing?”

You can play with the simulation in PHET to “visualize” electric field due to an oscillating charge. Note that the electric field in the simulation is what is known as near field zone.
https://phet.colorado.edu/en/simulat...diating-charge

The far field can be seen from this simulation.
https://www.youtube.com/watch?v=uNLj6NjQsbw

If you are not overwhelmed by the complex result at first look, you deserved a thumb up. The mathematical description of electromagnetic radiation is complicated but Feynman had demonstrated an ingenious way of looking at equation of EM radiation. You can find it at the following link.
http://www.feynmanlectures.caltech.edu/II_21.html

Like what others had mentioned, to really “understand” EM waves, you may have to wait till you go to university to study electrodynamics. I would advise against the use of any analogy to learn EM waves, most of them are wrong and would hinder your understanding when you advance to higher level.
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PhysicsStudent17
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Thank you for the additional response

The stuff about near and far field zones has me a bit confused, but I understand the gist of Maxwell's equations, so I'll leave it there for now

There was something else that confused me a bit, on the subject of EM waves

The attached image: it seems to show that the energy of an EM wave increases directly with frequency.

My issues:

1) In classical Physics, I thought energy was directly proportional to amplitude squared? It seems to show all the different parts of the spectrum as having the same amplitude

2) In quantum Physics, the energy of a photon is given by E = hf, hence higher energy photons occur at higher frequencies. But this picture seems to suggest that a higher frequency of radiation will dictate a higher overall energy of the wave?

Am I just over-analyzing?
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have
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(Original post by PhysicsStudent17)
Thank you for the additional response

The stuff about near and far field zones has me a bit confused, but I understand the gist of Maxwell's equations, so I'll leave it there for now

There was something else that confused me a bit, on the subject of EM waves

The attached image: it seems to show that the energy of an EM wave increases directly with frequency.

My issues:

1) In classical Physics, I thought energy was directly proportional to amplitude squared? It seems to show all the different parts of the spectrum as having the same amplitude

2) In quantum Physics, the energy of a photon is given by E = hf, hence higher energy photons occur at higher frequencies. But this picture seems to suggest that a higher frequency of radiation will dictate a higher overall energy of the wave?

Am I just over-analyzing?
If Energy is proportional to amplitude squared, that doesn't mean that there aren't other factors that affect energy. Waves with the same amplitude can have different energies.
Hence "Proportional to" and not equal to.
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Ljg2015
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(Original post by have)
If Energy is proportional to amplitude squared, that doesn't mean that there aren't other factors that affect energy. Waves with the same amplitude can have different energies.
Hence "Proportional to" and not equal to.
Where does the proportionality of energy and amplitude squared come from? :/
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Farhan.Hanif93
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(Original post by Ljg2015)
Where does the proportionality of energy and amplitude squared come from? :/
In the case of an EM wave, it comes from computing the energy density of the classical electromagnetic field (a calculation that you, again, won't be able to do without an understanding of Maxwell's equations and vector calculus). The end result is:

U = \dfrac{\epsilon_0 |\mathbf{E}|^2}{2} + \dfrac{|\mathbf{B}|^2}{2\mu_0}

Where U is the aforementioned energy density, \mathbf{E}, \mathbf{B} are the electric/magnetic vector fields respectively, and \epsilon_0, \mu_0 are the so-called electric/magnetic constants (traditionally called the permittivity and permeability of free space respectively, such that the speed of light is c^2 = (\epsilon_0 \mu_0)^{-1}).

In the case of a plane wave, the electric and magnetic field configurations satisfy |\mathbf{E}|=c|\mathbf{B}| where |\mathbf{E}| can be interpreted as the amplitude. Plugging this all in and simplifying the expression for U above, we see that U=\epsilon_0 |\mathbf{E}|^2 i.e. the energy density is proportional to the square of the amplitude, as required.

As should be apparent, there's very little of that which can be understood based on school physics alone. This further emphasises the point that you're not quite in the position to answer these questions yet. Nonetheless, keep the mindset of questioning everything, all the way down to the basics - being able to ask such questions will undoubtedly form the foundation for your understanding of physics in the years to come.

(Original post by PhysicsStudent17)
There was something else that confused me a bit, on the subject of EM waves

The attached image: it seems to show that the energy of an EM wave increases directly with frequency.

My issues:

1) In classical Physics, I thought energy was directly proportional to amplitude squared? It seems to show all the different parts of the spectrum as having the same amplitude

2) In quantum Physics, the energy of a photon is given by E = hf, hence higher energy photons occur at higher frequencies. But this picture seems to suggest that a higher frequency of radiation will dictate a higher overall energy of the wave?

Am I just over-analyzing?
That picture is indeed very misleading, and I suggest limiting it's use for the purposes of GCSE/A-Level only. Beyond, you will have a much more concrete way to describe this phenomena both classically and quantum mechanically.
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Ljg2015
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(Original post by Farhan.Hanif93)
In the case of an EM wave, it comes from computing the energy density of the classical electromagnetic field (a calculation that you, again, won't be able to do without an understanding of Maxwell's equations and vector calculus). The end result is:

U = \dfrac{\epsilon_0 |\mathbf{E}|^2}{2} + \dfrac{|\mathbf{B}|^2}{2\mu_0}

Where U is the aforementioned energy density, \mathbf{E}, \mathbf{B} are the electric/magnetic vector fields respectively, and \epsilon_0, \mu_0 are the so-called electric/magnetic constants (traditionally called the permittivity and permeability of free space respectively, such that the speed of light is c^2 = (\epsilon_0 \mu_0)^{-1}).

In the case of a plane wave, the electric and magnetic field configurations satisfy |\mathbf{E}|=c|\mathbf{B}| where |\mathbf{E}| can be interpreted as the amplitude. Plugging this all in and simplifying the expression for U above, we see that U=\epsilon_0 |\mathbf{E}|^2 i.e. the energy density is proportional to the square of the amplitude, as required.

As should be apparent, there's very little of that which can be understood based on school physics alone. This further emphasises the point that you're not quite in the position to answer these questions yet. Nonetheless, keep the mindset of questioning everything, all the way down to the basics - being able to ask such questions will undoubtedly form the foundation for your understanding of physics in the years to come.


That picture is indeed very misleading, and I suggest limiting it's use for the purposes of GCSE/A-Level only. Beyond, you will have a much more concrete way to describe this phenomena both classically and quantum mechanically.
That's amazing, cheers. We'll be going into Maxwell's equations in Sept when I'm back for my second year. Did you use latex to write those equations?
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Farhan.Hanif93
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(Original post by Ljg2015)
Did you use latex to write those equations?
Yes. See here for an introduction.
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PhysicsStudent17
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Thanks everyone
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Eimmanuel
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(Original post by PhysicsStudent17)
Thank you for the additional response

The stuff about near and far field zones has me a bit confused, but I understand the gist of Maxwell's equations, so I'll leave it there for now

There was something else that confused me a bit, on the subject of EM waves

The attached image: it seems to show that the energy of an EM wave increases directly with frequency.

My issues:

1) In classical Physics, I thought energy was directly proportional to amplitude squared? It seems to show all the different parts of the spectrum as having the same amplitude

2) In quantum Physics, the energy of a photon is given by E = hf, hence higher energy photons occur at higher frequencies. But this picture seems to suggest that a higher frequency of radiation will dictate a higher overall energy of the wave?

Am I just over-analyzing?

You are not over-analyzing. It is a valid question and in fact the "two pictures of light" had been "reconciled" by the physicists long time ago.

… That picture is indeed very misleading, and I suggest limiting it's use for the purposes of GCSE/A-Level only.

I don’t totally agree or disagree this comment. I don’t think the “two-extremes” view of light is misleading, because they have been "reconciled" in quantum field theory.

The following is an excerpt from a chapter called “THE QUANTIZATION OF WAVE FIELDS” in a quantum mechanics book that did the reconciliation.
Name:  QFT_01.jpg
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One can find such discussion in physics stackexchange as well.
https://physics.stackexchange.com/qu...-single-photon


The picture can be misleading if one has read the article by Prof Art Hobson, he stated the following

There are overwhelming grounds to conclude that all the fundamental constituents of quantum physics are fields rather than particles.

Rigorous analysis shows that, even under a broad definition of “particle,” particles are inconsistent with the combined principles of relativity and quantum physics. Photons, in particular, cannot be point particles because relativistic and quantum principles imply that a photon cannot “be found” at a specific location, even in principle….

Some queries about his paper can be found online.
https://www.quora.com/Whats-your-tak...m-field-theory


I agree that most A level physics students don’t have the knowledge or maths tool understand the connection.

If any A level students are confused, just ignore what you have read.
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