Electromagnetic wave HD

The propagation of a linearly polarized electromagnetic wave: the vectors of the electric field E and the magnetic field H are perpendicular to each other and to the direction of the wave propagation. Another principle of electro-magnetic wave propagation is that the vectors E and H oscillate in phase, i.e. they achieve the maximum value in the same points of the space.

[jlin]

@TopGunMan In case you have not seen the explanation yet. It says: curl(B) = dE/dt + J. Which means greatest curl, not greatest strength. Curl is a function of spatial derivative of B. So greater curl means B varies more in space (ie dB/dx ), which happens in the same place as greatest dE/dt, so they are in phase.

[CACBCCCU]

r8448, if you prefer, let's just use Maxwell's equations and forget about experiment:

Using the modern Heaviside form of Maxwell's equations in the vacuum or a chargeless medium, del cross E is proportional to the rate of change of the magnetic field B, and del cross B is proportional to the rate of change of the electric field E. So it seems that if the incident beam is in free space, it apparently should have the E and B fields 90 degrees out of phase with each other.

[jethomas]

This is a traditional visualization of one way an EM wave could go while fitting Maxwell's equations. There might not be any such thing as EM radiation, maybe it's all some kind of weird particles, but Maxwell's equations do fit the data at least as well as Newton's equations fit relativity.

[CACBCCCU]

r8448, we're talking about light here, and sure light is made of bosons, and that's a reference to quantum physics, but light quanta (EM quanta) are the only quanta of interest here, the energy of each light quantum (photon) is quite low compared to the total energy of an electron. The generation and detection of light are where electrons definitely come into the picture, so I'd suggest using laser heating to keep the picture simple, much like the relation between photon energy and frequency.

[jethomas]

This is not the only way it can go. This is linearly polarized light. Circularly polarized light looks very different. Circularly polarized light is the sum of two different linearly polarized waves that are out of phase. Some people say that this means that linearly polarized light is fundamental and everything else is combinations of that. But I think linearly polarized light is the sum of two different circularly polarized waves that are out of phase too. So which is more basic?

[CACBCCCU]

"There is currently a need for (moving) elektrons to create a H field."

I'm told an oscillating E field generates an oscillating H field. I'm also told the E field and H fields are not in phase at a transmitting antenna.

Seems the photon energy confusion I generated here could be resolved for example by viewing each photon as being something analogous to at least a co-moving half-cycle of the moving wave shown in this video, not just one thin slice of the wave in space and time.

[maaszaxxs]

@CACBCCCU isn't del cross B equal A, so that divergence of B should be equal to zero, Maxwell's four elementary eqns

[LOLipopz]

This is so cool! I am a kinesthetic learner, but I visualizing stuff like this really helps, also. Thanks a lot! I am going to take my physics final tomorrow... Thank you for making this vid, though. This will really help. :0)

[CACBCCCU]

That's a great question, r8448. I have thought the same thing.

The H field is apparently ignored in most applications, and having E and H out of phase apparently would make total photon energy a constant, which is apparently an assumption of quantum physics (Energy = hf, f constant frequency) so I suspect you may be right.

[CACBCCCU]

I'm not sure how one would measure variations in the magnetic field from an electromagnetic quanta, but I suppose the heating energy of a laser beam on a thin nonreflecting absorber should not vary by phase with distance, so again it seems the E and H fields are out of phase so as to maintain the vector sum magnitude constant, and also that both fields are converted into heating energy with equal efficiency.

[CACBCCCU]

After writing that, I had to go look it up in one of my old books on electromagnetism, and apparently if one goes through all the math properly, using Maxwell's equations, the E and H fields are indeed correctly shown to be in phase in this video.

Sorry about the confusion, it's been a long time since I've looked closely at this subject.

[TikoSmiles]

this like is in my 11th grade physics text book. :) Good job

[CACBCCCU]

It doesn't seem that one has to consider electrons in the vacuum to answer the question of photon E-H phase relations.

As I mentioned before, the amount of heat generated in a flat absorber is presumably not phase-dependent under heating by a phase-coherent beam of photons. For example a 400 nanometer laser would heat a chunk of powdered coal equally as well from one inch away as it would from one inch plus 100 nanometers distance.

[CACBCCCU]

The same observation of fixed laser heating rate over various distances would presumably apply to, say for example, a 100 nanometer thick sheet of gold sandwiched with graphite and carbon dust, to limit the total absorber distance from the 400 nanometer laser output port to a precise range.

[CACBCCCU]

@j0lin101 Nice explanation. Thank you.

The E and B vectors point laterally, perpendicular to direction of light propagation, and oscillate at the wave frequency at a stationary point, spin 1 in that only full lateral-plane instantaneous rotation brings it back. In gravity the vector will point in the opposite direction of propagation and does not normally oscillate in any direction, hypothetical gravity waves being an exception oscillating laterally with bilinear symmetry, a spin-2 property.

[jethomas]

This is the traditional way to show the wave but it is too simple. It graphs the electric field and the magnetic field along points on a line. But electric fields are scalar. They don't have a direction, except for the gradient. For the picture to make sense you would need to show enough of the surrounding radiation to see the gradient. That would be a lot more complicated.

[mikety]

@z1zaz

1. Birds falling was due to fireworks concussion -- mechanical waves, not electromagnetic waves
2. EM waves heat things up -- that's a proven fact (just go out in the sun!). I don't know why that's so important to mention here, other than it highlights an obvious safety problem with "beaming" lots of power all over the place.

[r]

It seems like nobody really knows for sure. The question is then, how can EM-waves travel trough vacuum. Are there electrons in vacuum? There is currently a need for (moving) elektrons to create a H field.

[TopGunMan]

Can someone who truly understands this explain why the magnetic and electric fields of an EM wave are in phase, even though maxwell's law states that an oscillating electric field produces a magnetic field with the highest magnetic strength at the greatest rate of change of the electric field intensity. (out of phase)

[zzaz]

@TopGunMan
Does the transverse magnetic EM 'wave' component exist in the first place ?
Your comment is valid for a resonant circuit; but magnetism is a reciprocal *concept* only !
Tesla-1919-
"The Hertz wave theory of wireless transmission may be kept up for some while, but I do not hesitate to say that in a short time it will be recognised as one of the most remarkable and inexplicable abberations of the scientific mind which has ever been recorded in history."
Some 'short' time; Hey ?