# Electromagnetic waves

inmyblood
So what i understand from my professor about the electromagnetic waves is this :

an electric charge has an electric field E
a moving electric charge induces a magnetic field B
The electromagnetic wave produced is due to the moving charge which has both components E and B...

But my question is, why does this ONLY work in an accelerating charge? doesn't a regular charge not accelerating (such as a current) produce a magnetic field as well as an electric field ?

Thanks

PhilDSP
Hi inmyblood,

A non-accelerating charge produces merely a displacement current or perturbations in the E and B fields that are not traveling waves. You can think of them as evanescent waves, disturbances that dissipate very quickly if the charge stops moving.

A nice equation showing how both charge velocity and acceleration affect the magnetic field can be found in "Classical Electomagnetism via Relativity" by W. G. V. Rosser p. 38-41 (referenced from Jefimenko's book "Causality, Electromagnetic Induction and Gravitation"):

$$H = \frac{q}{4 \pi s^3}([v][1 - \frac{v^2}{c^2}] - \frac{1}{c[r]}[r]\ \mathsf x \ (R\ \mathsf x \ [a]))\ \mathsf x \ [r]$$

(H is related to B through the ratio given by the vacuum permittivity constant $\mu_0$)

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xAxis
Steady current produces constant magnetic field, which doesn't induce Electric field. Accelerating charge however produces variable magnetic field, which then in turn produces variable Electric field.

PhilDSP
Accelerating charge however produces variable magnetic field, which then in turn produces variable Electric field.

Please be careful there. Strictly speaking the moving charge produces both the electric and magnetic fields or rather, their fluctuation. (Sorry, I omitted the related equation for the electric field)

$$E = \frac{q}{4 \pi \epsilon_0 s^3}(R[1 - \frac{v^2}{c^2}] + \frac{1}{c^2}[r]\ \mathsf x \ (R\ \mathsf x \ [a]))$$

Though as a side effect, some other charge (or a photon) could be producing magnetic field fluctuations that move the charge in the equation resulting in added E and B field components. But we're probably most interested in the one charge to not complicate and confuse things, aren't we?

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inmyblood
Thanks PhilDSP!

So If i understand correctly an EM wave has to have both the magnitude of the Electric Field as well as its Magnetic field varying in space to be considered an EM wave ?

Also regarding the Magnetic waves may you check my question about mutual inductance ?

Thanks :)

xAxis
Thanks PhilDSP!

So If i understand correctly an EM wave has to have both the magnitude of the Electric Field as well as its Magnetic field varying in space to be considered an EM wave ?
Is that what PhilDSP sad? I didn't get it. I thought you asked why steady current doesn't produce EM wave.

PhilDSP
So If i understand correctly an EM wave has to have both the magnitude of the Electric Field as well as its Magnetic field varying in space to be considered an EM wave ?

That's probably not an especially good way to classify an EM wave. I think generally a single non-accelerated moving charge will also have E and B fields varying in space or across space. We should notice the curl in the equation for the H field. That means the field values will curve across space.

But a conglomerate of moving charges, a current, might be steady over a certain region of space and then produce fields that don't varying in time at one position.

A different classification is that the EM wave will be self-propelled while displacement current will not be.

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elias2010
The antenna TV amplifier feds up electricity to the antenna.Does eletricity pull EM waves?

elias2010
Provided that photons have not mass,why only SW reflected by ionospere?
Why radars works only with UHF waves? Why submarines communicate with LW?
And many more questions arise.

elias2010
Why when you put an AM radio close to an elektricity conductor the signal becomes stronger?

Gold Member
Provided that photons have not mass,why only SW reflected by ionospere?
Why radars works only with UHF waves? Why submarines communicate with LW?
And many more questions arise.

You can only answer these questions by getting to know the basics of EM theory. There is no quick 'arm waving' way of explaining something so complicated. You will be disappointed if you hope for a simple explanation.

Gold Member
2021 Award
The antenna TV amplifier feds up electricity to the antenna.Does eletricity pull EM waves?

no it doesnt.
A power supply feeds electricity up to an amplifier at the antenna
that DC voltage is used to make the amplifier work.

there is no electrical pulling of EM waves

Dave

Gold Member
2021 Award
Why when you put an AM radio close to an elektricity conductor the signal becomes stronger?

the conductor acts as a long antenna and helps to pick up the radio signal better,
it is then induced into the radio's internal antenna when the radio is brough close to the conductor.
The conductor ( a piece of wire) doesn't need electricity in it for it to act as an antenna
any long piece of wire will have the same effect

Dave

elias2010
why different colours have different refractive index?

Gold Member
The speed of the light through a medium will depend on how it interacts with the structure it passes through. Different colours (at least, the spectral colours) just correspond to different wavelengths. When EM waves interact with matter it is always wavelength dependent so it is not surprising that different wavelengths of light should be slowed down by different amounts as they pass through a transparent medium - as far as the WHOLE of the EM spectrum ("DC to Daylight") is concerned, there is a vast range of effects that a given medium can have on the different wavelengths involved.

elias2010
no it doesnt.
A power supply feeds electricity up to an amplifier at the antenna
that DC voltage is used to make the amplifier work.

there is no electrical pulling of EM waves

Dave

If you touch antenna you will feel the sting.Amplifier is just a frequency filter.

Gold Member
2021 Award
If you touch antenna you will feel the sting. Amplifier is just a frequency filter.

no that's not quite correct. The amplifier AMPLIFIES the signal, else it wouldn't be called an amplifier. Generally mast head amplifiers are used in weak signal areas or where a single antenna feeds a block of accomodation units.

That tingle you feel is coming from the TV. Many of the TV's that have a voltage feeding out of them will be enough for you to feel a slight tingle.

Some of the older TV's that had live chassis, had quite a substantial voltage on the antenna socket. And altho the sockets had bypass and DC blocking capacitors, you could still get a strong tingle. This had nothing to do with feeding power to amplifiers.

cheers
Dave

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Gold Member
2021 Award
I will also take the opportunity to answer a couople of your earlier questions
well give you a little insight anyway...

Why radars works only with UHF waves?

but the main reason why they use microwave frequencies, usually greater than 1000MHz, is because as the frequency is increased the resolution of the RADAR gets better. That is they are able to discriminate between smaller and smaller objects.

I remember many years ago, the local airport near where I used to live had RADAR on 600MHz.
up till 1999, FAA in the USA used 1330 and 1350 MHz freq's for airport radars. don't know how much that's changed in the last 10 odd years

Weather radar commonly uses 3000MHz (3GHz)

Why submarines communicate with LW?

from what I understand, VLF ( Very Low Frequency) radio signals penetrated the sea much easier than higher freq's. This allowed submarines to remain submerged and still receive communications.

cheers
Dave

Gold Member
If you touch an antenna and you "feel the sting" then your head amplifier power supply is faulty - that's the mains you can feel. You might think about changing it!
Some basics: An amplifier is an amplifier and a frequency filter is a frequency filter: two different beasts. The head amp is a wide band device (no filtering) which handles all the possible channels you might want to receiv with your selective receiver. The reason that you put a head amp up at the aerial is that it has a better noise figure than the receiver and is providing amplification at the far end of the feeder. Both of those functions improve the carrier to noise ratio at the input to the set when your received signal is below the desirable level.
The voltages on a T V receiving antenna are in the region of a few tens of milivolts. No "sting" possible.

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2021 Award
... The voltages on a T V receiving antenna are in the region of a few tens of milivolts. No "sting" possible.

not entirely correct. having serviced TV's for more than 30 years. I can definitively tell you that you will receive a tingling from the antenna socket or any antenna connected to many many makes and models of TV's. This is normal ... it is NOT a fault condition
I will qualify that by saying that generally it would be usually felt by the more sensitive skin areas say, under the arms, rather than on the thicker / drier skin of the hands or fingers

Dave

Gold Member
The volts come from the TV and not from the aether. They aren't sucked in from elsewhere.

I was discussing the situation with a naked antenn and head amp.

That tingle can also be felt on microphones and electric kettles when there's poor earthing. Did you ever measure those volts, btw?

elias2010
I will also take the opportunity to answer a couople of your earlier questions
well give you a little insight anyway...

but the main reason why they use microwave frequencies, usually greater than 1000MHz, is because as the frequency is increased the resolution of the RADAR gets better. That is they are able to discriminate between smaller and smaller objects.

I remember many years ago, the local airport near where I used to live had RADAR on 600MHz.
up till 1999, FAA in the USA used 1330 and 1350 MHz freq's for airport radars. don't know how much that's changed in the last 10 odd years

Weather radar commonly uses 3000MHz (3GHz)

from what I understand, VLF ( Very Low Frequency) radio signals penetrated the sea much easier than higher freq's. This allowed submarines to remain submerged and still receive communications.

cheers
Dave
I have known all that you mentioned,my question was exactly about EM waves properties:
Why UHF and microwaves are more reflected by metal surfaces and other objects?
Why MW and LW are more penetrating?
Why only SW reflected from the ionosphere?
If we consider that the photons have size or mass,then the above and many more can be explained.

Gold Member
I have known all that you mentioned,my question was exactly about EM waves properties:
Why UHF and microwaves are more reflected by metal surfaces and other objects?
Why MW and LW are more penetrating?
Why only SW reflected from the ionosphere?
If we consider that the photons have size or mass,then the above and many more can be explained.

For starters, photons cannot have mass or they couldn't travel at c, could they?

The other questions cover virtually the whole of em theory. There isn't a simple answer to include them all at once.

elias2010
For starters, photons cannot have mass or they couldn't travel at c, could they?
Why not? In a transparent material velocity is lower.

PhilDSP
If photons have rest mass it must be exceedingly small. Mass in an electromagnetic particle causes frequency dispersion of radiation when the particle interacts with another particle. Experimentally, in a vacuum no dispersion has been found to very, very small limits.

Staff Emeritus
Why not? In a transparent material velocity is lower.

For one thing it would break a few well tested theories. On top of that, all experiments to date have shown that light travels at c and has no mass. It is possible that we simply haven't had the required accuracy to measure the mass of a photon if it does have mass, but it is not expected to.

elias2010
For one thing it would break a few well tested theories. On top of that, all experiments to date have shown that light travels at c and has no mass. It is possible that we simply haven't had the required accuracy to measure the mass of a photon if it does have mass, but it is not expected to.

If we consider photon as a particle with mass, then de Broglie equation applies to it: f=mc2/h. So, it becomes obvious that frequency expresses the photon mass or size and thereby there are various photon sizes. That’s why its mass is not defined yet.
My theory, whitch is not allowed me to present, matches the experimental results and explains almost everything.On the contrary,the prevailing theory leads to the acceptance of 70% dark energy (or is it 90%? who the measured and how?)

Gold Member
There is a proverb which involves angel fearing to tread.
so it becomes obvious
is not a statement that can be readily applied to modern Physics theories.
How does your hypothesis tie in with relativistic effects, for instance? The existing one seems to do that very well and you would need to sort it out before being confident that your 'new' approach is at all valid. For instance, do you have any solid evidence to back it up (at the risk of getting the thread locked )?

mufa
De Broglie equation has experimentally confirmed.Why we have to reject the hypothesis photon have a mass instead of the possibility being wrong the relativistic formula for energy?

Gold Member
De Broglie equation has experimentally confirmed.Why we have to reject the hypothesis photon have a mass instead of the possibility being wrong the relativistic formula for energy?

I think its something to do with the the fact that the existing model explains more phenomena than your alternative one. You are proposing an idea that is so different that you would have to go through the whole of observed Science in detail, justifying it, point by point and showing where the present model is wrong. Just asking why it's not accepted, is not good enough.

Having just listened to a 'Scientist' on the radio, talking obout the principle of falsifiability, I guess I should say that I would be pleased if you managed to show you were right.

mufa
I appreciate your condescension.Unfortunately,I am not a student and I have not free time to study in depth.That's why I threw the ball to you.

Gold Member
I appreciate your condescension.Unfortunately,I am not a student and I have not free time to study in depth.That's why I threw the ball to you.

Sorry about my 'tone'. You seemed fairly well informed and a bit 'combative' (?) and I was a bit too ready to pick up my cudgel.
Any way, why should Photons have no mass?
I think that there is experimental evidence in which it has been attempted to infer their rest mass. This is based on splitting their Energy into two components
E = p2c2 + mrestc2
where p is the momentum (which can be measured)
Looking for a discrepancy between the measured E and the energy due to the momentum gives an upper limit of the mrestc2 component of 10-17eV, (very small but, of course, an experiment can't yield a totally zero value).
So that's a fairly good justification for assuming it's zero and it would seem to go along with relativity as a model.