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"):

[tex]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][/tex]

(H is related to B through the ratio given by the vacuum permittivity constant [itex]\mu_0[/itex])

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

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)

[tex]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]))[/tex]


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?

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 ?

showthread.php?t=603580

Thanks :)

xAxis

Is that what PhilDSP sad? I didn't get it. I thought you asked why steady current doesn't produce EM wave.

PhilDSP

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.

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?

sophiecentaur

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.

davenn

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

davenn

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) doesnt 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?

sophiecentaur

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

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

davenn

no thats not quite correct. The amplifier AMPLIFIES the signal, else it wouldnt 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