sci.physics.electromag

"Szczepan Bia³ek" < @ > wrote in message
news:fae4id$j1g$1@ ...
>
> "Bill Miller" <
>> "Szczepan Bia³ek" <
>>>>>
>>>>> In current-carrying bodies the electrons behave like gas. Have you any
>>>>> objections?
>>>>> If no, you should throw away the four ways and try the new (new in the
>>>>> sense of textbooks - engineering know that)
>>>>
>>>> I'm sorry. I do not understand your point. Can you please expand or
>>>> re-state?
>>>
>>> I must use citations. Here is the "Hydraulic analogy":
>>> /wiki/Hydraulic_analogy . Todays no the "Gas
>>> analogy" (compressible gas) for currents in conductors. I suggest to
>>> try such way. It means for the wire that there are not only the gradient
>>> of the voltage but also the gradient of charge. The all "hydraulic"
>>> equations are here useless. The columb method should be used.
>>> S*
>>
>> I'm sorry, but I do not see the relevance here. Just to illustrate one
>> major difference, if we have a hose connected to a mechanical valve and
>> source, it takes a finite -- and long -- time for the fluid to make its
>> way from the source to a "bucket" at the hose's exit.
>
> Wrong. The "electrical" hose is always full. The water hose can be also
> full. So it does not take time,

Well... yes and no. In a disconnected wire, the electrons are in motion, but
the overall effect of their motion is electrically neutral. If the hose is
full, and the valve is closed, then there is nothing to keep the fluid from
flowing out (unless the valve is at the end of the hose, and then we have
changed the conditions of the example)
>>
>> In contrast, suppose we connect an electrical power source to a switch,
>> some wires and (say) a light bulb on the other side of the room. We
>> activate the switch and the light will illuminate essentially instantly
>> although electrons themselves flow so slowly that most people can walk
>> faster than they move!
>>
>> There are many other important differences, such as AC vs DC (Fluid flow
>> is DC whereas with AC, the electrons almost NEVER get from the source to
>> the load), RF skin effect (Electron flow is at the surface) vs fluid flow
>> (zero flow at the perimeter of the pipe, maximum at the center) etc. etc.
>
> The above are commonly known. The hydraulic analogy is goood enough for
> steady currents. But for oscillating no. To adopt the hydraulic equations
> for oscillating currents the displacement current was added. At that time
> "gas equations" did not exist.
> You often wrote about antenas. What electrons are like in antenas? Water
> or gas. (In water only the pressure is changing. in gas also density -
> imagine what it means)

Actually, in antennas, it seems that electron flow has nothing to do with
radiation. Instead, the metal seems to "guide" the ready-to-radiate EM
energy to where it can do so. An excellent example of this is a microwave
"horn" radiator, fed with waveguide. The "horn" can be formed by slowly
expanding the dimensions of the waveguide until the Zo of the guide matches
that of free space (377 ohm). At that point, the microwave energy radiates
from the open end of the waveguide into free space.

A similar phenomenon exists in non-radiating structures, but that is the
subject of another -- and MUCH longer discussion. This was understood in the
19th century and first (?) espoused by Prof. Poynting in his seminal paper
that introduced us to what we now call the Poynting Vector. Please see:

/~mcdonald/examples/EM/

As Poynting explains, current flow is *always* associated with fields. The
same cannot be said for fluids.

All the best!
Bill Miller
> S*
>