Difference between electricity and electromagnetic waves

[h3x3n]

Hello,

Is there any difference between 50/60 AC current and a 50/60 Hz photon stream/electromagnetic wave propagating through a medium ??

Regards,
Hex

 

[mfb]

Where is the similarity between those two, apart from the common frequency?

 

[h3x3n]

Where is the similarity between those two, apart from the common frequency?

My confusion started because of the following lines in a textbook i am reading (skilling - fundamentals of electric waves) where there is quote along the lines of
"circuit theory may be considered as a special case of more general theory of electromagnetic fields" from page numbered 1.

 

[mfb]

Circuits work with electromagnetic fields and electric currents and charges - but they require a completely different analysis compared to electromagnetic waves in vacuum or a medium.

 

[nsaspook]

My confusion started because of the following lines in a textbook i am reading (skilling - fundamentals of electric waves) where there is quote along the lines of
"circuit theory may be considered as a special case of more general theory of electromagnetic fields" from page numbered 1.

The special case is mainly non-radiation of EM energy in free-space, the energy from point A to point B remains in the near-field and it's field components can be computed accurately using point measured voltage/current magnitudes and phases. It's mainly a matter of circuit size and signal wavelength that determines when "circuit theory" stops being accurate not a absolute frequency.

At the level of EM energy there is not a difference because in both cases the actually energy is carried in fields (near-field with the wire creating a reactive impedance space around it or the far-field free-space resistive impedance for the Em wave).

 

[h3x3n]

@mfb and @nsaspook : thanks for the reply.
Quoting from http://www.capturedlightning.com/frames/Non-Herzian_Waves.html
"but one particularly useful consequence is the tendency of long thin conductors to guide an EM wave along themselves. When an EM field is guided in this way, we call it electricity flowing along a wire,"
It is from the section titled "How do charges behave in a conductor?"
This confuses me even more.

 

[nsaspook]

@mfb and @nsaspook : thanks for the reply.
Quoting from http://www.capturedlightning.com/frames/Non-Herzian_Waves.html
"but one particularly useful consequence is the tendency of long thin conductors to guide an EM wave along themselves. When an EM field is guided in this way, we call it electricity flowing along a wire,"
It is from the section titled "How do charges behave in a conductor?"
This confuses me even more.

You will be confused if you think that "electricity" is different from EM energy.
From your link:

Tesla would be entitled, as any electrician is, to say that the energy is carried by the current. But he fails to appreciate that the current will only flow in conjunction with the associated field. The current will not flow without the field, because by definition the field is that thing which applies a force to a particle through the particle's property of charge. Short of proposing a fundamental new interaction involving charge (which he doesn't seem to be doing) Tesla's proposals can only be based on a fairly elementary failure to appreciate just what charges, currents and fields are all about.

This is, indeed, a rather common misconception, and often occurs amongst those trained in electronics and electrical engineering. It is natural (and perfectly reasonable within those domains) to think of the energy as flowing through wires, carried by currents. Voltage is seen as a kind of pressure and current is regarded as a flow of 'electricity', much like a flow of water in a pipe. Fields are regarded as incidental, sometimes a nuisance, and electricians are happy enough with any circuit until the wires reach an antenna, at which point a mysterious thing called radiation happens. The electrician's explanations tend to disolve into handwaving at that point as he struggles to address the question of just where and how is the energy transferred from the circuit to the field.

The physicist sees things differently by viewing nature from a wider perspective - one which isn't constrained by the limited and artificial view of nature given by circuit theory. When looking at an electrical circuit, the physicist sees a complex pattern of E and H fields formed into a rich and marvelously functional 3-D pattern by the boundary conditions imposed by lots of carefully arranged conductors and dielectrics. The technology of arranging those conductors and dielectrics is called electronics. Here and there those conductors might be formed into shapes that allow the fields to spread out in certain useful ways, and the electrician would call one of those bits the antenna.

The physicist's view of an electronic circuit as an intricate pattern of ripples and tensions in the EM field, anchored in place by a beautifully crafted assemblage of materials, is a wonder to behold and helps us to remember that no change in the physics occurs when we move from 'energy in the circuit' to 'energy in the air', and that such a move is merely a convenient change of descriptive language.

By the way, note that it is not necessary for us to say that the current 'causes' the field, or that the field 'causes' the current. There's no cause and effect process in EM interactions at the fundamental level. EM doesn't contain an 'arrow of time'. It is only when thermodynamics steps in, and you start to look at the so called entropy of the system that it becomes meaningful to say that energy flows from A to B. As far as the particle/field is concerned it is just A interacting with B. It is only when you move outside EM and consider the thermodynamic implications of the A-B interaction that you can begin to identify a cause-effect relationship. Only then can you unambiguously declare one to be the 'source' and the other to be the 'load'. The important thing is that charge and field are inseparable, you can't choose between one or the other as a choice of energy transfer mechanism, but you can choose between them as descriptions of the transfer process, and of course, both give the same correct answers.

 

[h3x3n]

Where is the similarity between those two, apart from the common frequency?

You will be confused if you think that "electricity" is different from EM energy.
From your link:

Sorry for bringing it up again, but could you please clear this confusion

So "electricity" is not electromagnetic wave but it is electromagnetic energy ??

 

[andrien]

electricity is moving charges(mainly electrons in circuitry) and electromagnetic waves are produced by charges in motion(not same as charges,but say some photons!)

 

[nsaspook]

Sorry for bringing it up again, but could you please clear this confusion

So "electricity" is not electromagnetic wave but it is electromagnetic energy ??

My standard "what is electricity" link: http://amasci.com/miscon/whatis.html

For the same reason, we will never find a simple answer to the question "what is electricity?" because the question itself is wrong. First we must realize that "electricity" does not exist. There is no single thing named "electricity." We must learn that, while several different things do exist inside wires, people wrongly call all of them by a single name.

http://amasci.com/elect/elefaq1.html#ae

 

[BruceW]

So "electricity" is not electromagnetic wave but it is electromagnetic energy ??

"electricity" is a very vague term that doesn't mean anything specific. I would say it is mostly associated with how electromagnetism is useful to us humans. For example, it is used in terms like "electrical device". Also, it is strongly associated with electromagnetic energy because one of the important uses of electromagnetism for us humans is to turn EM energy into other forms (such as light).

 

[h3x3n]

Hello i got another doubt,

If electricity is electromagnetic energy, what is the need for photovoltaic ??
Cant sunlight for example be used directly as electricity ??

 

[mfb]

You cannot connect a cable to sunlight.
It has been proposed to directly use the oscillating electromagnetic field from sunlight, but that is an engineering challenge.

 

[Dale]

If electricity is electromagnetic energy, what is the need for photovoltaic ??
Cant sunlight for example be used directly as electricity ??

A given electrical device works on a fairly narrow range of currents and potentials as well as a narrow range of frequencies. It cannot utilize energy that doesn't match its operating specifications quite closely. Sunlight typically has frequencies much too high, and voltages and currents much too low.

 

[nsaspook]

You cannot connect a cable to sunlight.
It has been proposed to directly use the oscillating electromagnetic field from sunlight, but that is an engineering challenge.

The main problem is not the size and structure of the "wiring" to contain the EM fields at light frequencies but the fabrication of diodes to convert the energy into a DC voltage like is done with utility frequency energy. Nanoscale EM structures can also be used to focus light energy to increase the conversion efficiency of some types of solarcells.

http://arxiv.org/pdf/1204.0330.pdf
http://www.sciencedaily.com/releases/2011/12/111222142459.htm
http://www.researchgate.net/publication/228801855_Optical_nano-antennas_and_metamaterials

 

[h3x3n]

Thanks to everyone who replied and helped (esp mfb and nsaspook)
A related web page is at http://www.Newton.dep.anl.gov/askasci/phy99/phy99272.htm

 

[anorlunda]

If you put a pulse in one end of a 1000 mile power line, how long does it take to come out the other end? A wire is not free space so the answer is not 1000/c, but it is still very fast close to 800/c. One needs to solve the wave equations to calculate that.

But the wave effects die away quickly, leaving other equations to better describe what happens on the scale of milliseconds or seconds. Ohm's law, rather than the wave equation is a better tool for those time scales.

The point is that for engineering analysis we almost always work with equations that are simplified forms of more general equations. That doesn't make the general equations invalid, or suggest any conflict. The wave equations for electromagnetic propagation and Ohm's law are both valid and each is more practical for studying a subset of the problem.

As a power engineer, I've had to deal with electrical phenomena with time scales of nanoseconds up to and including decades. Every analysis chooses to neglect faster effects (assume them to be instantaneous) and slower effects (assume them to be constant) and to deal with what remains in the middle. In other words, we partition analysis by time duration, often coming up with completely different sets of equations for each duration. But there is no conflict between these equations, each is a specialized subset of the whole.

I suspect that is part of the source of your confusion. Both electromagnetic waves and Ohm's law are valid but useful in different circumstances.

 

[Dale]

As a power engineer, I've had to deal with electrical phenomena with time scales of nanoseconds up to and including decades.

That is interesting. What kind of electrical phenomena has a time scale of decades in power engineering?

 

[RonL]

That is interesting. What kind of electrical phenomena has a time scale of decades in power engineering?

Maybe this ?

http://en.wikipedia.org/wiki/Centennial_Light

 

[anorlunda]

That is interesting. What kind of electrical phenomena has a time scale of decades in power engineering?

Generation planning: making sure there is adequate generation of the right type at the right place to serve future demands.

I'm sure physicists will sneer at the idea, but a power engineer has to deal with physics and human behavior at the same time. We may approach quanta at the fast end of the time spectrum and demographics/politics at the slow end. The point is that it is a continuum. In some regions of the time domain behavioral and physical models overlap and interact. Engineers, but not scientists, are forced to deal with both.

For example, in the region of days-months, the dominant equations are the economics of the energy/capacity markets but everything bought or sold must conform to what the grid and the power plants are physically able to do. An operating plan must also be prepared for any weather nature might deliver, plus a spectrum of equipment failures and repairs.

These things can not be divided up and solved separately; they interact. We do however, break up the problem into different time domain regions. We partition by duration not by discipline.

Roughly speaking, anything less than 15 minutes is almost purely the comfortable domains of physics/chemistry/nuclear.