In this post AC means <Alternating Current> unless otherwise mentioned like AC Voltage, which is not <Alternating Current> Voltage but more like <Alternating Voltage>. Let's see the generally accepted idea of AC (Alternating Current).

Alternating current

From Wikipedia, the free encyclopedia (13-Dec-2013)

This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (April 2009)

Alternating Current (green curve). The horizontal axis measures time; the vertical, current or voltage.

In alternating current (AC, also ac), the flow of electric charge periodically reverses direction. In direct current (DC, also dc), the flow of electric charge is only in one direction.
The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.^[1] ^[2]
AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.

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If you think for a while about the above explanation on Alternating Current, some simple questions arise.

Question - 1)

"In alternating current (AC, also ac), the flow of electric charge periodically reverses direction."

The direction of AC reverses, which meas the direction changes periodically (back and forth) and usually in very precisely in terms of time - sine wave.

"AC is the form in which electric power is delivered to businesses and residences."

If this be true how electric power is delivered to businesses and residences (seems to one direction, from a power station to businesses and residences) despite AC changes the direction periodically (back and forth).

Question - 2)

"Audio and radio signals carried on electrical wires are also examples of alternating current."

If this be true how Audio and radio signals are carried on electrical wires (seems to one direction, from a radio antenna through electrical wires to a loudspeaker or ear phone) despite AC changes the direction periodically.

We have some answers to Question -2)

Radio signals (in Electromagnetic Wave form) in space travel far to every direction (all directions), not change direction periodically. You can find more detailed explanations easily but usually with many math equations, which seem very convincing due to the massive math.

But how about Audio and radio signals on electrical wires after coming into through an antenna? By using DC bias the direction of signals changes from AC (changing direction periodically back and force) to pulsating DC (not changing direction periodically but changing the magnitude periodically). (see the chart at the top)

<wave> may be a hint when considering the direction of AC (Alternating Current)t.

Additional Question - Direction of time.

I have found few people mentioning this but the direction of the arrow of the horizontal lime indicates that the direction of time is forward not backward. This contradicts the concept of time in our daily life, which is backward. Time is passing through us - from front to back.

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Transmission, distribution, and domestic power supply

Wik on AC (13-Dec-2013) continued

AC voltage may be increased or decreased with a transformer. Use of a higher voltage leads to significantly more efficient transmission of power. The power losses in a conductor are a product of the square of the current and the resistance of the conductor, described by the formula

$P_{\rm L} = I^2 R \, .$

This means that when transmitting a fixed power on a given wire, if the current is doubled, the power loss will be four times greater.
The power transmitted is equal to the product of the current and the voltage (assuming no phase difference); that is,

$P_{\rm T} = IV \, .$

Thus, the same amount of power can be transmitted with a lower current by increasing the voltage. It is therefore advantageous when transmitting large amounts of power to distribute the power with high voltages (often hundreds of kilovolts).

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Like other explanation on AC (Alternating Current), the story changes to AC Power with the equations and explanation like above The explanations usually do not mention the direction of AC (Alternating Current). Electrical Power (as well as Electrical Energy) is not a vector quantity but scalar quantity so does not have direction. Transmission means the electrical power is transmitted from one place (power station) to another (to businesses and residences）so it must have one direction.

Effects at high frequencies

Wik on AC (13-Dec-2013) continued

Main article: Skin effect

A direct current flows uniformly throughout the cross-section of a uniform wire. An alternating current of any frequency is forced away from the wire's center, toward its outer surface. This is because the acceleration of an electric charge in an alternating current produces waves of electromagnetic radiation that cancel the propagation of electricity toward the center of materials with high conductivity. This phenomenon is called skin effect.
At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for power transmission (50–60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost.
Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I²R loss).

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"A direct current flows uniformly throughout the cross-section of a uniform wire." As a belief it is OK but some explanation with evidences are required.

"This is because the acceleration of an electric charge in an alternating current produces waves of electromagnetic radiation that cancel the propagation of electricity toward the center of materials with high conductivity. This phenomenon is called skin effect."

This explanation is too simple as no math equations but suggest some important things on our concern the direction of AC (Alternating Current).

The key ward may be again <waves>.

Techniques for reducing AC resistance

Wik on AC (13-Dec-2013) continued

For low to medium frequencies, conductors can be divided into stranded wires, each insulated from one other, and the relative positions of individual strands specially arranged within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current throughout the total cross section of the stranded conductors. Litz wire is used for making high-Q inductors, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher radio frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.

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The last sentence (not main clause but subordinate clause) suggests rather implicitly and partly explicitly <wires carry radio frequency current (a kind of AC and current in electromagnetic wave form)> but does not mention the direction of the current but likely a kind of AC - the current changing the direction or magnitude periodically.

Techniques for reducing radiation loss

As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated is lost. Depending on the frequency, different techniques are used to minimize the loss due to radiation.

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The first sentence is very suggestive as an answer to our question <Does Alternating Current flow ?>. What is < periodic acceleration> ? (meanwhile <As written above> is strange as this is never mentioned above>). The explanation is too simple here.

Twisted pairs

At frequencies up to about 1 GHz, pairs of wires are twisted together in a cable, forming a twisted pair. This reduces losses from electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signalling system, so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively cancelled by radiation from the other wire, resulting in almost no radiation loss.

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This statement suggests <signal current moves back and forth>.

Coaxial cables

Coaxial cables are commonly used at audio frequencies and above for convenience. A coaxial cable has a conductive wire inside a conductive tube, separated by a dielectric layer. The current flowing on the inner conductor is equal and opposite to the current flowing on the inner surface of the tube. The electromagnetic field is thus completely contained within the tube, and (ideally) no energy is lost to radiation or coupling outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 5 GHz. For microwave frequencies greater than 5 GHz, the losses (due mainly to the electrical resistance of the central conductor) become too large, making waveguides a more efficient medium for transmitting energy. Coaxial cables with an air rather than solid dielectric are preferred as they transmit power with lower loss.

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This explanation also suggests <<signal current moves back and forth>.

Waveguides

Waveguides are similar to coax cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an electric current, but rather by means of a guided electromagnetic field. Although surface currents do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide.
Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so they are only feasible at microwave frequencies. In addition to this mechanical feasibility, electrical resistance of the non-ideal metals forming the walls of the waveguide cause dissipation of power (surface currents flowing on lossy conductors dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.

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This explanation on Waveguide is very suggestive on our concern - again the direction of AC (Alternating Current).

Fiber optics

At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.

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This statement is also very interesting.

Mathematics of AC voltages

Wik on AC (13-Dec-2013) continued

Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:

$v(t)=V_\mathrm{peak}\cdot\sin(\omega t)$ ,

where

$\displaystyle V_{\rm peak}$ is the peak voltage (unit: volt),
is the angular frequency (unit: radians per second)
- The angular frequency is related to the physical frequency, $\displaystyle f$ (unit = hertz), which represents the number of cycles per second, by the equation $\displaystyle\omega = 2\pi f$ .
$\displaystyle t$ is the time (unit: second).

Power and root mean square

The relationship between voltage and the power delivered is

$p(t) = \frac{v^2(t)}{R}$ where $R$ represents a load resistance.

Rather than using instantaneous power, $p(t)$ , it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value, written as $V_{\rm rms}$ , because

$P_{\rm time~averaged} = \frac{{V^2}_{\rm rms}}{R}.$

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These equations remind us

The power losses in a conductor are a product of the square of the current and the resistance of the conductor, described by the formula

$P_{\rm L} = I^2 R \, .$

This equation is relatively easy to understand intuitively as the current flows (sorry for using <flow> here through resistor, which use power. But

$P_{\rm time~averaged} = \frac{{V^2}_{\rm rms}}{R}.$

is unti-intuitive as V is generally potential. But when we consider that voltage and the unit of voltage - volt (s) are defined as

Voltage = Energy / Charge. 1 volt = 1 Joule / 1 coulomb

Energy comes in.

Anyway among the Wiki explanation on AC we cannot find any explicit explanation on our concern - the direction of AC (Alternating Current). So we must continue to seek.

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My very old Penguin dictionary of Electronics (printed in 1980) simply explains

in <wave>

An alternating current propagated through a long chain of network or filter behaves as if it were wave. Elementary particles, such as electrons, have associated wavelike characteristics. See also Doppler effect.

I think the word <to propagate> is more appropriate than <to flow> when to show the direction of Alternating Current.

sptt

Asnetcomp Tutorials

Friday, December 13, 2013

Does Alternating Current flow ?

In this post AC means <Alternating Current> unless otherwise mentioned like AC Voltage, which is not <Alternating Current> Voltage but more like <Alternating Voltage>. Let's see the generally accepted idea of AC (Alternating Current).

Alternating current

Transmission, distribution, and domestic power supply

Wik on AC (13-Dec-2013) continued

Effects at high frequencies

Wik on AC (13-Dec-2013) continued

Techniques for reducing AC resistance

Wik on AC (13-Dec-2013) continued

Twisted pairs

Coaxial cables

Waveguides

Fiber optics

Wik on AC (13-Dec-2013) continued

Power and root mean square

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