Electrical Engineering

Signal in a copper wire is simply continuous electric current (commonly referred to as electricity), whether alternating current (AC) or direct current (DC). It is not different than the electric power transmission lines that carry electric energy (or power) from power plants to distribution companies and houses.

Continuous electric current is the net motion of free electrons that are a part of the atomic structure of every conductor (i.e. metal). In the absense of an electric field, free electrons are in random/chaotic/thermal motion within the conductor, i.e. they are moving randomly and very fast. When an electric field is applied, they start moving under the influence of the applied electric field (i.e. in a net direction), extremely slowly compared to their random motion, but moving nonetheless. This net motion of free electrons is what is referred to as "continuous electric current".

If signal is continuous electric current, and continuous electric current is the net motion of free electrons, then signal attenuation is fading of this net motion.

What resists to the net motion of free electrons is the fixed atoms on the lattice structure. The "flowing" free electrons collide with fixed lattice atoms and cause Ohmic heating (a.k.a. Joule heating). This explains the resistance of the cable, which is the reason of signal attenuation. As the signal travels on the cable, the voltage level will decrease due to "copper losses" (i.e. ohmic heating). Resistivity increases with length (and decreases with diameter), therefore the longer the cable, the higher the resistance (and attenuation).

Near-field electromagnetic fields around a current carrying wire traps the free electrons to the outer shell of the wire. This is referred to as "skin effect". This effectively decreases the cross-sectional area, which in turn increases the resistance, and which in turn increases the signal attenuation. Skin effect becomes more effective at higher frequency signals flowing through the wire. Therefore, higher frequency signals flows through a higher resistance and thus gets attenuated more.

Note that fading (i.e. attenuation) of the signal is not the only problem in signal transmission. When a signal is sent, it should arrive at the load (i.e. destination) above some treshold power (i.e. not attenuated too much), and it should arrive undistorted (i.e. not interfered with too much). These are the two concerns:

- attenuation

- interference.

Reactance of the conducting medium (i.e. the cable used) or external electromagnetic fields can interfere with the signal. Cable's reactance comes from capacitance and inductance of the multiple wires isolated (to a degree) from each other within the cable.

That's it for now, my friend ;]

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Voltage is not intended to be applied to a thermocouple. Heat is, and that will result is a voltage being developed which can be used to measure the heat.

Actually what will happen depends on the polarity of the voltage applied to the thermocouple. One polarity will heat the thermocouple, the opposite polarity will cool the thermocouple. This phenomenon can be used to make a tiny refrigeration system needing no refrigerant fluid or moving parts (other than a fan to expel the unwanted heat) called a thermoelectric cooler.

A practical thermoelectric cooler is not built with ordinary thermocouples used to measure temperature or generate electricity. Instead it uses large thermocouples having a contact area between the dissimilar metals measured in square inches. Two of these large thermocouples are connected back to back in opposite polarity with a thermal insulator between them. When power is applied, one gets cold while the other gets hot.

Thermoelectric coolers are used in microprocessor heatsinks and in small portable refrigerators that are powered from a car's cigarette lighter outlet. In fact most of these small portable electric refrigerators have a switch that reverses power to the thermoelectric cooler, making it a heater instead; thus they can keep food either cold or hot, whichever is appropriate.

In a transmission line, voltage is stepped up to a very high value. This is so as to take advantage of lower line current and smaller conductor size. At these potentials1, however, like charges repel, so most of the current flow is along the skin of the conductor. Stranded conductors, even wound wire over insulating rope, is more efficient, and more cost effective, than solid conductors.

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1 Actually, this effect occurs at any voltage, but it is most notable at the very high voltages used in power line transmission systems.

Stranded conductors are used because of their flexibility compared with solid conductors. Normally, transmission lines consist of a steel-wire core (which provides strength) around which are wound stranded aluminium conductors.

Resistance depends on the thickness and length of the wire used, as well as the conductor used. For example, a short, thick wire made of copper will conduct electricity better than a long, thin wire made of, say, iron.

Resistance is directly proportional to the length and inversely proportional to the cross-sectional area (not 'thickness') of a material. Its constant of proportionality is called resistivity which is affected by temperature -so temperature indirectly affects resistance.