MRC_Hans
Penultimate Amazing
- Joined
- Aug 28, 2002
- Messages
- 24,961
Power lines do not radiate EM waves, for the reasons Moulder mentions. But they are surrounded by electromagnetic fields. Energy can be, and is, coupled out of the power lines, but, as Moulder notes, not to an extent that it is considered when calculating the losses in a line.cogreslab said:Moulder’s question 5 reads:
5) Do power lines produce electromagnetic radiation?
His answer reads as follows:
"To be an effective radiation source an antenna must have a length comparable to its wavelength. Power-frequency sources are clearly too short compared to their wavelength (5,000 km) to be effective radiation sources. Calculations show that the typical maximum power radiated by a power line would be less than 0.0001 microwatts/cm^2, compared to the 0.2 microwatts/cm^2 that a full moon delivers to the Earth's surface on a clear night. The issue of whether power lines could produced ionizing radiation is covered in Q21B.
This is not to say that there is no loss of power during transmission. There are sources of loss in transmission lines that have nothing to do with "radiation" (in the sense as it is used in electromagnetic theory). Much of the loss of energy is a result of resistive heating; this is in sharp contrast to radiofrequency and microwave antennas, which "lose" energy to space by radiation. Likewise, there are many ways of transmitting energy that do not involve radiation; electric circuits do it all the time".
Moulder appears to be saying here that ELF power lines do not radiate, and though there has been some dispute about this I am inclined to agree with him. This is not to say that their effects cannot have action at a distance, however.
Energy coupling through electromagnetic AC fields can happen in two ways:
Capacitive coupling: This is the electric field; any conductor near the power wire will form a capacitor together with the wire, and an AC current is coupled from the wire, through the capacitance, and to ground.
Inductive coupling: This is the magnetic field. In any conductor that is inside the magnetic field, an EMF (electromotoric force) is induced. If the conductor is so rranged that the EMF can make a current run, the energy used for this is coupled out of the power wire (slightly increasing its impedance), like in a transformer.
Hans