Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 09:34 PM 3/27/2007, Tesla list wrote:
Original poster: "Drake Schutt" <drake89@xxxxxxxxx>Well I always watch this debate whenever it comes up, but this time I have a question.Can someone please explain to me how TC's transmit electricity and why they're so inefficient, in terms that someone with a basic knowledge physics (AP next year!) and precalculus could understand?
#1 - any form of electromagnetic radiation, once you're sufficiently far from the source, expands spherically: inverse square law. The receiver can only "intercept" some fraction of the total power radiated. Yes, you can have a directive antenna, but even with that, the beam spreads out.
#2 - the conversion of "wall plug power" to electromagnetic energy that can propagate has fairly low efficiency in a TC. In a suitable high power RF amplifier or source (like a microwave oven magnetron or a Class C or Class E/F amplifier) one might get the efficiency up into the 60-70% range.
#3 - conversion of electromagnetic energy back into a usable form is also inefficient (perhaps with the exception of conversion to heat, as in a solar collector)
The stack up of #2, #1, and #3 makes long distance transmission of large amounts of power by EM radiation less than wonderful, although systems have been built to deal with certain aspects, and accepting the limitations.. If you build a very large (many wavelengths) antenna at each end, put one end in orbit with a bunch of solar cells, and you might break even against putting all the solar cells on the surface (because the satellite could be illuminated 24/7, but on the ground, only an average of 5-8 hours a day, so as long as the whole solarcell/satellite transmitter/inverse square loss/ground receiver/conversion to power is better than 25% efficient, you're ahead of the game... )
Please note.. it's not that power can't be transmitted wirelessly (it's done all the time, in the form of radio), it's that it can't be done in a way that is even remotely economically competitive with conventional wires. You're up against a technology that is well understood, and can transmit Gigawatts from one place to another 1000 km away with better than 90% efficiency. Not only that, but having all that power nicely contained with in a transmission line reduces the liklihood of undesirable side effects: power going places you don't want it.
Wires are also convenient because they contain the energy, providing a means by which one can easily measure it, and charge for it, as well as disconnecting a customer if they don't pay for it, all of which provides a means to pay for the capital investment required to build the system in the first place.
An important note.... if the transmitter and receiver are *close* together, then you can couple power by capacitance or mutual inductance, and actually transmit useful amounts of power. For instance, the RFID chip is powered by RF energy coupled to it from the reader. Inductive charging coupling is used for things like electric toothbrushes, etc. In these cases, the distance between the two "antennas" is small compared to the size of the antenna.
Also what is it that proponents of wireless power transmission propose we do to 'light up the world' and why is this "against the laws of physics", as i believe a lot of people say?
One last question- I remember hearing that when a coil is tuned for maximum energy transfer it produces minimal sparks, is this true?thanks for any responses, I'm sure there are answers in the archives but I'm too lazy to sort through all of the flame wars.Drake Schutt