There's No Such Thing as an Electromagnetic Free Lunch
Martin_H
Posts: 4,051
When a load is attached to the secondary coil of a transformer, the current that flows through that circuit comes from the primary via the inductive coupling between the coils. I've heard that if you put an amp meter on the primary you can measure the increased current draw in the primary as the load on the secondary increases.
What this means is that a wireless charger consumes more power when a phone is charging than when a phone is not.
Here's the weird thought.
A radio station broadcasts by moving electrons up and down an antenna to broadcast an electromagnetic wave. This seem a bit like a transformer with one turn. If I have a crystal radio its antenna is acting like the secondary coil in this one to one transformer. There is now a current flowing through the circuit and providing sound. Granted it is a few milliwatts, but it is not zero, and it had to come from somewhere, so obviously it came from the radio station (the primary).
Here's the weird questions.
Is this correct and could the additional load be measured? For example assume we turn on ten thousand crystal radios at once, that starts to add up!
tl;dr Does an unlisted radio station consume less power than a popular one?
What this means is that a wireless charger consumes more power when a phone is charging than when a phone is not.
Here's the weird thought.
A radio station broadcasts by moving electrons up and down an antenna to broadcast an electromagnetic wave. This seem a bit like a transformer with one turn. If I have a crystal radio its antenna is acting like the secondary coil in this one to one transformer. There is now a current flowing through the circuit and providing sound. Granted it is a few milliwatts, but it is not zero, and it had to come from somewhere, so obviously it came from the radio station (the primary).
Here's the weird questions.
Is this correct and could the additional load be measured? For example assume we turn on ten thousand crystal radios at once, that starts to add up!
tl;dr Does an unlisted radio station consume less power than a popular one?
Comments
I don't think turning on a bunch of crystal radios would increase the load at the transmitting station rather I think the extra radios would greatly reduce the distance the signal travels. I don't mean to imply the signal will reduce overall but any signal passing through a crystal radio's antenna would be greatly reduced in power so the extra receivers would act as a small barrier to the propagating signal.
At least this the way I've made sense of it in my head.
Hopefully someone with a better understanding of this stuff will chime in.
Edit: I have very little confidence in my explanation. I'm starting to think Martin_H is likely correct in thinking popular radio stations would require more power than unpopular stations.
In the transformer it is magnetic flux that transfers the energy. In radio it is radio frequency waves.
Popular radio stations can afford bigger transmitters and antennas, better sound for the listener. Static free, able to be uninterrupted around tall buildings and under overpasses. But not to interfere with other stations in other cities on the same frequency. There is a lot of interferance to overcome, radio's everywhere.
OK, I think I understand this problem. Radio waves are electromagnetic waves which are detached from their source and radiating away, while the magnetic flux is part of the electromagnetic field around the coil.
So the radio station spent all its energy when it sent the signal out, even if no one listens to it. But I think Duane has a point that an antenna blocks antennas behind it, much like one solar panel shades any behind it. Granted that with diffraction reduces the effect, but there's still no free lunch, some power is lost.
But we've all been warned about living in close proximity to high tension power lines. I'm sure if you set up a long wire antenna under one of those it would act like a transformer secondary and you could draw a bit of power from the lines. Would probably be very hard to detect with all the other line losses. I look at the giant steel towers holding the lines up via insulators and wonder how much energy they may be absorbing. Are those towers grounded?
WRT energy, other fun things to ponder are Free power radios, which rectify all the other signals you're NOT listening to, and use that energy to power an amplifier. Not much, but something. See #153 at http://www.crystalradio.net/crystalplans/
See also http://www.ke3ij.com/nopower.htm
http://www.intio.or.jp/jf10zl/PLTX.htm
People have been caught and prosecuted for stealing power that way:
http://www.industrytap.com/electromagnetic-harvesters-free-lunch-or-theft/1805
-Phil
Power company equipment detected the drain of energy and went to investigate. The farmer was arrested for using electricity from the power company without a meter.
Last month, a guy was walking through our neighborhood pointing a very sci-fi "gun" at houses & power lines. The wife alerted me to him. Gamma rays? Shy young (?) me went out to welcome/confront the fellow. He was friendly enough and said he was with Frontier Communications (who just took over for Verizon in our 'hood) and was sniffing for a detected signal loss atop the newly-replaced phone poles behind our houses. Lot sizes here in God's Country are only ~50' wide by 100' deep, so we & our neighbors take an interest in who's sniffing what.
I can understand the legal basis that is involved, if it is the farmers land, and the electric company leased or bought the property the towers stand on.
But if HV overhanging wires cross your property between towers or run alongside, the electric company may use eminent domain to cross your land, and you are not compensated for that infraction. What is stopping you from trying to grab a few watts through the air.
I don't think they can stop you from setting up a SW antenna, not that it could be used for that being in close proximity to what? 50, 000 Volts.
EDIT: You can always wear some kind of electromagnetic clothing to harness some power. If it is going through your body anyway, maybe there is enough to keep your phone charged.
If anyone thinks that claim is possible, would you please explain where my assumptions are wrong?
While the first claim might be possible, I seriously doubt the practicality of it. Don't forget his barn would be in a mixed three-phase field, not at all like each phase being individually fed to transformer windings. I doubt that one could extract useful AC mains power from such a field with nothing but coils. You might get some power, but I would think you'd have to run a motor-generator set or something to get the usual single-phase power that houses run from.
People HAVE tapped power illegally, but as far as I'm aware, it's done with direct connections (i.e. wire), not induction.
In other news the radio works, but its selectivity isn't good as it seems to receive multiple channels at once. It was supposed to be tunable by moving a pickup up and down the coil, which should change the inductance to favor one channel. That kinda works.
However, if you move your hands in various places near the coil your hand capacitance seems to partially favor one channel over another. So I imagine it might be improved if I added a variable capacitor, but my junk drawer is fresh out of them.
-Phil
I've had several physics instructors talk about people stealing power using induction coils. It might not be a common issue but I'm pretty sure it does happen.
[edit] I've searched a bit, and can't find any examples of something practical. Lots of comments about how it should be possible, but those comments seem to conveniently ignore the fact that there are three phases.
[edit 2] I did find this https://user.physics.unc.edu/~deardorf/phys25/rwp/exam1rwpsolution.html Even assuming a 230 kV single-phase line, it still wasn't even close to practical. I don't know that a physics exam question is much of an analysis, but it shows that it isn't as easy as it sounds. When the fields from three phases combine, I think the result is an even weaker field due to some cancellation.
Close enough: http://www.ebay.com/itm/Variable-Capacitor-Dual-18-420-pF-18-135-pF-3-8-shaft-Ham-Radio-/252608094870
http://www.ebay.com/itm/Neat-Homebrew-Crystal-Set-Cat-Whisker-Detector-f-Old-Ham-Radio-Receiver-Tuner-/391630580603
Those are great questions. They remind me of why I thought the magnetism and electricity semester of physics was so hard.
Those questions sure make it seem like steeling power with a coil isn't very efficient.
Thanks for taking the time to find the link.
Just don't forget the right hand rule.
Or was it the left hand rule?
Or both.
Must be good money, I could think of better ways to enjoy a view.