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Given an almost unlimited budget, would it be safe and possible to have wireless charging everywhere within a town?

For example I could be walking down the street, phone in my pocket, constantly being charged. Never having to worry about it running out?

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  • $\begingroup$ What you could do, which is similar in concept, is to put contact based, plug free, induction chargers all over the place, so that you could plop your phone on one anywhere in town and get it charged for free. This would require very minimal technological innovation and wouldn't be terribly expensive. Most of the cost would be installation which could be financed by making it part of the building code in a new part of town, and the actual electrical cost would be minimal can could be financed by the town if desired by giving everyone in town a 1% rebate on their electrical bills or some such. $\endgroup$
    – ohwilleke
    Nov 17, 2016 at 23:57
  • $\begingroup$ Too bad for your almost unlimited budget because all resources would be depleted before wireless charging technology can justify its worth... $\endgroup$
    – user6760
    Nov 18, 2016 at 5:08

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The problem with wireless charging is technical and not economic.

The amount of money necessary to buy the electrical juice necessary for something like that is trivial. An iPhone6 uses a little less than 50 cents (U.S.) of electricity per year (about 1 U.S. cent of electricity per phone per week). If people just charged cell phones and there were 5,000 people in your little town, the electricity cost of providing free phone charging to everyone in the town would be about $200 a month. You could finance this program by having local school kids recycle can or look under seat cushions in public places for loose change. Hell, it is common right now to let people charge their electric cars for free in public parking lots.

The problem is that when you are transmitting energy through the air, the highly energetic EM fields are likely to be harmful to you, unlike a radio wave which is so low powered that it is hardly more harmful than background radiation. Essentially, a wireless recharging connection is like shooting a high powered laser beam or high powered X-ray (or gamma ray) from the wireless charging source to your device in more or less a straight line. At a minimum this would drastically increase cancer risks for everyone in the vicinity. In a worst case scenario, people would get painfully zapped and it might start fires as well. Wires have insulation for a reason, and you can't insulate the wireless transfer of power associated with a more than de minimus range wireless charger.

Usually, today, wireless chargers rely on induction of fields between two objects that are basically touching, so there is no danger zone between them that can hurt the users.

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  • $\begingroup$ RFID tags are often passive, thus relying on induction field to power it up so it can answer. Yet reading them from a distance is common. I'm not living in NA, but I think Walmart reads your whole cart at once, which implies an induction field going through the whole thing. Not that it really matters anyway, induction fields are harmless unless you use high enough a frequency that it starts heating up water (around 2GHz). $\endgroup$
    – spectras
    Nov 18, 2016 at 7:33
  • $\begingroup$ A passive RFID (radio frequency identification) tag is, by definition using low powered radio waves, and further gets away with a very weak signal by limiting the range of the low energy signal of a passive tag to 0.1 to 12 meters or so. en.wikipedia.org/wiki/Radio-frequency_identification and only operating for the tiny fraction of a second that it takes to send 96 bits of data. It doesn't take much energy to transmit a very short signal (considerably less than a single text message), at radio frequencies, for a very short distance, which makes it harmless. $\endgroup$
    – ohwilleke
    Nov 18, 2016 at 8:02
  • $\begingroup$ Specifically, it takes on the order of a millisecond to read an RFID tag. rfidjournal.com/blogs/experts/entry?10736 This is orders of magnitude less than what a cell phone uses in a second. $\endgroup$
    – ohwilleke
    Nov 18, 2016 at 8:24
  • $\begingroup$ My point was just: induction fields are not harmful per se, unless they use high enough a frequency that they start heating up water. If you want a more powerful example, induction stoves inject kilowatts and are still harmless — unless you're wearing some metallic stuff, especially if that's iron. The main issue with a strong city-wide EM field would be all metallic stuff in the city, not direct harm to humans. $\endgroup$
    – spectras
    Nov 18, 2016 at 9:50
  • $\begingroup$ …and the examples you give are misleading (XRay and laser) because they are high frequency and that's what makes them dangerous: laser energy gets released at the point it touches the skin (within a few mm actually). XRay is even worse because its frequency is high enough that it's ionizing. You don't have this problem with, say, a 25kHz induction field, as it goes right through the body without delivering its energy in it. $\endgroup$
    – spectras
    Nov 18, 2016 at 9:54
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No. Near-field resonate coil power transfer has a very short range. There is simply no practical phenomenon as you describe. It’s not a matter of spending money on infrastructure and standards—what you ask for simply does not exist.

See the summary on Wikipedia

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  • $\begingroup$ Near-field resonance is not the only way to wirelessly charge devices. Sure, most common now, but don't have to stay that way. $\endgroup$
    – Mołot
    Nov 18, 2016 at 0:27
  • $\begingroup$ Radiated energy would go everywhere and not just the receiver. What did you have in mind? $\endgroup$
    – JDługosz
    Nov 18, 2016 at 0:38
  • $\begingroup$ Maser, for example, was considered as a longer-distance method. $\endgroup$
    – Mołot
    Nov 18, 2016 at 0:56
  • $\begingroup$ I don’t think aiming a micowave laser at your pocketed devices is a good idea. The receiving antenna needs to be larger, and it needs to handle spill over radiation. $\endgroup$
    – JDługosz
    Nov 18, 2016 at 1:18
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    $\begingroup$ @kingledion> it has to be a minimum frequency to cook. If the frequency is lower, it will go through your meat without heating it up. $\endgroup$
    – spectras
    Nov 18, 2016 at 7:36

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