# Wireless Electricity **URL:** \ **Category:** Factual Questions\ **Created:** [March 9, 2010, 9:17am UTC](https://boards.straightdope.com/t/wireless-electricity/531832 "2010-03-09T09:17:58Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [March 10, 2010, 6:30pm UTC](https://boards.straightdope.com/t/wireless-electricity/531832/21 "2010-03-10T18:30:59Z")
> [@CurtC](#): > > It would be _possible_ for the banks to all get rid of ATM machines, and instead dispense cash by putting powerful blowers at centralized locations, and blowing hundred-dollar bills in every possible direction - you just grab the cash you need. I’ve stopped at those a couple times — but the withdrawal fees are _outrageous._ ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [March 10, 2010, 8:12pm UTC](https://boards.straightdope.com/t/wireless-electricity/531832/22 "2010-03-10T20:12:48Z")
At the CES 2010 trade show, there was a device on display which claimed to recharge batteries for your portable devices by harvesting power from nearby WiFi radio energy: [http://www.ohgizmo.com/2010/01/09/ces2010-rca-airnergy-charger-harvests-electricity-from-wifi/](http://www.ohgizmo.com/2010/01/09/ces2010-rca-airnergy-charger-harvests-electricity-from-wifi/) Comments on that story are fun: > [@Cynake](#): > > Here’s some math. Long story short, by my calculations, 100% efficiency and absorption at 5 feet away from a 100mW home router, (reasonable figures), it would take 34.5 years to charge that blackberry battery. > [@thinker0002](#): > > You are not taking into account that… it may harvest MULTIPLE hotspots… > [@Daniel Koch](#): > > Well, if there were 12,579 hotspots nearby, all only 5 feet away, then yeah, it might charge in a day (still assuming 100% efficiency). ---
**Author:** ![Fridgemagnet](https://avatars.discourse-cdn.com/v4/letter/f/22d042/32.png) [@Fridgemagnet](https://boards.straightdope.com/u/Fridgemagnet)\ **Post date:** [March 10, 2010, 8:21pm UTC](https://boards.straightdope.com/t/wireless-electricity/531832/23 "2010-03-10T20:21:40Z")
Re: **JoelUpchurch’s** links - I despair at the nonsense that people will invest in. The Korean electric pram is an absolute non-starter and I’m surprised that it’s got as far as a prototype. Aside from the massive infrastructure costs and terrible efficiency, it falls down flat on practicality. What happens when you want to deviate from the power rail? I doubt a vehicle would have much leeway for suspension travel at 12 cm, yet alone 1 cm. I assume it exists as showcase technology rather than anything that will be taken up soon, as Korean technology can be world-class, if not world-leading (like their 100 Mbps internet infrastructure). The train might see the light of day in cities that want to be perceived as cutting-edge, or in places where live rails or overhead cables are impractical, but I see no mention at all of efficiency figures buried in all the glossy corporate cockery. There will be a very good reason for this. One major aspect of inductive coupling that I’ve never seen mentioned anywhere is electromagnetic pollution. If people kicked up a fuss about radiation from their mobile phones, imagine how concerned they’d be if they thought the EM power levels on the street were many millions of times greater. Which is what they would be. The radiated power levels may even break local safety laws, are there are regulations setting upper limits (linked to exposure times) of the electric and magnetic field strengths that a person may be exposed to. > [@Chronos](#): > > Actually, it [the inverse square law] does apply to laser beams, too, just with a much smaller coefficient. A laser beam that didn’t spread out would violate the Heisenberg uncertainty principle. It is quite true that laser collimation is never perfect, and there will always be some divergence when attempting to produce a fixed-width beam, but it’s not an inverse-square law, that would imply power density dropping to a quarter with every doubling of distance. I didn’t know divergence was down to Heisenberg though (I’ve always lazily assumed it was down to imperfect optics), so now I can blame him for that too, along with the theoretical impossibility of Star Trek-style transporters and replicators. What a spoilsport! ---
**Author:** ![awldune](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@awldune](https://boards.straightdope.com/u/awldune)\ **Post date:** [March 10, 2010, 8:33pm UTC](https://boards.straightdope.com/t/wireless-electricity/531832/24 "2010-03-10T20:33:09Z")
My mother has a neurostimulator implant for chronic pain and it uses some sort of wireless charger to restore the batteries. I think the range is limited to a few inches, but it is certainly an improvement over having a surgical battery replacement. I think that similar tech exists for pacemakers. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 10, 2010, 8:50pm UTC](https://boards.straightdope.com/t/wireless-electricity/531832/25 "2010-03-10T20:50:07Z")
> [@](#): > > It is quite true that laser collimation is never perfect, and there will always be some divergence when attempting to produce a fixed-width beam, but it’s not an inverse-square law, that would imply power density dropping to a quarter with every doubling of distance. This is true, over long distances. ---
**Author:** ![JoelUpchurch](https://avatars.discourse-cdn.com/v4/letter/j/f05b48/32.png) [@JoelUpchurch](https://boards.straightdope.com/u/JoelUpchurch)\ **Post date:** [March 10, 2010, 10:50pm UTC](https://boards.straightdope.com/t/wireless-electricity/531832/26 "2010-03-10T22:50:17Z")
> [@Fridgemagnet](#): > > Re: **JoelUpchurch’s** links - I despair at the nonsense that people will invest in. The Korean electric pram is an absolute non-starter and I’m surprised that it’s got as far as a prototype. Aside from the massive infrastructure costs and terrible efficiency, it falls down flat on practicality. What happens when you want to deviate from the power rail? I doubt a vehicle would have much leeway for suspension travel at 12 cm, yet alone 1 cm. I assume it exists as showcase technology rather than anything that will be taken up soon, as Korean technology can be world-class, if not world-leading (like their 100 Mbps internet infrastructure). I think it would be obvious that there would be feedback systems to adjust the horizontal and vertical position of the receiver on the vehicle to keep it in the sweet spot for most efficient power transfer. There would also be sensors to detect debris on the road and lift the receiver out of harms ways. Such a system would also be capable of automatically steering the vehicle, which could save 10,000 lives a year if we ever got it past the lawyers. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 11, 2010, 1:43am UTC](https://boards.straightdope.com/t/wireless-electricity/531832/27 "2010-03-11T01:43:56Z")
> [@Fridgemagnet](#): > > It is quite true that laser collimation is never perfect, and there will always be some divergence when attempting to produce a fixed-width beam, but it’s not an inverse-square law, that would imply power density dropping to a quarter with every doubling of distance. > [@Chronos](#): > > This is true, over long distances. It’s true once you’re into the far field, but that can be a large distance from the laser. For a beam width of diameter D, for light with a wavelength L, that distance is on the order of D[sup]2[/sup]/L. For red light, L is about 7E-5 cm. If D = 0.5 cm, this distance is 0.5[sup]2[/sup]/7E-5 or about 30 meters or 100 feet. [Previous page](https://boards.straightdope.com/t/wireless-electricity/531832.md?page=1)