Can We Power Things Wirelessly?

[yebvas1]

In attempting to resolve a quibble between a friend and myself, I need an answer to the following question. Is there such a thing as powering something wirelessly? E.g. is it possible to recharge a cell phone or other object, or light a lightbulb wirelessly?

 

[mgb_phys]

Is there such a thing as powering something wirelessly?

Yes

E.g. is it possible to recharge a cell phone or other object,

At close distances - yes reasonably easily

or light a lightbulb wirelessly?

You can light a fluorescent tube by just standing under a high voltage transmission line.
You can send enough power to run a regular light bulb a couple of metres with about 50% efficiency, but you need some fairly large equipment for the transmitter and receiver.


ps - not sure why this is a poll?

 

[Dyad]

Definitely.

It's been demonstrated by an MIT group last year:
http://web.mit.edu/newsoffice/2007/wireless-0607.html

 

[yebvas1]

Thanks for the replys (turns out I am right ; ) ), and yes the poll has no point

 

[AstroRoyale]

Those new-fangled induction cooktops are an example of power transmission sans wires.

 

[mgb_phys]

Those new-fangled induction cooktops are an example of power transmission sans wires.

As are those old fangled hot coal stoves.

 

[berkeman]

As are those old fangled hot coal stoves.

 

[Pumblechook]

The is nothing and new or mysterious with wireless power.

I was lighting small bulbs over a distance of a few metres in the 60s. The problem is poor efficiency. Converting AC mains to DC and then to RF and then transmitting the RF from A to B results in losses and then converting RF back to DC and then maybe back to AC...losses all the way. The aerials or coils you need to use to tend be nearly as big as the gap you are trying to bridge so it all non-starter really and is only used where a direct wire connection is not easy...e.g powering implanted devices in humans or animals.

I needed 25 Watts or so of AC mains power to light a bulb of less than a Watt at a distance of 2 metres using UHF. The two yagi aerials were a metre long each. Converted to DC the received power would have been 500 mW say... so overall efficiency was about 2% and would cost you a lot in equipment compared to just running a wire.

Then there is the problem of interference to other equipment and safety concerns.

 

[LURCH]

For a more long-distance application, look up "Crystal Radios." A fun project you can do at home.

 

[mgb_phys]

I needed 25 Watts or so of AC mains power to light a bulb of less than a Watt at a distance of 2 metres using UHF. The two yagi aerials were a metre long each. Converted to DC the received power would have been 500 mW say... so overall efficiency was about 2% and would cost you a lot in equipment compared to just running a wire.

It's got a bit better, the MIT group managed to transfer 60w with 45% efficiency a couple of meters with coils 0.6m in diameter. http://www.physorg.com/news100445957.html

Short range inductively coupled works a lot better, so the same system that charges your toothbrush could charge a cell phone or a wireless mouse

 

[Pumblechook]

The 45% will be the coupling efficiency RF-RF and NOT the overall efficiency which will be much lower. Those coils are ridiculously large.

Even if 45% was the overall figure it would be not good. You would taking 133 Watts from the mains to light a 60 W bulb... 73 Watts wasted.

I suspect the overall efficiency with coupled resonant coils of that size and spacing working at HF without final conversion back to AC would be around 20% at best.

Makes more sense to just run a wire a get 99.9% efficiency.

I don't see why anybody thinks there is anything new in all this.

I have been working on resonant receiving loops (coils) over the last few weeks and receiving signals from North and S America in the UK on LF (long wave) and MF (medium wave..AM band). One was 8 x 17 metres and one is 3 x 3 metres. It is 1920s technology.

http://oak.cats.ohiou.edu/~postr/MRT/Clarod.htm