We have recently set up our house with a wireless network. While using her laptop, my girlfriend asked why the computers power couldn't be wireless also. I dismissed the idea by saying something along the lines of 'I wouldn't like to be standing between the router and the laptop in that case'.

But I'm not sure now, is it actually a silly idea? Would sending enough electrical energy to power a laptop (presumably with some kind of electromagetic radiation) be dangerous? Would it require line of sight?

__________________
"I worry that, especially as the Millennium edges nearer, pseudo-science and superstition will seem year by year more tempting, the siren song of unreason more sonorous and attractive." - Carl Sagan, 1995

More trouble than it's worth, IMO. you'd need to broadcast a lot more power than the laptop needs to power it, unless you direct it at the laptop. you also need to broadcast on a frequency which will not interfere with any electronics, human tissues or anything else valuable in your home. Or your neighbours home. Or the street outside. And the laptop needs to be able to receive this power. If you direct the power to the laptop, most of the problems go away, but then you are stuck in line of sight of the power supply.
It would be nice, though

My initial reaction was based on the assumption that it would be beamed directly, and based in the radio/microwave spectrum, thus my worries about getting a warm and not-so-fuzzy feeling inside if I happened to be standing in the wrong place.

Just broadcasting it in all directions would be even more scary.

__________________
"I worry that, especially as the Millennium edges nearer, pseudo-science and superstition will seem year by year more tempting, the siren song of unreason more sonorous and attractive." - Carl Sagan, 1995

I had the idea, a couple of years ago, about having coils under the pavement to recharge people's mobile phones as they walked along. I don't know if this would be possible. I wouldn't need to be as powerful as it would just be for recharging.

Didn't Tesla try this? And wasn't it impractical because of the arcing he got? I seem to recall him blowing up a barn.

__________________
Quaeso quousque humi defixa tua mens erit? Nonne aspicis, quae in templa veneris?

Hard to say, because that was the result of most of the things Tesla tried

__________________
"I worry that, especially as the Millennium edges nearer, pseudo-science and superstition will seem year by year more tempting, the siren song of unreason more sonorous and attractive." - Carl Sagan, 1995

Yup, and this is exactly why Tesla's idea was impracticle. It could be done through magnetic induction using coils as Frog march suggested. It would have to be done over very short distances, and the coils would have to be polarized with respect to each other for this to be efficient at all.

Speaking of which, I remember watching a news tidbit on Daily Planet, which is a science based news program on the Discovery channel in Canada. They showed these really thin floor mats with coils built into them that were installed under your flooring. They didn't go into too much detail, and I've been unable to find any more info on the technology on the web, but it looks like magnetic induction to me. They said that the technology is much safer than the wall warts that we use today, and that appliances would need to have coils built into them as well. When I started thinking about it, I realized that you wouldn't need big bulky transformers in your appliances anymore, as you'd be able to upstep or downstep the power based on the size of the appliance coil.

__________________
Obedience brings victory
Victory is life
That is the order of things

Heh, true enough!

Could that be practical? The distance is probably short enough, but how much power would you need to pump into that?

__________________
Quaeso quousque humi defixa tua mens erit? Nonne aspicis, quae in templa veneris?

Assuming that the Coils aren't more than an inch or so apart, there shouldn't be any noticable loss at all. Magnetic induction is a very efficient means of transfering power over very short distances.

You'd probably want to keep it at the standard 50Hz that AC power uses in the UK.

__________________
Obedience brings victory
Victory is life
That is the order of things

I presume, then, that this sub-floor induction technology is only good for floor applicances.

__________________
Quaeso quousque humi defixa tua mens erit? Nonne aspicis, quae in templa veneris?

Not necessarily. If you wanted to take things to the extreme, one could conceivably put coils in the bottom of the table. You could then run wires up to a coiled mat on the surface of the table. Loss would still remain minimal. What's particulary interesting about magnetic induction is the fact that the electrostatic field (E field) is nearly non existant.

Check out this page for more info.

The above page applies to communication technology. In the case of AC power though, there isn't a need for transmitters or amplifiers. You're just transfering raw power directly through the inductive reactance applied by the AC currents frequency.

__________________
Obedience brings victory
Victory is life
That is the order of things

Well, huh. All that I can say is "neat!"

__________________
Quaeso quousque humi defixa tua mens erit? Nonne aspicis, quae in templa veneris?

A strong enough field to power appliances would play havoc with your computer, though... and I bet your TV would look funny, too.

__________________
"It's turtles all the way down."

That can be shielded against. Now we're starting to talk BIG money, though.

__________________
Quaeso quousque humi defixa tua mens erit? Nonne aspicis, quae in templa veneris?

Household AC isn't strong enough to cause any major interference by itself. Every peice of wire in your house that has a current going through it produces a magnetic field. Coiling the wire localizes the magnetic field. When two inductors (coils) are aligned with respect to each other, the coupling between the two inductors is maximized, and full power can be transfered between the two coils with little loss.

See this page for more info on mutual inductance. You may notice that they use the transformer as a prime example of inductance.

__________________
Obedience brings victory
Victory is life
That is the order of things

People are working on the problem.

__________________
Do try not to take me too seriously.

I think you'd often be better off coating the device you wanted to have wireless power with photoelectric cells connected to a small rechargable battery. When your device needs extra power you could have a nice efficent LED spotlamp programmed to shine light on whatever needs it. This has the advantage of letting you use your devices anywhere there is light, not just wireless power, and it could be much more efficient and won't cause interference.

That's not really a runner. If i want to power my laptop with pv cells, they'll be bigger than it is. And you're getting pretty big losses in normal use. And since you need line of sight, that seems (to me) like as big or bigger a limitation as a power cable.

__________________
"Last Thursday, in a major breakthrough for the science of astrology, everyone born under the sign of scorpio was run over by milk trucks...."

Let's see, my laptop lid has a surface area of about 0.1 of a square meter, assume the highest efficency cells yet constructed, 40% and a directed LED light as bright as sunlight, then that would supply 40 watts, enough for a laptop. Since a laptop is unlikely to be used 24 hours a day the battery can be charged when you're not using it which would enable you to use less efficent solar cells, a smaller area, less bright light, etc.

With a 90% efficent LED light and 40% efficent photoelectric cells it would have an efficency of 36%, which isn't too bad, although more realistic 20% efficiency cells will cut that in half to 18%.

This is much less than what is possible with microwaves, but it does have the advantage of being able to use any light anywhere and people may feel more comfortable with light than with microwaves.

Whoa, wait a minute. You said there would be pretty big losses with photoelectrics, so I assumed you were talking about a directed microwave system. If you're not using line of sight for microwaves that's going to have pretty big losses too.

But neither method is probably as easy as putting down a mat on your desk that you plug in and powers your devices through induction. For devices you carry around with you, you just dump them on the mat before you go to bed and they charge up overnight.