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Nearly 125 years late, wireless recharging arrives | by subbywan | 2008-08-22 08:58:19 |
| Let's see -- 60W, 75% efficiency... |
by bwkaz |
2008-08-22 09:31:55 |
That means they transmitted 80W to begin with. But electric field strength drops off proportional to the square of the distance. So the equation for P (received power) in terms of P' (transmitted power), r (distance), and a constant is:
P = (P' * C) / r**2
Or more usefully:
P/P' = C / r**2
Eff = C / r**2
In this case:
Eff = C / 9
C = 9 * .75 = 6.75
But the efficiency will fall with the square of the distance. It's 75% at three feet; at six feet, it'll be 6.75/36 = 18.75%. At twelve feet, it'll be 6.75/144 = 4.6875%.
(At twelve feet, P = P' * Eff = 80 * .046875 = 3.75W.)
In short -- it's going to be a while before you can get any appreciable amount of energy at a distance comparable to what you can get 802.11 to do, for instance. (Yes, 802.11 has the same inverse-square relationship. The difference is, wireless hardware can work with a much lower received power level, because it isn't using the received power to do work directly.)
Either you need to increase the 6.75 factor (which is probably what MIT did here -- it probably started out lower than 6.75), or you need to increase the 80W transmit power (like radio stations do). In either case, you're increasing a linear factor of the equation, but you're working against something that decreases the received power by an inverse *square*. Good luck... ;-)
(Unless I'm missing something obvious, that is.) |
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Inverse square doesn't apply. | by AndyA | 2008-08-22 11:05:38 |
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