reply to post by Im a Marty
thats the specifics with what I'm saying
You may have a little problem, then -- quite apart from the technical difficulty pointed out by The Redneck.
You're depending on the
kinetic energy of these photons to provide energy -- you're not relying on photovoltaic effects of any kind.
How much kinetic energy do photons have?
We can do the calculation. The force
f exerted by solar photons on a perpendicular surface is given by the power
W of the sunlight
divided by the speed of light
c. So
f = W/c
How do we get this equation? It is derived from good old E = mc^2. Exactly how is explained, step by step, on
this page.
In space, at the mean distance of Earth from the Sun, the power of sunlight is roughly 1,400 watts per square metre
(
here's where I got the figure). It sounds quite impressive, but how
much force is exerted by that light? Applying
f = W/c, we get...
...a paltry
4.6 microPascals* (microNewtons per square metre).
Now, even your one-atom-thick cone material has got to have some mass. Besides, it has to carry an induction coil, which would add to its mass, and as
the coil moved within the field of the surrounding magnet it would encounter electromotive resistance which the light pressure would have to overcome.
But let's forget all those little quibbles, shall we, and just say that the cone has an area of one square meter and weighs... one milligramme, one
thousandth of a gramme. Is that light enough for you?
Then, since force = mass x acceleration,
F = ma
the light pressure of 4.6 x 10^-6 Newtons (4.6 microNewtons) acting on a mass of 10^-6 kilogrammes (one milligramme) will give us an acceleration of
4.6 x 10^-6 / 10^-6 or 4.6 metres per second per second.
Suppose that the cone is allowed to accelerate in one direction for
a whole second (which it wouldn't, obviously). Then its kinetic energy at
the end of that second, given by the Newtonian equation
E = 1/2mv^2
where
m is the mass and
v the terminal velocity, would be
1/2*10^-6*4.6*4.6
or roughly
0.00001 joules.
Even if you took the whole mass of the cone into account (not just the mass of the incorporated coil, which obviously would be much less), and assumed
an impossible 100% efficiency of transduction, the maximum energy that could be induced in the 'speaker' coil would be 0.00001 joules per second;
that is to say,
one hundred thousandth of a watt.
Which, I am sure you will agree, is not very much.
But would you even get this? To begin with, there's the problem of air resistance, which The Redneck has already mentioned. Speakers work by moving
air; that's what sound is: air vibrating. Your almost weightless, light-powered speaker wouldn't be able to move at all for air resistance. As The
Redneck says, it would probably rip itself apart trying.
Lastly, there's the added complication that sunlight is continuous. Once your speaker cone has reached the limit of its travel, how will you return
it to its starting position? You'd have to switch off the sunlight and then use some other force to push it back again. Let me think...
Eureka! We could use an ingenious arrangement of swivelling mirrors and some kind of high-frequency shutter device...
...but where would we find the power to run them? Bet
that would take a bit more than a hundred thousandth of a watt...
Bedtime for me. I think.