Photon and Photovoltaic question!!! Can they power a 10W laptop given this number of photons!!!?

Question

Rate at which photons with wavelengths of 700nm, interact with photosystem 1 reach the surface of the earth is of the order of 10^21 photons per square metre per second. Assuming the power requirement of a laptop to be 10W carry out rough calculations including assumptions to test the feasibility of providing this power in this method.!!

cheers :)

10pts given

Answer 1

Look at it this way:

E=h*v = 6.6e-34 *(2*pi/700e-9) = 6e-27 Joules per photon * 10^21 photon/m^2/sec = 6e-6 Watts/m^2.

So you only have 6micro-watts available in that square meter, if you only use 700 nm light.  

The problem with the way this is stated is that no solar cell will use such a narrow band.  The total solar irradiance at the surface of the earth is about 1000 Watts/m^2!.  I think the question was probably trying to give you some information on quantum efficiency, that you would use along with a planck curve, to find the amount of power available to a solar cell.  But, the way the question is worded makes no sense.

Answer 2

IF your numbers are correct, and each photon kicks out 1 electron to create a current.

1 amp is 6e18 electrons/sec

for 1 square meter, 10e21 electrons/sec is about 1000 amps.

Assuming you need 10 amps for the laptop, then an area of 0.01 m² or 100 cm² is enough.

But I know that you will never get 1000 amps out of a square meter of anything. So the assumptions are wrong.

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