1.

The average energy flux of sunlight is `1.0 kW m^-2`. This energy of radiation is falling normally on the metal plate surface of area `10 cm^2` which completely abosbs the energy. How much force is exerted on the plate if it is exposed to sunlight for 10 minutes?

Answer» Correct Answer - `3.3xx10^-9N`
Here, energy flux,`=1.0kWm^-2`
`=1.0xx10^3Wm^-2`,
`A=10cm^2=10xx10^-4m^2=10^-3m^2`
`t=10min=10xx60s=600s`.
Total energy falling on the plate.
U=energy flux x area x time
`=(1xx10^3)xx(10^-3)xx(600)J=600J`
Momentum of the radiation
`p=U/c=600/(3xx10^8)=2xx10^-6kgms^-1`
Since the plate absorbs the entire energy, so
momentum delivered to the plate= change in
momentum `(Deltap)=2xx10^-6kgms^-1`
Force on the plate,
`F=(Deltap)/t=(2xx10^-6)/60=3.3xx10^-9N`


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