Earth receives `1400 W m ^-2` of solar power. If all the solar energy falling on lens of area `0.2m^2` is focussed on to a block of ice of mass 280 g , then what is the time (in min) taken by the ice to melt completely ? `[L_("fusion")=3.3xx105 J kg ^-1]`

Answer

`5.5`

Explanation

Heat required to melt the block
`= (0.28 kg) ( 3.3 xx 10 ^(5) J //kg) = 9.24 xx 10^(4) J`
Here received per second `= (14000 J // m ^(2) ) ( 0.2 m ^(2)) = 280 J`
Time taken (in minutes) `= ( 9.24 xx 10 ^(4))/( 280 xx 60) = 5. 5`