1. If the same source as in question 1 (i.e., 10 W) is placed at the centre of a sphere 10.0 m in radius, what is the intensity at a distance of 10 m from the source?

There are 2 ways to solve this:

 1. Similar to the solution of Q1 (b)
I =P/A = 10/[4p(10)2] = 7.96X10-3 W/m2

 2. Remembering from Q1 (b) that the intensity at 1 m was 0.796 W/m2 and using the inverse square law:
I =P/A = 0.796/102 = 7.96X10-3 W/m2

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