3. (8%) On a cloudless day, the sunlight that reaches the surface of the earth h

Question

3. (8%) On a cloudless day, the sunlight that reaches the surface of the earth has an intensity of a What is the electromagnetic energy contained in 5.5 m of space just above the earth's surface? 4. (10%) A speeder is pulling directly away and increasing his distance from a police car that is moving at 20 m/s with respect to the ground. The radar gun in the polilce car emits an electromagnetic wave with a frequency of 6.8 x 10 Hz. The wave reflects from the speeder's car and returns to the police car, where its frequency is measured to be 340 Hz less than the emitted frequency, Find the speeder's speed with respect to the ground.

Explanation / Answer

Given : I = 1.5 ×103 W/m2 ;volume of space = 5.5 m3

Consideration: The base of space = 1 m2

Solution:

The height of space can be given as : h = 5.5 m3/1m2

h = 5.5 m

Time taken for all the light in the space to travel down 5.3 m :

t = h/c = (5.5 m)/ (3×108 m/s) = 1.83 ×10-8 s

Energy reaching 1m2 base is given by :

I×(1m2)= 1.5 ×103 W

The energy (E) travelling through this space is given as :

E = (1.5 ×103 W)× (1.83×10-8 s) = 2.745 ×10-5 J

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