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Light with an energy flux of 40W//cm^(2)...

Light with an energy flux of `40W//cm^(2)` falls on a non-reflecting surface at normal incidence. If the surface has an area of `20cm^(2)`, find the average force exerted on the surface during a `30min` time span.

A

`3.4 xx 10 ^(-6) N `

B

`4.5 xx 10 ^(-7) N `

C

`1.2 xx 10 ^(-6) N `

D

`1.2 xx 10 ^(-7) N `

Text Solution

Verified by Experts

The correct Answer is:
C
Energy flux` = 18 W cm ^(-2) = 18 J s ^(-1) cm ^(-2)`
Area `= 20 cm^(2),` Time `=30 min = 1800s`
Total energy falling on the surface,
U = energy flux ` x` time `xx` area
`= (18 J s ^(-1) cm ^(-2)) xx 1800 s xx 20 cm ^(2) = 6.48 xx 10 ^(5) J`
The toal momentum delivered to the surface,
`p =(U)/(c) = ( 6.48 xx 10 ^(5) J )/( 3 xx 10 ^(8) ms ^(-1)) = 2. 16 xx 10 ^(-3) kg m s ^(-1)`
The average force exerted on the surface,
`F = (p )/(t) = (2. 16 xx 10 ^(-3))/( 1800 ) = 1.2 xx 10 ^(-6) N `
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