Light with an energy flux of 18 W/cm2 falls on a non-reflecting surface at normal incident. If the surface has an area of 20 cm2, find the total momentum delivered (for complete absorption) during a 30 minute time span?

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  1. 2.16 kg m/s
  2. 2.16 × 10-3 kg m/s
  3. 2.16 × 10-5 kg m/s
  4. 2.16 × 10-2 kg m/s

Answer (Detailed Solution Below)

Option 2 : 2.16 × 10-3 kg m/s
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Detailed Solution

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Concept:

Formula:

Energy flux = Power incident/Normal Area

Momentum delivered during complete absorption at normal incidence,

p = E/c = PΔt/c

Power incident, P = Energy flux × Area 

Calculations:

Given:

Energy flux = 18 W/cm2

θ = 90°

Area of the surface = 20 cm2

t = 30 minutes = 30 × 60 s

Power incident = Energy flux × Area 

⇒ Power incident = 18 × 20  = 360 W

∴ Momentum delivered in 30 minutes = PΔt/c

= (360 × 30 × 60)/(3 × 108)

= 2.16 × 10-3 kg m/s

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