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Young's Modulus of Elasticity

Electromagnet...

Question

Electromagnetic radiation with an energy flux of $10 {W cm}^{- 2}$ falls on a reflecting surface at normal incidence. The surface has an area of $20 {cm}^{2}$ , what is average force exerted on surface during a $30$ minute time span ?

1.2 \times 10^{- 6} N

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2.4 \times 10^{- 6} N

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1.33 \times 10^{- 6} N

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2.66 \times 10^{- 6} N

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Solution

The correct option is C $1.33 \times 10^{- 6} N$
For reflecting surface, the force exerted by radiation is given by
$F = P_{r a d} . A$
Where $P_{r a d} = \frac{2 I}{c}$
$\Rightarrow F = \frac{2 I A}{c} = \frac{2 \times 10 \times 20}{3 \times 10^{8}} = 1.33 \times 10^{- 6} N$

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