JEE Main 2014 · 12 Apr · Shift 1 · Physics Q22

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PhysicsSingle correct+4 / -1Question 22 of 90

A lamp emits monochromatic green light uniformly in all directions. The lamp is 3% efficient in converting electrical power to electromagnetic waves and consumes 100 W of power. The amplitude of the electric field associated with the electromagnetic radiation at a distance of 5 m from the lamp will be nearly :

1.34 V/m

2.68 V/m

4.02 V/m

5.36 V/m

Answer and worked solution

Official answer: B

Radiated power 3 W. At 5 m:
$I=\frac3{4\pi(25)}=9.55\times10^{-3}\text{ W/m}^2,\qquad E_0=\sqrt{\frac{2I}{c\epsilon_0}}=\sqrt{\frac{2\times9.55\times10^{-3}}{3\times10^8\times8.85\times10^{-12}}}\approx2.68\text{ V/m}.$

Topics:Energy and momentum of EM waves

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JEE Main 2014 · 12 Apr · Shift 1, Physics question 22