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Home  >>  JEEMAIN and NEET  >>  Physics  >>  Class12  >>  Electromagnetic Waves

S1: In an electromagnetic wave, the average energy density of electric field is equal to the average energy density of the magnetic field \[\] S2: Electric and magnetic fields are related as $E = cB$

$\begin {array} {1 1} (a)\;\text{S1 and S2 are true; S2 is a correct explanation of S1.}\\ (b)\;\text{S1 and S2 are true; S2 is not a correct explanation of S1.} \\ (c)\;\text{S1 is true; S2 is false} \\ (d)\;\text{S1 is false; S2 is true.} \end {array}$

1 Answer

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Energy density in electric field is
$u_E = \large\frac{1}{2}$$ \in_oE^2$
Energy density in magnetic field is
$ u_B = \large\frac{1}{(2\mu_o)}$$ B^2$
We know $E = cB$ and $c = \large\frac{1}{\sqrt{(\mu_o \in_o)}}$
So, $u_E =\large\frac{1}{2}$$ \in_oE^2$
$=\large\frac{1}{2}$$ \in_o(cB)^2$
$=\large\frac{1}{2}$$ \in_o \times \large\frac{1}{(\mu_o \in_o)}$$ \times B^2 = \large\frac{B^2 }{2\mu_o}$
So, $u_E = u_B$
Ans : (a)
answered Mar 3, 2014 by thanvigandhi_1

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