Verified Solution • 2025–26 Session moderate

QJEEM19Ph3c90: JEE Main 2019 Physics - Class 12 JEEM19Ph3c90 Solution: The energy associated with electric field is (U E ) and with…

The energy associated with electric field is (UE) and with magnetic field is (UB) for an electromagnetic wave in free space. Then :

Multiple Choice Options

  • A UE=UB2{U_E} = {{{U_B}} \over 2}
  • B UE>UB{U_E} > {U_B}
  • C UE<UB{U_E} < {U_B}
  • D UE=UB{U_E} = {U_B}

Step-by-Step Verified Solution

Energy density of magnetic field (UB) = B22μ0{{{B^2}} \over {2{\mu _0}}}

Also,

Energy density of electric field

UE = 12{1 \over 2} ε\varepsilon 0E2

= 12{1 \over 2} ε\varepsilon 0 B2 C2     [as    EB{E \over B} = C ]

= 12{1 \over 2} ε\varepsilon 0 B2 ×\times (1ε0μ0)\left( {{1 \over {{\varepsilon _0}{\mu _0}}}} \right)     [as   C2 = 1ε0μ0{{1 \over {{\varepsilon _0}{\mu _0}}}} ]

= B22μ0{{{B^2}} \over {2{\mu _0}}}

∴\therefore   UB = UE

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