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JEE Main20199 Jan 2019Evening ShiftPhysicsElectromagnetic WavesActual

The energy associated with electric field is U E and with magnetic field is U B for an electromagnetic wave in free space. Then:

Options

  1. AU E > U B
  2. BU E = U B
  3. CU E = U B 2
  4. DU E < U B

Correct answer

B. U E = U B

Step-by-step solution

The average energy density of electric field is given by u E = 1 2 &#949; E 0 2 Here, E 0 is the maximum value of electric field. The average energy density of magnetic field is given by u B = B 0 2 2 &#956; Here, B 0 is the maximum value of magnetic field. The speed of light is given by c 2 = 1 &#956; &#949; Also, E 0 = c B 0 So, u E = 1 2 c 2 &#956; c B 0 2 = B 0 2 2 &#956; = u B As the energy density of electric field and magnetic field are equal, the energy associated for equal volume will also be equal.

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