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JEE MainPhysicsElectromagnetic Waves

A point source radiates electromagnetic waves isotropically in a vacuum with a total average power P . The average electric energy density at a distance r from the source is: (where c is the speed of light in vacuum)

Options

  1. AP 8 r^2 c
  2. BP 4 r^2 c
  3. CP 2 r^2 c
  4. DP 16 r^2 c

Correct answer

A. P 8 r^2 c

Step-by-step solution

The intensity I of the electromagnetic wave at a distance r from an isotropic point source is given by the power divided by the surface area of a sphere of radius r : I = P 4 r^2 The intensity is related to the total average energy density u by the relation I = u c . Therefore, the total average energy density is: u = I c = P 4 r^2 c In an electromagnetic wave, the total average energy density is equally divided between the electric and magnetic fields. The average electric energy density u_E is half of the total a

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