85. A charged particle oscillates harmonically along the x-axis as shown above. The radiation from the particle is detected at a distant point P, which lies in the xy-plane. The electric field at P is in the

(A) z direction and has a maximum amplitude at =90

(B) z direction and has a minimum amplitude at =90

(C) xy-plane and has a maximum amplitude at =90

(D) xy-plane and has a minimum amplitude at =90

(E) xy-plane and has a maximum amplitude at =45

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## another problem of electromagnetism

propagation of radiation) has the following dependence on theta: S~sin(theta)^2 , so it has a maximum amplitude at theta=90. The same applies to E. So the correct answer is C.

You may want to have a look at the electric dipole radiation pattern. Just focus on the S vector and you will be able to answer a lot of similar problems.

Now, in this means when your observation point P is on the x-axis you will not see any field because there is no perpendicular component to the acceleration as viewed from this point. Similarly, when you view the accelerating particle at 90 degrees offset from the x-axis the total acceleration is the same as the perpendicular component. Therefore the radiation has a maximum at 90 degrees from the x-axis and the correct answer is C, as danty indicated.