The electromagnetic wave of the radiation field from a wire antenna travels towa

Question

The electromagnetic wave of the radiation field from a wire antenna travels toward the plane of the paper (which is in the -z direction). At time t = 0 s, the wave strikes the paper at normal incidence. The magnetic field vector at point O in the figure points in the -y direction and has a magnitude of 4.0 * 10-8 T. The frequency of the wave is 1.0 * 1016 Hz.

What is the magnitude of the magnetic field at point O at time t = 5.0 * 10^-17 s? Note: Assume that the figure shows the peak value of B

Ans: 4.0 *10^-8 T

Explanation / Answer

The magnetic field at O is given by.

B(t) = - Bmax *cos(2**f*t) y^

Bmax = 4.0*10^-8 T
f = 1.0 * 10^16 Hz ,
t = 5.0 * 10^-17 s
Substituing Values,
B(t) = - 4.0*10^-8 *cos(2**1.0 * 10^16*5.0 * 10^-17)
B(t) =  - 4.0*10^-8 * (-1)
B(t) = 4.0 * 10^-8 T

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