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An AM radio station broadcasts with a wavelength of 484 m. Its transmitting antenna is vertical, resulting in vertically polarized radio waves (i.e, the electric field of the waves is vertical). You observe the station's signal at a point where it's propagating due east, and where the amplitude of the wave's electric field is 347 mV/m. Find (a) the wave frequency, (b) the amplitude of the wave's magnetic field, and (c) the direction of the magnetic field.

Question

An AM radio station broadcasts with a wavelength of 484 m. Its transmitting antenna is vertical, resulting in vertically polarized radio waves (i.e, the electric field of the waves is vertical). You observe the station's signal at a point where it's propagating due east, and where the amplitude of the wave's electric field is 347 mV/m. Find (a) the wave frequency, (b) the amplitude of the wave's magnetic field, and (c) the direction of the magnetic field.

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Solution

(a) To find the wave frequency, we can use the relationship between the speed of light (c), wavelength (λ), and frequency (f). The speed of light in a vacuum is approximately 3×108m/s3 \times 10^8 \, \text{m/s}.

c=λf c = \lambda \cdot f

Rearranging to solve for frequency (f):

f=cλ f = \frac{c}{\lambda}

Given: λ=484m \lambda = 484 \, \text{m} c=3×108m/s c = 3 \times 10^8 \, \text{m/s}

f=3×108m/s484m f = \frac{3 \times 10^8 \, \text{m/s}}{484 \, \text{m}}

f6.20×105Hz f \approx 6.20 \times 10^5 \, \text{Hz}

So, the frequency of the wave is approximately 620kHz620 \, \text{kHz}.

(b) To find the amplitude of the wave's magnetic field, we use the relationship between the electric field (E) and the magnetic field (B) in an electromagnetic wave:

E=cB E = c \cdot B

Rearranging to solve for B:

B=Ec B = \frac{E}{c}

Given: E=347mV/m=347×103V/m E = 347 \, \text{mV/m} = 347 \times 10^{-3} \, \text{V/m} c=3×108m/s c = 3 \times 10^8 \, \text{m/s}

B=347×103V/m3×108m/s B = \frac{347 \times 10^{-3} \, \text{V/m}}{3 \times 10^8 \, \text{m/s}}

B1.16×109T B \approx 1.16 \times 10^{-9} \, \text{T}

So, the amplitude of the wave's magnetic field is approximately 1.16nT1.16 \, \text{nT}.

(c) The direction of the magnetic field in an electromagnetic wave can be determined using the right-hand rule. For a wave propagating due east (positive x-direction) with a vertically polarized electric field (positive y-direction), the magnetic field will be in the direction that completes the right-handed coordinate system.

Using the right-hand rule:

  • Point your thumb in the direction of wave propagation (east, or positive x-direction).
  • Point your index finger in the direction of the electric field (vertical, or positive y-direction).

Your middle finger, perpendicular to both, will point in the direction of the magnetic field. In this case, it will point south (negative z-direction).

So, the direction of the magnetic field is south.

This problem has been solved

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