Electric and Magnetic Fields

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G1SG2

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A changing electric field will generate a magnetic field. Which of the following best describes the direction of the magnetic field that accompanies the transmission of an FM signal as described in the passage?

A. Parallel to the ground and perpendicular to the direction of propagation.
B. Perpendicular to the ground and perpendicular to the direction of propagation
C & D-wrong.

Explanation: The electric field vector, the magnetic field vector, and the vector that points in the direction of propagation are always mutually perpendicular. If the FM signal is plane-polarized with the electric field E parallel to the ground (as stated in the last paragraph of the passage), then the answer must be A.

Last paragraph of the passage: The AM broadcast band includes frequencies from….while the FM bands includes frequencies from….it's also known that AM signals tend to have greater range than Fm signals. The audible spectrum contains frequencies between 20 Hz and 20,000 Hz, although waves with frequencies greater than 10,000 Hz are not transmitted effectively by radio signals. FM signals traveling across the surface of the Earth are plane polarized perpendicular to the ground.

Where does it say in this paragraph that the "FM signal is plane-polarized with the electric field E parallel to the ground?" What's the relationship between the bolded sentences?

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FM signals traveling across the surface of the Earth are plane polarized perpendicular to the ground.

By implication, this sentencetells u that the perpendicular (up & down) component of the electromagnetic wave has been polarized leaving only horizontal (side to side) oscillations which are parallel to the ground.

With EM waves, qualitatively speaking, it's better to consider only one of the of 2 fields (electric or magnetic) and ignore the other, otherwise things get unnecessarily complicated. The one field I consider, I assume that its behavior is representative of both individual fields.

Having said this and given the perpendicular polarization, the correct answer has to the FM signal parallel to the ground. All EM waves oscillate perpendicularly to direction of propagation so the second portion of the answer choices is trivial.

Now I don't see anything in the quoted paragraph that suggested precisely which component (E or B) was polarized and which component remained. Since I only consider one of the 2 fields, I don't get caught up on whether it's E or B. That fact is immaterial to answering this question. To me E & B are interchangeable but technically its a single electromagnetic phenomena.
 
Last paragraph of the passage: The AM broadcast band includes frequencies from….while the FM bands includes frequencies from….it's also known that AM signals tend to have greater range than Fm signals. The audible spectrum contains frequencies between 20 Hz and 20,000 Hz, although waves with frequencies greater than 10,000 Hz are not transmitted effectively by radio signals. FM signals traveling across the surface of the Earth are plane polarized perpendicular to the ground.

Where does it say in this paragraph that the "FM signal is plane-polarized with the electric field E parallel to the ground?" What's the relationship between the bolded sentences?
According to my physics text, the statement "light is polarized in the y direction" would mean that the corresponding electric field vector would in the y direction. Basically direction of polarization refers to the direction of the electric field vector.
 
FM signals traveling across the surface of the Earth are plane polarized perpendicular to the ground.

By implication, this sentencetells u that the perpendicular (up & down) component of the electromagnetic wave has been polarized leaving only horizontal (side to side) oscillations which are parallel to the ground.

With EM waves, qualitatively speaking, it's better to consider only one of the of 2 fields (electric or magnetic) and ignore the other, otherwise things get unnecessarily complicated. The one field I consider, I assume that its behavior is representative of both individual fields.

Having said this and given the perpendicular polarization, the correct answer has to the FM signal parallel to the ground. All EM waves oscillate perpendicularly to direction of propagation so the second portion of the answer choices is trivial.

Now I don't see anything in the quoted paragraph that suggested precisely which component (E or B) was polarized and which component remained. Since I only consider one of the 2 fields, I don't get caught up on whether it's E or B. That fact is immaterial to answering this question. To me E & B are interchangeable but technically its a single electromagnetic phenomena.

According to my physics text, the statement "light is polarized in the y direction" would mean that the corresponding electric field vector would in the y direction. Basically direction of polarization refers to the direction of the electric field vector.

Thanks a lot, guys. So, because the passage said that "FM signals traveling across the surface of the Earth are plane polarized perpendicular to the ground", the E field is pointing in the y direction, and the magnetic field is parallel to the ground because the electric and magnetic fields are always perpendicular to each other, right?
 
Thanks a lot, guys. So, because the passage said that "FM signals traveling across the surface of the Earth are plane polarized perpendicular to the ground", the E field is pointing in the y direction, and the magnetic field is parallel to the ground because the electric and magnetic fields are always perpendicular to each other, right?
That is correct. You might know this but with electromagnetic waves you can figure out the position of the different field vectors by another right-hand rule. Point your fingers in the direction of the electric field and curl towards the magnetic field, so that your thumb points in the direction of the propagation of the wave. (This might be a bit more than one needs to know).

Now I don't see anything in the quoted paragraph that suggested precisely which component (E or B) was polarized and which component remained. Since I only consider one of the 2 fields, I don't get caught up on whether it's E or B. That fact is immaterial to answering this question. To me E & B are interchangeable but technically its a single electromagnetic phenomena.
Perhaps I'm misunderstanding you, but it sounds like you're saying that E and B point in the same plane (i.e. their parallel). E and B are perpendicular to each other.

If the question had asked about the electric field with the same exact answer choices, B would have been correct.
 
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