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A wave is travelling through a medium until it hits the end of the medium and there is nothing but vacuum beyond. What happens to a mechanical wave? Electromagnetic wave?

  1. reflects backward, continues on
  2. reflects backward, reflects backward
  3. continues on, continues on
  4. stops, continues on

(a)

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You’re on the moon, skipping around, and your radio breaks. What would be the best way to communicate this problem to your friend, who is also skipping around on the moon: yelling or flashing a light? Why?

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Given the waveform in [link] (d), if T = 3.0 × 10 9 s and E = 5.0 × 10 5 N / C , which of the following is the correct equation for the wave at the antenna?

  1. ( 5 .0 × 10 5 N / C ) sin ( 2 π ( 3 .0 × 10 9 s ) t )
  2. ( 5 .0 × 10 5 N / C ) sin ( 2 π t ( 3 .0 × 10 9 s ) )
  3. ( 5 .0 × 10 5 N / C ) cos ( 2 π ( 3 .0 × 10 9 s ) t )
  4. ( 5 .0 × 10 5 N / C ) cos ( 2 π t ( 3 .0 × 10 9 s ) )

(d)

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Given the waveform in [link] (d), if f = 2.0 GHz and E = 6.0 × 10 5 N/C , what is the correct equation for the magnetic field wave at the antenna?

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In Heinrich Hertz’s spark gap experiment ( [link] ), how will the induced sparks in Loop 2 compare to those created in Loop 1?

  1. Stronger
  2. Weaker
  3. Need to know the tuner settings to tell
  4. Weaker, but how much depends on the tuner settings.

(d)

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The sun is far away from the Earth, and the intervening space is very close to empty. Yet the tilt of the Earth’s axis of rotation relative to the sun results in seasons. Explain why, given what you have learned in this section.

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Section summary

  • Electromagnetic waves are created by oscillating charges (which radiate whenever accelerated) and have the same frequency as the oscillation.
  • Since the electric and magnetic fields in most electromagnetic waves are perpendicular to the direction in which the wave moves, it is ordinarily a transverse wave.
  • The strengths of the electric and magnetic parts of the wave are related by
    E B = c , size 12{ { {E} over {B} } = ital " c"} {}

    which implies that the magnetic field B size 12{B} {} is very weak relative to the electric field E size 12{E} {} .

Conceptual questions

The direction of the electric field shown in each part of [link] is that produced by the charge distribution in the wire. Justify the direction shown in each part, using the Coulomb force law and the definition of E = F / q size 12{E= {F} slash {q} } {} , where q size 12{q} {} is a positive test charge.

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Is the direction of the magnetic field shown in [link] (a) consistent with the right-hand rule for current (RHR-2) in the direction shown in the figure?

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Why is the direction of the current shown in each part of [link] opposite to the electric field produced by the wire’s charge separation?

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In which situation shown in [link] will the electromagnetic wave be more successful in inducing a current in the wire? Explain.

Part a of the diagram shows an electromagnetic wave approaching a long straight vertical wire. The wave is shown with the variation of two components E and B. E is a sine wave in vertical plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a horizontal plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves. Part b of the diagram shows an electromagnetic wave approaching a long straight vertical wire. The wave is shown with the variation of two components E and B. E is a sine wave in horizontal plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a vertical plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves.
Electromagnetic waves approaching long straight wires.
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In which situation shown in [link] will the electromagnetic wave be more successful in inducing a current in the loop? Explain.

Part a of the diagram shows an electromagnetic wave approaching a receiver loop connected to a tuner. The wave is shown with the variation of two components E and B. E is a sine wave in vertical plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a horizontal plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves. Part b of the diagram shows an electromagnetic wave approaching a receiver loop connected to a tuner. The wave is shown with the variation of two components E and B. E is a sine wave in horizontal plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a vertical plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves.
Electromagnetic waves approaching a wire loop.
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Should the straight wire antenna of a radio be vertical or horizontal to best receive radio waves broadcast by a vertical transmitter antenna? How should a loop antenna be aligned to best receive the signals? (Note that the direction of the loop that produces the best reception can be used to determine the location of the source. It is used for that purpose in tracking tagged animals in nature studies, for example.)

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Under what conditions might wires in a DC circuit emit electromagnetic waves?

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Give an example of interference of electromagnetic waves.

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[link] shows the interference pattern of two radio antennas broadcasting the same signal. Explain how this is analogous to the interference pattern for sound produced by two speakers. Could this be used to make a directional antenna system that broadcasts preferentially in certain directions? Explain.

The picture shows an overhead view of a radio broadcast antenna sending signals in the form of waves. Two waves are shown in the diagram with concentric circular wave fonts. The crest and trough are marked as bold and dashed circles respectively. The points where the bold circles of the two different waves meet are marked as points of constructive interference. Arrows point outward from the antenna, joining these points. These arrows show the directions of constructive interference.
An overhead view of two radio broadcast antennas sending the same signal, and the interference pattern they produce.
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Can an antenna be any length? Explain your answer.

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Problems&Exercises

What is the maximum electric field strength in an electromagnetic wave that has a maximum magnetic field strength of 5 . 00 × 10 4 T size 12{5 "." "00"×"10" rSup { size 8{-4} } " T"} {} (about 10 times the Earth’s)?

150 kV/m

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The maximum magnetic field strength of an electromagnetic field is 5 × 10 6 T size 12{5 times "10" rSup { size 8{ - 6} } T} {} . Calculate the maximum electric field strength if the wave is traveling in a medium in which the speed of the wave is 0.75 c size 12{c} {} .

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Verify the units obtained for magnetic field strength B in [link] (using the equation B = E c ) are in fact teslas (T).

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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