Electromagnetic Waves
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AP Physics 2 › Electromagnetic Waves
A radar pulse travels through air toward an aircraft. At a point in space, the electric field points upward while the magnetic field points to the right, and the pulse moves forward. Which statement best describes electromagnetic waves?
They have electric and magnetic fields parallel to each other so the wave can move forward.
They consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of travel.
They carry charge from the transmitter because an electric field implies moving charge.
They travel only in matter because the fields need particles to oscillate.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation. The radar pulse shows the electric field pointing upward, the magnetic field pointing to the right, and the pulse moving forward, demonstrating the required perpendicular orientations. These three directions form a right-handed coordinate system characteristic of EM waves. Choice C incorrectly claims that EM waves carry charge, confusing the presence of an electric field with actual charge transport. Remember that EM waves transport energy and momentum but not electric charge through space.
A microwave beam in air travels horizontally. A detector shows the electric field oscillates vertically while the magnetic field oscillates into and out of the page. Which statement best describes electromagnetic waves?
They require a medium and therefore cannot propagate in a vacuum.
They carry net charge because the fields oscillate through space.
They have mutually perpendicular electric and magnetic fields, both perpendicular to the direction of travel.
They have electric and magnetic fields parallel to each other at every point.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves are characterized by electric and magnetic fields that oscillate perpendicular to each other and to the direction of wave propagation. The problem describes the electric field oscillating vertically and the magnetic field oscillating into/out of the page while the beam travels horizontally, confirming all three directions are mutually perpendicular. This transverse wave nature is fundamental to all electromagnetic radiation. Choice A incorrectly claims EM waves need a medium, confusing them with mechanical waves that require matter for propagation. To identify EM waves, always check that the E and B fields are perpendicular to each other and both are perpendicular to the propagation direction.
An EM wave travels in the $+z$ direction through vacuum. A probe measures the electric field oscillating along $+x$ and the magnetic field oscillating along $+y$. Which statement best describes electromagnetic waves?
They transport net charge since electric fields imply moving charges through space.
They are longitudinal because both fields oscillate along the direction of travel.
They are transverse with perpendicular electric and magnetic fields, each perpendicular to the propagation direction.
They require a medium because the fields must vibrate within matter.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves are transverse waves with electric and magnetic fields oscillating perpendicular to each other and to the propagation direction. The measurement shows the wave traveling in +z direction with the electric field along +x and magnetic field along +y, confirming all three directions are mutually perpendicular. This transverse nature distinguishes EM waves from longitudinal waves like sound. Choice A incorrectly describes EM waves as longitudinal, misunderstanding that the fields oscillate perpendicular to, not along, the direction of travel. To verify an EM wave, check that E, B, and the propagation direction form a right-handed coordinate system.
A plane electromagnetic wave moves through vacuum. The electric field oscillates along $+\hat y$ while the magnetic field oscillates along $-\hat z$. Which direction does the wave propagate?
In the $+\hat z$ direction.
In the $+\hat x$ direction.
In the $+\hat y$ direction.
In the $-\hat x$ direction.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves have electric and magnetic fields perpendicular to each other and to the propagation direction, with the direction given by E × B. With E along +ŷ and B along -ẑ, the cross product (+ŷ) × (-ẑ) = -x̂, indicating propagation in the -x direction. Choice D represents the misconception that the wave travels along one of the field directions rather than perpendicular to both. Use the right-hand rule with E × B to find the propagation direction of electromagnetic waves.
An electromagnetic wave travels through glass. The wave’s electric field oscillates in one transverse direction, and the magnetic field oscillates in a different transverse direction, with both perpendicular to the propagation direction. Which statement best describes electromagnetic waves?
They carry charge through the glass because the fields push electrons forward continuously.
They can have electric and magnetic fields that are parallel if the wave is in a medium.
They cannot propagate in a vacuum and therefore must always travel in matter.
They are transverse waves with perpendicular electric and magnetic fields, even while traveling in a medium.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves maintain their fundamental character as transverse waves with perpendicular electric and magnetic fields even when traveling through a material medium like glass. The electric field oscillates in one transverse direction while the magnetic field oscillates in another transverse direction, both perpendicular to the propagation direction. This perpendicular relationship between E and B fields is a universal property of EM waves regardless of the medium. Choice D incorrectly states that EM waves cannot propagate in vacuum, contradicting the fundamental property that distinguishes them from mechanical waves. Always remember that EM waves can travel through vacuum and maintain their transverse nature in any medium.
A visible-light wave moves through space toward an astronaut. The electric field oscillates north–south while the magnetic field oscillates east–west; both are perpendicular to the direction of travel. Which statement best describes electromagnetic waves?
They consist of perpendicular oscillating electric and magnetic fields and can travel through empty space.
They have electric and magnetic fields that are always parallel to each other.
They require air or another medium because fields cannot exist in empty space.
They carry a net flow of charge in the direction of propagation.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of propagation. The visible light wave shows the electric field oscillating north-south, the magnetic field oscillating east-west, and both perpendicular to the travel direction toward the astronaut. These waves can propagate through the vacuum of space without any material medium. Choice A incorrectly claims that fields cannot exist in empty space, reflecting a misconception that confuses electromagnetic waves with mechanical waves. Remember that EM waves can travel through vacuum because the oscillating fields sustain each other through electromagnetic induction.
A probe in deep space detects a radio signal whose electric field oscillates along the $+y$ direction while the magnetic field oscillates along the $+z$ direction. The signal’s intensity decreases with distance from the source, but it is still detected in vacuum. Which statement best describes electromagnetic waves?
They consist of electric and magnetic fields that are parallel to each other as the wave travels.
They propagate by transporting net electric charge from the source to the detector.
They consist of oscillating electric and magnetic fields perpendicular to each other and to the direction of travel.
They require a material medium to propagate and cannot travel through vacuum.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation. In this problem, the electric field oscillates along +y, the magnetic field along +z, so the wave must travel along +x (or -x), with all three directions mutually perpendicular. The wave travels through vacuum, demonstrating that EM waves do not require a material medium for propagation. Choice A incorrectly states that EM waves need a medium, reflecting the misconception that all waves behave like mechanical waves. Remember that electromagnetic waves are self-sustaining oscillations of electric and magnetic fields that can propagate through empty space.
A visible-light wave travels through vacuum toward a detector. The electric field oscillates left–right while the magnetic field oscillates up–down, always perpendicular to the direction of travel. The detector is moved to twice the distance from the point source, with no absorption in between. Compared to the original location, the detected intensity is:
twice as large because the fields add constructively in vacuum.
unchanged because electromagnetic waves do not spread out in space.
one-half as large because frequency decreases with distance traveled.
one-fourth as large because intensity spreads over a larger spherical area.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of propagation. For a point source emitting uniformly in all directions, the intensity follows an inverse square law: intensity is proportional to 1/r². When the distance doubles from r to 2r, the intensity becomes 1/(2r)² = 1/4r², which is one-fourth the original intensity. Choice B incorrectly claims frequency changes with distance, confusing the Doppler effect with intensity spreading. The key principle is that electromagnetic wave intensity from a point source decreases as 1/r² due to energy spreading over larger spherical surfaces.
A radar wave propagates through air; its electric field oscillates north–south while its magnetic field oscillates up–down, both perpendicular to the propagation direction. The wave enters a region where its speed decreases, but its frequency remains the same. Which statement best describes what changes?
Its frequency decreases because electromagnetic waves slow down by losing charge.
Its polarization changes because the electric and magnetic fields become parallel.
Its wavelength increases because $\lambda=vf$ and $v$ decreases.
Its wavelength decreases because $\lambda=v/f$ and $v$ decreases.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves consist of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation. When an EM wave enters a medium where its speed decreases, the frequency remains constant (determined by the source), but wavelength must decrease according to λ = v/f. Since v decreases and f is constant, λ decreases proportionally. Choice C incorrectly claims frequency changes, misunderstanding that frequency is a source property that doesn't change when crossing boundaries. Remember that when electromagnetic waves enter a denser medium, speed and wavelength decrease together while frequency remains constant.
A microwave beam passes through air and then enters a block of glass. The electric field oscillates vertically while the magnetic field oscillates horizontally, each perpendicular to the beam’s direction. The frequency of the wave is measured to be unchanged as it enters the glass. Compared to in air, in the glass the wave has:
a smaller wavelength and a larger speed.
a smaller wavelength and a smaller speed.
a larger wavelength and a smaller speed.
a larger wavelength and a larger speed.
Explanation
This question tests understanding of electromagnetic waves. Electromagnetic waves consist of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation. When an EM wave enters a denser medium like glass, its speed decreases while its frequency remains constant (frequency is determined by the source). Since v = fλ, and v decreases while f stays constant, the wavelength λ must decrease proportionally. Choice B incorrectly suggests wavelength increases, reflecting confusion about the wave equation relationship. Remember that when electromagnetic waves enter a denser medium, they slow down and their wavelength decreases, but frequency always remains unchanged.