A 572-Hz longitudinal wave in air has a speed of 345 m/s. How much time is required for the phase to change by 90° at a given point in space?
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18. Waves & Sound
Phase Constant
Problem 29c
Textbook Question
A 572-Hz longitudinal wave in air has a speed of 345 m/s. At a particular instant, what is the phase difference (in degrees) between two points 4.4 cm apart?

1
Determine the wavelength of the wave using the formula: , where is the speed of the wave (345 m/s) and is the frequency (572 Hz).
Convert the distance between the two points (4.4 cm) into meters by dividing by 100, since 1 cm = 0.01 m.
Calculate the fraction of the wavelength that corresponds to the distance between the two points using the formula: , where is the distance (in meters) and is the wavelength.
Convert the fraction of the wavelength into a phase difference in degrees using the formula: , since one full wavelength corresponds to 360 degrees.
Express the phase difference as the final result, ensuring it is within the range of 0 to 360 degrees (if necessary, subtract multiples of 360 to bring it into this range).

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Wave Speed
Wave speed is the distance a wave travels per unit of time. In this case, the speed of the longitudinal wave in air is given as 345 m/s. This speed is crucial for determining how quickly the wave propagates through the medium and is essential for calculating the phase difference between two points.
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Wavelength
Wavelength is the distance between successive points of similar phase in a wave, such as crest to crest or trough to trough. It can be calculated using the formula λ = v/f, where v is the wave speed and f is the frequency. For the given wave, the wavelength can be determined, which is necessary for finding the phase difference over a specified distance.
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Phase Difference
Phase difference refers to the difference in the phase of two points in a wave, typically measured in degrees or radians. It indicates how far one point is ahead or behind another in the wave cycle. The phase difference can be calculated using the formula Δφ = (2π/λ) * Δx, where Δx is the distance between the two points, allowing us to find the phase difference for the specified distance of 4.4 cm.
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