Lesson Notes By Weeks and Term v5 - Grade 10
Waves, Sound and Light: longitudinal waves and sound – Week 4 focus
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Waves, Sound and Light: longitudinal waves and sound – Week 4 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Physical Sciences
Class: Grade 10
Term: Term 4
Week: 4
Theme: General lesson support
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This week, we delve into the fascinating world of sound waves, a type of longitudinal wave. Understanding sound is crucial because it's how we communicate, enjoy music, and even use technologies like sonar. In South Africa, sound plays a vital role in cultural expression through music, storytelling, and traditional ceremonies. From the rhythmic beats of a drum to the nuanced tones of spoken languages, sound shapes our experiences and connects us to our heritage. A deeper understanding of how sound behaves will help us appreciate and analyze these diverse soundscapes.
2.1 Longitudinal Waves A longitudinal wave is a wave where the displacement of the medium is in the same direction as, or the opposite direction to, the direction of propagation of the wave. Imagine pushing a slinky back and forth along its length. The coils bunch up (compression) and spread out (rarefaction) along the slinky, and this disturbance travels along the slinky's length. This is analogous to a longitudinal wave.
Key components of a longitudinal wave: Compression: A region where the particles of the medium are closer together than normal. The pressure is high here.
Rarefaction: A region where the particles of the medium are farther apart than normal. The pressure is low here. Wavelength (λ): The distance between two successive compressions or two successive rarefactions.
Amplitude: The maximum displacement of a particle from its equilibrium position. In sound waves, amplitude is related to loudness. 2.2 Sound Waves Sound waves are longitudinal waves that travel through a medium (solid, liquid, or gas) due to vibrations of the particles in that medium. Sound cannot travel through a vacuum because there are no particles to vibrate.
Production of sound: Sound is produced when an object vibrates, causing the surrounding air molecules to vibrate. These vibrations create compressions and rarefactions that propagate outwards as a sound wave. For instance, when you pluck a guitar string, it vibrates, pushing and pulling on the air around it, creating compressions and rarefactions.
Propagation of sound: Sound travels faster through denser materials because the particles are closer together and can transmit the vibrations more quickly. Solids are generally better conductors of sound than liquids, and liquids are better than gases. Temperature also affects the speed of sound. In general, the speed of sound increases with temperature.
Speed of sound: The speed of sound (v) depends on the medium through which it is traveling. It can be calculated using the following formula: v = fλ where: v = speed of sound (m/s) f = frequency (Hz) λ = wavelength (m) The speed of sound in air at approximately 20°C is 343 m/s. This value can vary slightly depending on temperature and humidity. 2.3 Frequency, Pitch, Amplitude, and Loudness Frequency (f): The number of complete vibrations (cycles) per second, measured in Hertz (Hz). Frequency determines the pitch of a sound. High frequency corresponds to a high pitch (like a whistle), and low frequency corresponds to a low pitch (like a bass drum).
Pitch: The perceived highness or lowness of a sound. It is directly related to frequency.
Amplitude: The maximum displacement of a particle from its equilibrium position. It's related to the energy the wave carries.
Loudness: The perceived intensity of a sound. It is related to the amplitude of the sound wave. A larger amplitude corresponds to a louder sound. Note that loudness is subjective, while intensity is objective. 2.4 Effects of Medium on Speed of Sound Density: The denser the medium, the faster the sound travels. This is because the particles are closer together, allowing for quicker transmission of vibrations. Sound travels much faster in steel than in air.
Temperature: The higher the temperature of the medium, the faster the sound travels. This is because the particles have more kinetic energy and vibrate more vigorously, allowing them to transmit vibrations more quickly. In air, the speed of sound increases by approximately 0.6 m/s for every 1°C increase in temperature.
Example 1: A sound wave has a frequency of 440 Hz and a wavelength of 0.78 m. Calculate the speed of the sound wave.
Solution:
Given: f = 440 Hz, λ = 0.78 m
Formula: v = fλ
Calculation: v = (440 Hz)(0.78 m) = 343.2 m/s
Answer: The speed of the sound wave is 343.2 m/s.
Example 2: The speed of sound in air is 345 m/s. A tuning fork vibrates at a frequency of 512 Hz. What is the wavelength of the sound wave produced by the tuning fork?
Solution:
Given: v = 345 m/s, f = 512 Hz
Formula: v = fλ, therefore λ = v/f
Calculation: λ = (345 m/s) / (512 Hz) = 0.674 m
Answer: The wavelength of the sound wave is 0.674 m.
Example 3: A learner shouts across a valley, and the echo returns 2.5 seconds later. Assuming the speed of sound is 340 m/s, how wide is the valley?
Solution:
Given: time for echo = 2.5 s, speed of sound = 340 m/s
Understanding: The sound travels to the other side of the valley and back, so the total distance traveled is twice the width of the valley. The time given (2.5s) is for the round trip. The time for a one-way trip is thus 2.5s / 2 = 1.25s
Formula: distance = speed x time
Calculation: distance = (340 m/s) (1.25 s) = 425 m
Answer: The width of the valley is 425 meters.
Guided Practice (With Solutions)
Question 1: What type of wave is a sound wave, and what are its characteristics?
Solution: A sound wave is a longitudinal wave. Its characteristics include compressions (regions of high pressure) and rarefactions (regions of low pressure). It requires a medium to travel and is produced by vibrating objects. The wavelength is the distance between two successive compressions or rarefactions.