Waves in air, fluids and solids

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수업 노트

Wave Basics

Waves transfer energy and information without transferring matter.
Waves are oscillations or vibrations about a fixed point.
Ripples on water cause particles to oscillate up and down; sound waves cause air particles to vibrate back and forth.
Objects floating on water (like a duck) bob up and down but do not travel with the wave, showing that matter is not transferred.

Describing Wave Motion

Amplitude is the distance from the undisturbed position to the peak or trough of a wave, measured in metres (m).
Wavelength is the distance from one point on a wave to the same point on the next wave, measured in metres (m).
Frequency is the number of waves passing a point each second, measured in hertz (Hz).
Time period is the time taken for one complete wave to pass a point, measured in seconds (s).
Frequency and period are related by: f = 1/T.
Wave speed is the speed at which energy is transferred through a medium.
Wavefronts are lines representing each wave; the distance between them is the wavelength.

Transverse wave motion in a rope: the vibration is at 90 degrees to the direction of wave motion and energy transfer, with the crest and trough labelled.

Transverse and Longitudinal Waves

Transverse waves vibrate at 90° (perpendicular) to the direction of energy transfer.
Examples of transverse waves: ripples on water, vibrations in a guitar string, S-waves, electromagnetic waves.
Longitudinal waves vibrate parallel to the direction of energy transfer.
Longitudinal waves consist of compressions (regions of higher density) and rarefactions (regions of lower density).
Examples of longitudinal waves: sound waves, P-waves, pressure waves in liquids or gases.
Transverse waves can travel in solids and on the surfaces of liquids; longitudinal waves can travel in solids, liquids and gases.
Only electromagnetic waves (transverse) can travel through a vacuum; longitudinal waves cannot.

Longitudinal and transverse waves.

The Wave Equation

Wave speed can be calculated using: v = f × λ (wave speed = frequency × wavelength).
v is wave speed in metres per second (m/s), f is frequency in hertz (Hz), λ is wavelength in metres (m).
Wave speed can also be calculated using: v = x / t (speed = distance / time).
The wave equation can be rearranged to find frequency (f = v/λ) or wavelength (λ = v/f).
All waves obey the wave equation.

Measuring Wave Speed

Speed of sound in air can be measured between two points: measure distance with a trundle wheel, time the sound with a stopwatch.
Using echoes: clap blocks in rhythm with echoes, measure distance to wall, time 20 claps, total distance = 20 × 2 × distance.
Using an oscilloscope: two microphones connected to an oscilloscope, measure time difference for sound to travel between them.
Speed of ripples on water: measure distance between two points, time how long a ripple takes to travel, repeat and average.
The oscilloscope method is most accurate because timing is automatic; the two-point method is least accurate due to short time intervals.

Measuring the speed of sound between two points

Reflection, Absorption and Transmission

When waves meet a boundary between two materials, they can be reflected, absorbed or transmitted.
A reflected sound wave is called an echo.
Reflection occurs at the boundary between two different materials.
Transmission is when a wave passes through a material; absorption is when a wave's energy is taken in by a material.
Ray diagrams can be used to illustrate the reflection of a wave at a surface.

Sound Waves

Sound waves are longitudinal waves consisting of compressions and rarefactions.
Sound waves cause pressure variations in the air, so they are pressure waves.
When sound waves hit a solid, the pressure variations cause the solid to vibrate in sync with the wave.
Sound travels fastest in solids, slower in liquids, and slowest in gases because molecules are closer together in solids.
The human ear detects sound when sound waves cause the eardrum and small bones to vibrate, which creates electrical impulses sent to the brain.
The range of human hearing is 20 Hz to 20 000 Hz (20 kHz).

Ultrasound and Infrasound

Ultrasound is sound with a frequency above 20 000 Hz (above the human hearing range).
Infrasound is sound with a frequency below 20 Hz (below the human hearing range).
Ultrasound is used in sonar, foetal scanning, and industrial imaging.
Infrasound is produced by earthquakes (P-waves) and can be used to study the Earth's core.
Ultrasound waves are partially reflected at boundaries between different media; the time for reflections to return can determine distance.

Frequencies of ultrasound

Seismic Waves and Earth's Structure

Earthquakes produce P-waves (longitudinal) and S-waves (transverse).
P-waves travel faster than S-waves, so P-waves are felt first.
P-waves can travel through solids and liquids; S-waves can only travel through solids.
S-waves cannot pass through the Earth's liquid outer core, which is why they are not detected on the opposite side of the Earth from an earthquake.
P-waves refract as they pass through different layers, creating shadow zones that provide evidence about the Earth's internal structure.
Seismic waves provide evidence that the mantle is solid, the outer core is liquid, and the inner core is solid.

Calculating Depth and Distance

If wave speed is known, distance can be calculated using: distance = speed × time (x = v × t).
Echo sounding uses ultrasound to detect objects underwater and measure water depth.
The sound wave travels to the ocean bottom and back, so total distance = 2 × depth.
The time for the echo to return is used to calculate depth: depth = (speed × time) / 2.
For example, a thunderclap heard 4 s after lightning: distance = 330 m/s × 4 s = 1320 m.

슬라이드

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연습 문제

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1.Which type of wave is a sound wave travelling through air?
Easy
- ATransverse
- BLongitudinal
- CElectromagnetic
- DStationary
2.In a longitudinal wave, the particles vibrate at right angles to the direction in which the wave transfers energy.
Easy
True or false?
3.Which row correctly names the two features labelled on a longitudinal wave?
Easy
- ACompression = region of lower density; rarefaction = region of higher density
- BCompression = region of higher density; rarefaction = region of lower density
- CCompression = highest point; rarefaction = lowest point
- DCompression = region of constant density; rarefaction = region of changing pressure
4.Which statement about the movement of particles in a sound wave travelling through air is correct?
Medium
- AAir particles travel along with the wave from the source to the listener.
- BAir particles vibrate about fixed positions and do not travel with the wave.
- CAir particles move at right angles to the direction of energy transfer.
- DAir particles are permanently pushed away from the source.
5.Sound waves travel fastest in solids and slowest in gases.
Easy
True or false?
6.What is the normal range of frequencies that a healthy human ear can hear?
Easy
- A2 Hz to 2000 Hz
- B20 Hz to 20 000 Hz
- C200 Hz to 200 000 Hz
- D20 Hz to 2000 Hz
7.What name is given to a sound wave that is reflected from a surface?
Easy
- AAn echo
- BA refraction
- CA rarefaction
- DAn ultrasound
8.Which of the following statements about sound waves are correct? (select all that apply)
Medium
- AThey are longitudinal waves.
- BThey consist of compressions and rarefactions.
- CThey can travel through a vacuum.
- DThey cause pressure variations in the air.
- EThey travel faster in gases than in solids.

더 많은 문제

9.Which of the following is an example of infrasound?
Medium
- AA sound wave with a frequency of 5 Hz
- BA sound wave with a frequency of 500 Hz
- CA sound wave with a frequency of 25 000 Hz
- DA sound wave with a frequency of 20 000 Hz
10.Which statement about P-waves and S-waves produced by an earthquake is correct?
Medium
- AP-waves are transverse and travel slower than S-waves.
- BP-waves are longitudinal and travel faster than S-waves.
- CS-waves are longitudinal and can travel through liquids.
- DS-waves are transverse and travel faster than P-waves.
11.S-waves from an earthquake can travel through the Earth's liquid outer core.
Easy
True or false?
12.Place the stages of how a sound wave is transmitted to the brain in the correct order, starting with the sound wave entering the ear.
Medium
- Sound wave travels down the auditory canal
- Eardrum vibrates
- Three small bones amplify and transfer the vibration
- Tiny hairs in the cochlea detect vibrations and create electrical impulses
- Electrical impulses travel along the auditory nerve to the brain
13.Match each wave type or term to its correct description.
Easy
- P-wave
- S-wave
- Ultrasound
- Infrasound
- Longitudinal seismic wave
- Transverse seismic wave
- Sound above 20,000 Hz
- Sound below 20 Hz
14.Which of the following statements about the transmission of sound are correct? (select all that apply)
Medium
- ASound travels fastest in solids because particles are closer together.
- BSound waves are transverse waves.
- CSound waves can travel through a vacuum.
- DWhen sound moves from one medium to another, its speed and wavelength may change.
- EThe frequency of a sound wave is fixed by the source when it changes medium.
15.A student claps two wooden blocks together in front of a wall and hears an echo after 0.6 s. The speed of sound in air is 340 m/s. How far is the student from the wall?
Medium
- A102 m
- B204 m
- C340 m
- D567 m
16.Ultrasound is defined as sound waves with a frequency above 20,000 Hz.
Easy
True or false?
17.Which of the following are uses of ultrasound? (select all that apply)
Medium
- ASonar for detecting objects underwater
- BFoetal scanning in medicine
- CExploring the Earth's core
- DMeasuring the speed of sound in air using echoes
- EProducing images of internal organs
18.A low-frequency sound wave is displayed on an oscilloscope as a transverse trace. The time base is set so that the horizontal axis shows time. The trace repeats every 0.005 s. What is the frequency of the sound wave?
Medium
- A0.005 Hz
- B200 Hz
- C5 Hz
- D5000 Hz
19.A surfer times the buoy moving from its highest point to the peak of the 7th wave, which takes 24.5 s. What is the frequency of the waves? Give your answer to 2 significant figures.
Medium
- A0.24 Hz
- B0.27 Hz
- C0.29 Hz
- D3.5 Hz
20.Water waves pass a buoy. It takes 2 minutes for 10 complete surface waves to pass the buoy. What is the period of one wave?
Medium
- A2 s
- B5 s
- C12 s
- D20 s
21.A buoy is 150 m from the shore. A lifeguard counts 12 waves between the buoy and the shore at any one time. The waves have a frequency of 0.083 Hz. What is the speed of the waves? Give your answer to 2 significant figures.
Medium
- A1.0 m/s
- B1.2 m/s
- C12 m/s
- D18 m/s
22.The wind picks up and the wave speed doubles, but the wavelength remains the same. What happens to the frequency of the waves?
Hard
- AIt halves
- BIt stays the same
- CIt doubles
- DIt quadruples
23.A sound wave has a frequency of 1020 Hz and the speed of sound in air is 340 m/s. What is the wavelength of the sound waves?
Medium
- A0.33 m
- B0.50 m
- C2.0 m
- D3.0 m
24.A student investigates waves on a rope attached to a vibration generator. The frequency is 40 Hz and the wavelength is 0.15 m. What is the wave speed?
Hard
- A0.00375 m/s
- B6.0 m/s
- C40.15 m/s
- D267 m/s
25.A surfer determines that the wave speed is between 3.5 m/s and 4.5 m/s. The frequency of the waves is 0.24 Hz. What is the possible range of wavelengths? Give your answers to 2 significant figures.
Hard
- ABetween 0.83 m and 1.1 m
- BBetween 15 m and 19 m
- CBetween 3.5 m and 4.5 m
- DBetween 0.068 m and 0.069 m
26.In a sound wave travelling through air, the air particles travel along with the wave from the source to the listener.
Easy
True or false?
27.When a sound wave passes from air into water, its frequency stays the same but its speed and wavelength change.
Medium
True or false?
28.Which of the following statements about P-waves and S-waves are correct? (select all that apply)
Medium
- AP-waves are longitudinal and can travel through solids and liquids.
- BS-waves are transverse and can travel through solids and liquids.
- CP-waves travel faster than S-waves, so they are felt first in an earthquake.
- DS-waves cannot travel through the Earth's liquid outer core.
- EP-waves are transverse and produce a side-to-side motion.
29.Match each term to its correct description.
Medium
- Compression
- Rarefaction
- Amplitude
- Wavelength
- A region of higher density in a longitudinal wave
- A region of lower density in a longitudinal wave
- The distance from the undisturbed position to a peak or trough
- The distance from one point on a wave to the same point on the next wave
30.Which letter on a cross-section diagram of the ripple tank shows the amplitude of a water wave?
Medium
- AThe arrow from the undisturbed position to a peak
- BThe arrow from one peak to the next peak
- CThe arrow from the trough to the next trough
- DThe arrow along the bottom of the tank
31.Which letter on a cross-section diagram of the ripple tank shows the wavelength of a water wave?
Medium
- AThe arrow from the undisturbed position to a peak
- BThe arrow from one peak to the next peak
- CThe arrow from the trough to the undisturbed position
- DThe arrow from the wooden bar to the first peak
32.The rope could snap during the investigation. Which of the following is the best way to reduce the risk of injury if the rope snaps?
Medium
- AWear safety goggles and stand clear of the rope
- BIncrease the frequency of the vibration generator
- CHold the rope tightly with both hands
- DMove closer to the rope to watch it more carefully
33.Which of the following is the most accurate method for measuring the speed of sound in air?
Hard
- AUsing two people with a stopwatch and wooden blocks 100 m apart
- BUsing echoes and clapping in time with the echoes
- CUsing two microphones connected to an oscilloscope
- DUsing a single stopwatch to time a clap
34.What happens to the loudness of the sound as the air is removed from the bell jar?
Hard
- AIt becomes louder because there is less air to absorb the sound
- BIt becomes quieter because sound needs particles to travel through
- CIt stays the same because sound travels through the glass
- DIt becomes higher in pitch because the frequency increases
35.Sound waves can travel through a vacuum.
Medium
True or false?
36.Why is ultrasound used for medical imaging rather than audible sound?
Medium
- AUltrasound has a longer wavelength and can penetrate deeper
- BUltrasound is non-invasive and is believed to be harmless
- CUltrasound travels faster than audible sound in tissue
- DUltrasound can only travel through bone
37.A student shines a ray of light at a boundary between air and glass. The light slows down as it enters the glass. What happens to the ray?
Hard
- AIt speeds up and bends away from the normal
- BIt slows down and bends towards the normal
- CIt slows down and bends away from the normal
- DIt continues in a straight line without changing direction
38.Which of the following statements about the transmission of sound is correct?
Medium
- ASound travels fastest in gases because the particles are far apart
- BSound travels fastest in solids because the particles are close together
- CSound travels at the same speed in all materials
- DSound travels fastest in a vacuum because there are no particles to slow it down
39.A sound wave has a frequency of 15 Hz. Which of the following is true?
Medium
- AIt is ultrasound and can be used for foetal scanning
- BIt is infrasound and is below the range of human hearing
- CIt is audible to humans because it is a low-pitched sound
- DIt is audible to humans because it has a long wavelength
40.Two students measure the speed of sound using the echo method. They stand 50 m from a wall and clap in time with the echoes. They time 20 claps and divide the time by 20. Which distance should be used for each clap-to-echo interval?
Medium
- A100 m, because the sound travels to the wall and back
- B50 m, because that is the distance to the wall
- C25 m, because the sound travels half the distance each way
- D200 m, because there are two students
41.Complete the sentence about the features of a longitudinal wave.
Easy
A sound wave consists of regions of higher density called ____ and regions of lower density called ____.
42.Complete the sentence about ultrasound imaging.
Medium
In foetal scanning, a ____ produces ultrasound waves and detects the reflected waves, which are called ____.
43.A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. Calculate its speed in m/s.
Medium
44.A clap of thunder is heard 4 seconds after the corresponding flash of lightning. The speed of sound in air is 330 m/s. How far away is the thunderstorm, in metres?
Hard
45.P-waves travel with an average speed of about 7000 m/s. Some P-waves are produced by a minor earthquake and detected a short while later by a seismometer 3000 km away. Calculate how long it will take the P-waves to reach the seismometer, in seconds.
Hard
46.An echo sounder on a boat sends an ultrasound pulse straight down to the seabed. The echo returns 0.40 s later. The speed of sound in water is 1500 m/s. Calculate the depth of the water, in metres.
Medium
47.Complete the sentence about sound waves.
Medium
Sound waves are ____ waves consisting of areas of higher density called compressions and areas of lower density called ____.
48.Name the type of seismic wave that is longitudinal and travels faster through the Earth.
Medium
49.The microphones are 5.0 m apart. The oscilloscope shows that the sound takes 0.015 s to travel between the two microphones. Calculate the speed of sound in air, in m/s.
Hard
50.The ultrasound pulse is reflected from a tissue boundary and returns to the transducer after 0.0001 s. The speed of sound in tissue is 1540 m/s. Calculate the distance from the transducer to the tissue boundary, in metres.
Hard
51.Complete the sentence about sound waves travelling through air.
Easy
Sound waves are ____ waves made up of regions of higher density called ____ and regions of lower density called ____.
52.A student measures the speed of sound using two microphones connected to an oscilloscope. The microphones are 5.0 m apart. The time difference between the sound reaching the first and second microphone is 0.015 s. Calculate the speed of sound in air.
Medium
53.Compare transverse and longitudinal waves.
Medium
54.Explain why sound travels further at night than during the day.
Hard
55.A student claps two wooden blocks together in front of a wall and hears an echo after 0.6 s. The speed of sound in air is 340 m/s. Explain how the student could use this method to determine the speed of sound, stating clearly what measurements they must take.
Hard
56.Describe a method the student could use to measure the wavelength of the wave on the rope accurately. Include what measurements to take and what equipment to use.
Hard
57.Explain how seismic waves provide evidence that the Earth's outer core is liquid.
Hard
58.A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. Calculate its speed in m/s.
Medium
- A374 m/s
- B408 m/s
- C340 m/s
- D306 m/s
59.A clap of thunder is heard 4 seconds after the corresponding flash of lightning. The speed of sound in air is 330 m/s. How far away is the thunderstorm, in metres?
Hard
- A1188 m
- B1584 m
- C1452 m
- D1320 m
60.P-waves travel with an average speed of about 7000 m/s. Some P-waves are produced by a minor earthquake and detected a short while later by a seismometer 3000 km away. Calculate how long it will take the P-waves to reach the seismometer, in seconds.
Hard
- A471.5 s
- B514.4 s
- C428.6 s
- D385.7 s
61.An echo sounder on a boat sends an ultrasound pulse straight down to the seabed. The echo returns 0.40 s later. The speed of sound in water is 1500 m/s. Calculate the depth of the water, in metres.
Medium
- A330 m
- B300 m
- C270 m
- D360 m
62.The microphones are 5.0 m apart. The oscilloscope shows that the sound takes 0.015 s to travel between the two microphones. Calculate the speed of sound in air, in m/s.
Hard
- A399 m/s
- B333 m/s
- C300 m/s
- D366 m/s
63.The ultrasound pulse is reflected from a tissue boundary and returns to the transducer after 0.0001 s. The speed of sound in tissue is 1540 m/s. Calculate the distance from the transducer to the tissue boundary, in metres.
Hard
- A1.077 m
- B0.077 m
- C2.077 m
- D-0.923 m
64.A student measures the speed of sound using two microphones connected to an oscilloscope. The microphones are 5.0 m apart. The time difference between the sound reaching the first and second microphone is 0.015 s. Calculate the speed of sound in air.
Medium
- A399.9 m/s
- B333.3 m/s
- C300 m/s
- D366.6 m/s

플래시카드

What is a wave, and what does it transfer?

A wave is a repeated oscillation or vibration about a fixed point. It transfers energy and information without transferring matter.

What is the difference between a transverse and a longitudinal wave?

In a transverse wave, points vibrate at 90° to the direction of energy transfer. In a longitudinal wave, points vibrate parallel to the direction of energy transfer.

What are the key features of a longitudinal wave?

Compressions (regions of higher density and pressure) and rarefactions (regions of lower density and pressure).

Give two examples of transverse waves and two examples of longitudinal waves.

Transverse: ripples on a water surface, vibrations in a guitar string, S-waves, electromagnetic waves. Longitudinal: sound waves, P-waves, pressure waves in a liquid or gas.

Define amplitude and give its unit.

The maximum displacement of a point on a wave from its undisturbed position. Measured in metres (m).

Define wavelength and give its unit.

The distance from one point on a wave to the equivalent point on the next wave, e.g. peak to peak. Measured in metres (m).

Define frequency and give its unit.

The number of waves passing a point each second. Measured in hertz (Hz).

Define the period of a wave and give its unit.

The time taken for a single wave to pass a point. Measured in seconds (s).

State the equation linking period and frequency.

T = 1 / f, where T is period in seconds and f is frequency in hertz.

State the wave equation.

wave speed = frequency × wavelength, or v = f λ, where v is in m/s, f in Hz and λ in m.

How can you calculate wave speed from distance and time?

wave speed = distance ÷ time (v = x / t), where x is the distance travelled in metres and t is the time in seconds.

What do wavefront diagrams show?

Each line represents one wavefront (one wave). The space between wavefronts is the wavelength, and the arrow shows the direction the wave travels.

How does a duck floating on water show that waves transfer energy but not matter?

The duck bobs up and down but does not travel along with the wave. The water particles oscillate about fixed positions, so only energy moves forward, not matter.

Why is sound a pressure wave?

Sound is a longitudinal wave made of compressions and rarefactions, which cause changes in air pressure that vary in time with the wave.

In which state does sound travel fastest, and why?

Fastest in solids, slowest in gases. The closer together the molecules, the faster the wave can transfer energy by molecules vibrating and knocking into neighbours.

What happens to a sound wave's speed and wavelength when it moves from one medium to another?

Both the wave speed and the wavelength change. The frequency stays the same because it is set by the source.

What is refraction, and what causes it?

A change in the direction of a wave caused by a change in its speed as it passes from one medium to another.

What is an echo?

A sound wave that has been reflected from a surface, such as a wall or cliff.

What is the normal range of human hearing?

20 Hz to 20 000 Hz (20 kHz).

Define ultrasound and infrasound.

Ultrasound is sound with a frequency above 20 000 Hz. Infrasound is sound with a frequency below 20 Hz.

How does the ear convert sound waves into the sensation of sound?

Pressure variations make the eardrum vibrate; the vibration passes to three small bones which amplify it and transfer it to fluid in the cochlea, where tiny hairs create electrical impulses sent to the brain.

Why does the ear only work over a limited frequency range?

Sound is transferred efficiently from air to the solid parts of the ear only for certain frequencies, which restricts the range of sounds humans can hear.

What is the natural frequency of a solid object?

The frequency at which a solid tends to vibrate most easily. Sound waves close to this frequency cause larger vibrations and are transferred more efficiently.

How does echo sounding measure the depth of water?

Ultrasound is sent down and reflects off the seabed. The total distance travelled is twice the depth, so depth = (speed × time) ÷ 2.

How is ultrasound used in foetal scanning?

A transducer sends ultrasound into the body; waves partially reflect at boundaries between different tissues. The time of each echo gives the distance to the boundary, and many measurements build up an image.

What are P-waves and S-waves?

Seismic waves from earthquakes. P-waves are longitudinal and faster; S-waves are transverse and slower.

What can P-waves and S-waves each travel through?

P-waves can pass through solids and liquids. S-waves can only travel through solids, not liquids.

Why are no S-waves detected on the opposite side of the Earth from an earthquake?

S-waves cannot travel through liquids, so they cannot pass through the Earth's molten (liquid) outer core. This is evidence that the outer core is liquid.

What do seismic shadow zones tell us about the Earth's core?

Refractions between layers cause shadow zones where no P-waves are detected. Their size and position suggest the inner core is solid.

Why do we rely on seismic waves to study the Earth's interior?

The interior cannot be observed directly — drilling only reaches about 12.2 km, while the Earth's radius is over 6000 km. Seismic waves provide evidence about its structure.

Why does sound travel further at night than during the day?

At night the air near the ground is cooler and denser, so sound travels faster there and refracts towards the ground, carrying it further.

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