Waves in Air, Fluids & Solids Flashcards

All 66 cards in this deck

What is a transverse wave?

A wave in which the oscillations are perpendicular to the direction of energy transfer.

What is a longitudinal wave?

A wave in which the oscillations are parallel to the direction of energy transfer.

In a longitudinal wave, what are compressions and rarefactions?

Compressions are regions where the particles are squashed close together; rarefactions are regions where they are spread further apart.

True or false? Sound waves in air are transverse, like ripples on a water surface.

False. Ripples on water are transverse, but sound waves in air are longitudinal.

What evidence shows that it is the wave, not the water, that travels across a water surface?

A floating object bobs up and down as ripples pass but is not carried along with them.

What evidence shows that air itself does not travel from a loudspeaker to your ear?

The air particles only vibrate about fixed positions — no wind or flow of air reaches you, only energy is transferred.

What is the amplitude of a wave?

The maximum displacement of a point on the wave away from its undisturbed position.

What is the wavelength of a wave?

The distance from a point on one wave to the equivalent point on the adjacent wave.

What is the frequency of a wave, and what is its unit?

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

What is the period of a wave, and what is the equation linking period and frequency?

The time taken for one complete wave to pass a point; $T = \dfrac{1}{f}$, with $T$ in seconds.

What is the wave speed?

The speed at which the energy is transferred (or the wave moves) through the medium, in m/s.

True or false? The amplitude of a wave is the distance from the top of a crest to the bottom of a trough.

False. The amplitude is the maximum displacement from the undisturbed position — half of the crest-to-trough distance.

State the wave equation, naming each quantity and its unit.

$v = f\lambda$: wave speed $v$ in m/s, frequency $f$ in Hz, wavelength $\lambda$ in m.

What are the steps to find the wavelength of a wave from its speed and frequency?
e.g. a wave of frequency 5 Hz travelling at 10 m/s

1. Write $v = f\lambda$
2. Rearrange: $\lambda = \dfrac{v}{f}$
3. Substitute: $\lambda = \dfrac{10}{5} = 2$ m

What are the steps to find the speed of a wave from its wavelength and period?
e.g. $\lambda = 6$ m, $T = 0.5$ s

1. Find frequency: $f = \dfrac{1}{T} = \dfrac{1}{0.5} = 2$ Hz
2. Use $v = f\lambda$
3. $v = 2 \times 6 = 12$ m/s

Which unit conversions must you do before substituting into $v = f\lambda$?

Frequency into Hz ($1$ kHz $= 1000$ Hz, $1$ MHz $= 1\,000\,000$ Hz), wavelength into metres ($1$ cm $= 0.01$ m) and speed into m/s ($1$ km/s $= 1000$ m/s).

True or false? If the frequency of a wave doubles while its speed stays the same, the wavelength halves.

True. In $v = f\lambda$, at constant $v$ the frequency and wavelength are inversely proportional.

Which equation gives wave speed from a measured distance and time?

$\text{speed} = \dfrac{\text{distance}}{\text{time}}$

What are the steps to measure the speed of sound in air?
e.g. two students standing 100 m apart, one banging cymbals

1. Measure the distance between them with a tape measure: 100 m
2. Time from seeing the cymbals hit to hearing the sound, repeating and taking a mean: about 0.30 s
3. speed $=\dfrac{100}{0.30}\approx 330$ m/s

What are the steps to measure the speed of ripples on a water surface without measuring the wavelength?
e.g. a ripple crossing a tank

1. Measure the distance the wave travels (the length of the tank) with a metre rule: 0.60 m
2. Time the wave to travel that distance with a stopwatch: 1.5 s
3. speed $=\dfrac{0.60}{1.5} = 0.40$ m/s

True or false? The speed of sound in air is about 330 m/s.

True. Accepted values are roughly 330–340 m/s.

Why are the timings repeated and a mean taken when measuring the speed of sound in air?

To reduce the effect of random errors, mainly human reaction time when starting and stopping the timer.

When a sound wave passes from one medium into another, which quantity stays the same?

The frequency — it is set by the source.

What happens to the wavelength of a sound wave when it passes into a medium where it travels faster?
e.g. sound going from air into water

The wavelength increases, because the frequency is unchanged and $v = f\lambda$.

True or false? Sound travels faster in solids than in air.

True. The particles are closer together, so vibrations are passed on more quickly.

Why can sound waves not travel through a vacuum?

They are mechanical waves and need particles of a medium to vibrate.

What are the steps to measure the frequency of ripples in a ripple tank?
e.g. 10 wave fronts pass a mark in 8 s

1. Time how long 10 wave fronts take to pass a fixed mark: 8 s
2. Period $=\dfrac{8}{10} = 0.8$ s
3. $f = \dfrac{1}{T} = 1.25$ Hz

What are the steps to measure the wavelength of ripples in a ripple tank?
e.g. 10 ripples spanning 20 cm

1. Use a lamp above the tank to project the ripple shadows onto paper below
2. Measure across 10 wavelengths with a ruler: 20 cm
3. Divide by 10: wavelength $= 2$ cm $= 0.02$ m

In RP8, how is the frequency of waves on a stretched string (waves in a solid) found?

Read it from the display of the signal generator driving the vibration generator.

True or false? Timing 10 ripples and dividing by 10 gives a more accurate period than timing a single ripple.

True. It reduces the percentage uncertainty caused by reaction time.

State the law of reflection.

The angle of incidence equals the angle of reflection.

What three things can happen to a wave at a boundary between two different materials?

It can be reflected, transmitted or absorbed.

What does it mean if a wave is absorbed at a material interface?

The wave's energy is transferred into the material's energy stores (usually thermal) rather than continuing as a wave.

What does it mean if a wave is transmitted at a material interface?

The wave passes through into the second material, often changing speed and direction.

True or false? The angle of incidence is measured between the incident ray and the surface.

False. It is measured between the incident ray and the normal.

What are the steps to draw a ray diagram for reflection at a plane surface?
e.g. a ray arriving at 30° to the normal

1. Draw the normal, perpendicular to the surface at the point where the ray hits
2. Draw the incident ray to that point and mark the angle of incidence: 30°
3. Draw the reflected ray on the other side of the normal at the same angle: 30°

What are the steps to investigate the refraction of light by a glass block (RP9)?
e.g. an angle of incidence of 40°

1. Draw round the block on paper and draw a normal where the ray will enter
2. Shine a narrow ray in at 40° to the normal and mark the ray going in and coming out with dots
3. Remove the block, join the dots and measure the angle of refraction from the normal with a protractor: e.g. 25°

Why should the ray from the ray box be as narrow as possible in RP9?

It is harder to judge the centre of a wider ray, which gives a larger uncertainty (more random error) in the angles measured.

What are the steps to estimate the uncertainty in a set of repeated angle measurements?
e.g. angles of refraction 28°, 25° and 22°

1. Find the range: $28 - 22 = 6$
2. Halve it: $6 \div 2 = 3$
3. Uncertainty $= \pm 3°$

Why is the room darkened when doing RP9 with a ray box?

The light ray is easier to see, so its path can be marked and its angle measured more accurately.

What is the range of normal human hearing?

20 Hz to 20 kHz (20 000 Hz).

What happens when sound waves travel through a solid?

They cause the solid to vibrate, passing the wave on through the material.

How do sound waves reaching the ear produce the sensation of sound?

They make the ear drum and other parts of the ear vibrate, and these vibrations are converted into the sensation of sound.

Why can humans only hear sounds within a limited frequency range?

The conversion of sound waves into vibrations of solid parts of the ear only works over a limited range of frequencies.

True or false? A healthy human can hear a sound of frequency 25 kHz.

False. It is above the upper limit of human hearing, 20 kHz.

Why can waves be used to explore structures that are hidden from direct observation?

They are partially reflected at boundaries between different materials, so the reflected waves reveal where the boundaries are.

Which three properties of waves differ between materials and so are used to explore hidden structures?

Velocity, absorption and reflection.

Give three uses of waves to detect or explore structures hidden from direct observation.

Ultrasound medical scanning, ultrasound flaw detection in metals, echo sounding of water depth and seismic waves probing the Earth's interior.

What is ultrasound?

Sound waves with a frequency higher than the upper limit of human hearing, i.e. above 20 kHz.

What happens to ultrasound when it meets a boundary between two different media?

It is partially reflected, and the time for the reflection to reach a detector shows how far away the boundary is.

What are the steps to find the distance to a boundary from an ultrasound echo time?
e.g. echo returns after 0.004 s at a speed of 1500 m/s

1. Total distance $= v \times t = 1500 \times 0.004 = 6$ m
2. This is there and back, so halve it
3. Distance to the boundary $= 3$ m

True or false? Ultrasound is a type of electromagnetic wave.

False. It is a longitudinal (mechanical) sound wave and needs a medium.

What is echo sounding?

Using high frequency sound waves (ultrasound) reflected from the sea bed or objects to measure water depth and detect objects in deep water.

Give two things echo sounding is used to detect in deep water.

The sea bed (water depth), and objects such as shipwrecks, submarines or shoals of fish.

Besides the echo time, what must be known to calculate water depth by echo sounding?

The speed of sound in water (about 1500 m/s).

True or false? In echo sounding, the water depth is equal to the speed of sound in water multiplied by the echo time.

False. That gives the total distance there and back — the depth is half of it.

Give an industrial use of ultrasound imaging.

Detecting cracks or flaws inside metal castings, pipes or welds.

How does an ultrasound scan build up an image of the inside of the body?

Ultrasound is partially reflected at each boundary between tissues; the return times are converted into distances and processed into an image.

How does ultrasound reveal a crack inside a metal block?

Ultrasound reflects off the crack, so an echo returns sooner than the echo from the far side of the block.

True or false? Ultrasound is preferred to X-rays for scanning a fetus because it is non-ionising.

True. Ultrasound does not ionise cells, so it is safer than X-rays.

What produces seismic waves?

Earthquakes.

What type of wave is a P-wave, and which materials can it travel through?

Longitudinal; it travels through both solids and liquids, at different speeds in each.

What type of wave is an S-wave, and which materials can it travel through?

Transverse; it travels through solids but cannot travel through a liquid.

What evidence from seismic waves shows that the Earth's outer core is liquid?

S-waves are not detected on the far side of the Earth (an S-wave shadow zone), because S-waves cannot travel through liquid.

True or false? S-waves travel faster than P-waves, so they arrive at a seismometer first.

False. P-waves are faster and arrive first.

Why is the study of seismic waves important to scientists?

It gave new evidence about the structure and size of the Earth's core, parts of the Earth that cannot be observed directly.