Waves in Air, Fluids & Solids (AQA GCSE Physics): Flashcards

Wavelength is the distance from one point on a wave to the same point on the next wave. It is given the symbol λ and measured in metres (m).

Frequency is the number of waves passing a point in a second. It is given the symbol f and measured in hertz (Hz).

The time period of a wave is the time taken for a single wave to pass a point (or one full cycle of a wave). It is given the symbol T and measured in seconds (s).

In a transverse wave, wavelength is measured from one peak to the next peak. In a longitudinal wave, wavelength is measured from the centre of one compression to the centre of the next.

Waves with a higher frequency transfer a higher amount of energy.

True.

The time period is just the inverse of the frequency.

Wave speed is the distance travelled by a wave each second. It is given the symbol v and measured in metres per second (m/s).

Where v = wave speed (m/s), f = frequency (Hz) and λ = wavelength (m).

Yes. Both transverse and longitudinal waves obey the wave equation.

Wave speed is the speed at which energy is transferred through a medium.

True.

Rearranging v = fλ gives λ = v ÷ f.

λ = v ÷ f

λ = 0.15 ÷ 0.5

Wavelength, λ = 0.30 m

Two people stand about 100 m apart (distance measured with a trundle wheel). One person bangs two wooden blocks together above their head; the second starts a stopwatch when they see this and stops it when they hear the sound. This is repeated and averaged, then speed = distance ÷ time.

A person stands about 50 m from a wall and claps two wooden blocks in rhythm with the echoes. A second person times 20 claps with a stopwatch. The total distance travelled is (20 × 2 × 50) m, so speed = distance ÷ time.

Two microphones are placed about 5 m apart (measured with a tape measure) and connected to an oscilloscope, which triggers when the first microphone detects a sound. A loud clap is made next to the first microphone, and the oscilloscope shows the time difference between the sound arriving at each microphone. This is repeated and averaged, then speed = distance ÷ time.

Two people stand a few metres apart with a tape measure. One person disturbs the water surface to create a ripple; the second times how long it takes the ripple to reach them with a stopwatch. This is repeated ten times and an average time calculated, then speed = distance ÷ time.

In each method for measuring wave speed, the speed is calculated by dividing the distance travelled by the time taken.

True.

Each clap-to-echo journey covers twice the distance to the wall (2 × 50 m), so for 20 claps the total distance is (20 × 2 × 50) m.

Sound waves transfer energy by molecules vibrating and colliding with neighbouring molecules. Solids have the most molecules present, so energy transfers fastest; gases have the fewest, so energy transfers slowest.

When a sound wave moves from one medium to another, its wave speed and wavelength change, but its frequency stays the same.

Refraction is the change in direction of a sound wave, caused by a change in its velocity as it moves from one medium to another.

Both the wavelength and the velocity of the sound wave decrease (the frequency stays the same).

Both the wavelength and the velocity of the sound wave increase (the frequency stays the same).

False.

On warm days, air molecules move faster and carry sound waves faster, increasing the speed of sound; on cold days, molecules move slower, decreasing the speed of sound.

Independent variable = frequency, f. Dependent variable = wavelength, λ.

By using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts.

By timing how long it takes for a given number of waves to pass a particular point, then dividing the number of wavefronts by the time taken.

In the vibrating string experiment, the frequency of the signal generator is adjusted until a stationary wave is produced.

A stroboscope flashed at the same frequency as the waves makes them appear stationary, so the wavefronts are easier to identify and the wavelength and frequency easier to measure accurately.

Use a rubber string instead of a metal wire, in case it snaps under tension. Wear safety goggles. Stand well away from the masses in case they fall. Place a crash mat under the masses.

False.

The depth and temperature of the water are control variables, kept constant; the independent variable is frequency and the dependent variable is wavelength.

Reflection occurs when a wave hits a boundary between two media and does not pass through, but instead stays in the original medium.

Transmission occurs when a wave passes through a substance.

Absorption occurs when energy is transferred from the wave into the particles of a substance.

Flat surfaces are the most reflective because the smoother the surface, the stronger the reflected wave. Rough surfaces are the least reflective because the light scatters in all directions.

Only red light has been reflected from the object; all other frequencies of visible light have been absorbed.

When a wave is transmitted through a material it is usually partially absorbed, so the transmitted wave has a lower amplitude.

The law of reflection states that the angle of incidence equals the angle of reflection.

The incident ray has an arrow pointing towards the boundary. The reflected ray has an arrow pointing away from the boundary.

False.

Angles are measured between the ray and a line at 90 degrees to the boundary, not the boundary surface itself.

Independent variable: angle of incidence (i). Dependent variable: angle of reflection (r).

Independent variable: angle of incidence (i). Dependent variable: angle of refraction (r).

Distance of the ray box from the mirror or block, width of the light beam, and using the same frequency/wavelength of light throughout.

When light enters a perspex block, the ray refracts towards the normal, so i is greater than r.

It refracts away from the normal, so the angle of incidence is less than the angle of refraction (i < r).

It passes straight through without refracting, so i = r.

If the mirror is distorted, this could affect the reflection angle. Use a mirror with little to no blemishes on it.

False.

i and r are always measured from the normal, not from the surface of the block.

Compressions are regions of higher density; rarefactions are regions of lower density, formed as a sound wave (a longitudinal wave) travels.

The compressions and rarefactions of a sound wave cause changes in pressure that vary in time with the wave.

The sound wave's vibrations are transferred to the glass, causing its surface to vibrate in sync with the wave. If the glass vibrates too much, this causes it to shatter.

Sound waves travel down the auditory canal towards the eardrum.

The eardrum (made of tissue and skin) and three small bones.

Vibrations pass from the eardrum to the three small bones, then to the inner ear, where nerve cells detect the sound and send a message to the brain.

False.

The range of frequencies a human can hear is 20 Hz to 20 000 Hz.

Echo sounding (ocean floor), ultrasound (inside the human body), ultrasound crack detection (rail tracks), reflection seismology (oil and gas underground) and seismic activity from earthquakes (structure of the Earth).

Reflection, absorption, transmission, and the speed of sound in the substance.

Sound waves travel fastest in solids, slower in liquids and slowest in gases.

Each type of substance produces different amounts of reflection, absorption and transmission, and transmits sound at a specific speed. By detecting the amount reflected and the speed, the hidden structure can be identified.

False.

Certain structures reflect a proportion of the sound wave and transmit the rest; only some substances absorb sound waves with very little reflection.

Ultrasound is sound waves with a frequency above the human hearing range of 20 000 Hz.

It may be partially reflected, partially transmitted, and partially absorbed.

The percentage reflected or transmitted depends on the two media at the boundary, e.g. an air-glass boundary reflects a different percentage to an air-wall boundary.

When the difference in speed of sound between the two media is large.

When sound in air hits a brick boundary, most of the sound is reflected because waves travel much faster in brick than in air.

True.

The human ear can hear frequencies between 20 Hz and 20 000 Hz; ultrasound is defined as sound above this range.

Echo sounding uses ultrasound to detect objects underwater. The sound wave is reflected off the ocean bottom, and the time it takes to return is used to calculate the depth of the water.

The wave must travel down to the ocean floor and then back up again to the ship, so the total distance is the depth there plus the depth back.

To find the distance travelled by a wave, rearrange the wave speed equation to x = v × t.

Distance travelled, x = v × t = 1500 × 0.12 = 180 m.

Depth, d = 180 ÷ 2 = 90 m.

False.

Echo sounding uses ultrasound to detect objects underwater; measuring the depth of the water is one example of this.

An ultrasound transducer is a device that can both emit and receive ultrasound waves.

Some of the waves are partially reflected, while the remainder are transmitted through the material.

Distance = speed × time. This is needed because ultrasound travels at different speeds through different media.

Unlike many other medical imaging techniques, ultrasound is non-invasive and is believed to be harmless.

Ultrasound can be used to construct images of a foetus in the womb, to generate 2D images of organs and internal structures not surrounded by bone and as a medical treatment, such as removing kidney stones.

The transducer emits ultrasound, which reflects off boundaries between tissues; the returning echoes generate electrical signals, and a series of measurements swept across an area is used to build up an image.

A crack causes some of the ultrasound waves to reflect earlier than the rest, producing an extra pulse on the oscilloscope trace between the start and end pulses.

True.

Ultrasound is only reflected at boundaries, and an uncracked bolt only has boundaries at its start and end.

P-waves are longitudinal seismic waves that can travel through both solids and liquids.

S-waves are transverse seismic waves that can only travel through solids.

P-waves travel faster than S-waves.

Infrasound is sound below the frequency of human hearing (<20 Hz). P-waves are very low frequency sound waves known as infrasound.

Seismic waves refract as they pass through the different layers of the Earth, which affects the regions in which the waves can be detected.

On the opposite side of the Earth to an earthquake, only P-waves are detected, not S-waves. This shows the mantle is solid (as both waves pass through it) and the outer core is liquid (since S-waves cannot penetrate it).

Refraction between layers causes two shadow zones where no P-waves are detected. The size and position of these zones indicate large refraction, showing the inner core is solid.

True.

By carefully timing the arrival of P-waves and S-waves at different seismometers around the world, both the location and magnitude of an earthquake can be determined.