Topic 4 - waves

Question 1

Explain how waves work

Answer 1

Waves transfer energy and information without transferring matter.

Evidence for this is that a water wave can be seen when a ball dropped into a pond bobs up and down, but the wave energy travels outwards towards as ripples across the surface of the pond

Question 2

Explain how waves can be described

Answer 2

FREQUENCY - number of waves passing a point each second, measured in Hertz (Hz)

SPEED - measured in metres per second (m/s)

WAVELENGTH and AMPLITUDE

PERIOD - the time taken for one wavelength to pass a point

PERIOD = 1/frequency

Question 3

Describe the difference between longitudinal and transverse waves by referring to sound, electromagnetic, seismic and water waves

Answer 3

SOUND waves and SEISMIC P WAVES are LONGITUDINAL waves. The particles in the material the sound is travelling through move back and forth along the same direction that sound is travelling. Particles in a longitudinal wave move along the same direction as the wave.

Waves on a WATER SURFACE, ELECTROMAGNETIC WAVES and SEISMIC S waves are all TRANSVERSE waves. The particles of water move in a direction at right angels to the direction the wave is travelling. Particles in a transverse wave move across the direction the wave is travelling.

Question 4

Describe how to calculate the speed/velocity of sound in air using an echo

Answer 4

1. Measure the distance from the sound to the reflecting surface (the wall)
2. Measure the time interval, with a stopwatch between the original sound being produced and the echo being heard.
3. Use speed (m/s) = distance (m) / time I (s)

Repeating the experiment a number of times over a range of distances will allow you to obtain accurate and precise results

Question 5

Describe how to calculate the speed/velocity of sound in air using two microphones and an oscilloscope

Answer 5

1. Set up the microphones one in front of the other at different distances in a straight line form a loudspeaker
2. Set the frequency of the sound from the loudspeaker to a known, audible value
3. Display the tow waveforms on the oscilloscope. Measure the distance between the microphones.
4. Move the microphones apart so that the waveforms move apart by 1 wavelength
5. Calculate the speed of sound using the equation : wave speed (m/s) = frequency (Hz) X Wavelength (m)

Question 6

Explain how to calculate the speed of ripples on water surfaces

Answer 6

You can work this out by using a ripple tank and a strobe.

Set the power supply to vibrate the paddle at a known frequency and use the strobe light to “freeze” the water waves so that you can measure the wavelength

Use the equation : wave speed = frequency X wavelength to calculate the speed or velocity of the water waves on the surface of the ripple tank

Question 7

Describe the effects of reflection

Answer 7

The amount of reflection that happens at the boundary between two materials depends on the densities of the materials. The greater the difference in density, the more sound energy will be reflected.

Sound is reflected when there is a big difference in densities of materials at an interface for example air and concrete

Question 8

Describe the effects of refraction

Answer 8

Sound waves, water waves and light waves can all be refracted. Refraction can result in a change of both speed and direction. The direction doesn’t change if the wavefronts travel perpendicular to the normal.

REFRACTION SPECIAL CASE:
When light, sound or water waves move from one material into another their direction DOESNT change if they are moving along the normal

Question 9

Describe transmission

Answer 9

Sound is transmitted through a material when the densities are similar

Question 10

Describe the effects of absorption

Answer 10

Sound can be absorbed by materials. The amount of absorption depends on the material and the wavelength of the sound

Question 11

Describe the effects of waves at different interfaces

Answer 11

• Sound waves travel slower in cooler, denser air than in warmer, less dense air
• water waves travel faster in deep water than in shallow water. They can also change direction.
• light waves can slow down and change direction when they pass from air to glass

Question 12

Describe how different substances may absorb, transmit, refract or reflect waves

Answer 12

different substances may absorb, transmit, refract or reflect waves in ways that vary with wavelength

Question 13

Describe the processes which convert wave disturbances between sound waves and vibrations in solids, and explain why such processes only work over a limited frequency range

Answer 13

When objects vibrate, sound waves are produced. These sound waves are a series of oscillations that transfer energy from the sound to the ear.

One example of this is the vibration a guitar string to produce a musical note. Once plucked, the string will vibrate at its natural frequency.

The human ear can only detect frequencies in the range 20 Hz -> 10 kHz. It doesn’t hear frequencies below 20 Hz or above 20 kHz.

Air molecules are forced to vibrate by the sound of the vibration. Energy travels via a longitudinal wave until it reaches the ear

Question 14

Explain the way the human ear works

Answer 14

Sound waves are channeled down the ear canal and cause the eardrum to vibrate. These vibrations pass through the ear as further vibrations are then converted to an electrical signal and carried to the brain.

Question 15

Explain how sound waves are produced in a drum

Answer 15

After the drum is hit, the vibration of the drum will cause a longitudinal wave to move outwards from it

Question 16

Describe ultrasound

Answer 16

Ultrasound is sound with frequencies greater than 20 000 hertz, Hz (10kHz)

Question 17

Describe infrasound

Answer 17

Infrasound is sound with frequencies lower than 20 hertz, Hz

Question 18

Explain the use of ultrasound in prenatal scanning

Answer 18

Ultrasound waves are used to make images of the inside of the body, they arent harmful so it is safe to use them to scan foetuses (unborn babies).

The ultrasound waves are sent into the woman’s body, and some of the sound is reflected each time it meets a layer of tissue with a different density to the one it has just passed through.

The scanner detects the echoes and a computer uses the information to make a picture

Question 19

Explain the use of ultrasound in sonar

Answer 19

Sonar uses pulses of ultrasound to find the depth below a ship.

The sonar equipment measures the time between sending the sound and detecting its echo.

The time is used to calculate the depth of the water, using distance = speed x time

Question 20

Explain the use of infrasound when exploring the earths core

Answer 20

Infrasound is believed to travel through the earth as shockwaves from tsunami , volcanoes and earthquakes. Infrasound is also produced when meteors enter the earths atmosphere. Scientists can detect this infrasound and track the path the meteor will take.

Infrasound waves can be detected from explosions under the ground. The infrasound waves can also help to determine the structure of rocks beneath the earths crust.

Question 21

Describe how changes, if any, in velocity, frequency and wavelength, in the transmission of sound waves from one medium to another are inter-related

Answer 21

When a sound wave moves from one material to another:

• the wave speed or velocity may change
• the wavelength may change

Wavelength is directly proportional to wave speed.
If the speed or velocity of a sound wave Increases, then its wavelength will increase. However its frequency does not change as the number of waves being produced per second isn’t affected

Question 22

Explain the method of investigating the suitability of apparatus to measure the speed, frequency, and wavelength of waves in a fluid

Answer 22

1. Set up the apparatus (ripple tank, strobe light)
2. Calculate the frequency of the waves by counting the number of waves that pass a point each second. Do this for a minute and then divide by 60 to get a more accurate value for the frequency of the water waves.
3. Use a stroboscope to ‘freeze’ the waves and find their wavelength by using a ruler. The ruler can be left in the tank or the waves can be projected onto a piece of A3 paper under the tank an the wave positions marked with pencil marks on the paper.
4. Calculate the wave speed

Question 23

Explain how water waves travel in a ripple tank experiment and their speed

Answer 23

Water waves will travel at a constant speed in a ripple tank when generated at different frequencies if the depth of the water is constant at all points.

This means that the equation wave speed = frequency X wavelength will give the same wave speed - if the frequency increases, then the wavelength will decrease in proportion

Question 24

Explain precautions to take when doing the investigation of suitability

Answer 24

• water and electricity are both being used here, both of which can be dangerous. Be careful to take this into consideration when planning your practical
• to obtain accurate results for the wavelength of the water wave it is best to find the distance between a large number of waves and then divide this value by the number of waves. This will reduce the percentage error in your value for the wavelength of the wave

Question 25

Explain the conclusion of the ripple tank suitability practical

Answer 25

A ripple tank can be used to determine values for the wavelength, frequency and wave speed of water waves. It is a suitable method, provided that small wavelengths and frequencies are used.