SP4 - Waves

Question 1

SP4a
1) What is a transverse wave?
2) What is a longitudinal wave?
3) Give 2 examples of transverse and longitudinal waves.
4) What do waves transfer?

Answer 1

1) A transverse wave is where the oscillations are perpendicular to the direction of energy transfer.
2) A longitudinal wave is where the oscillations are parallel to the direction of energy transfer.
3) Transverse waves: water waves, visible light waves, other electromagnetic wave examples (eg. Gamma rays, radio waves)
Longitudinal waves: sound waves, seismic P-waves.
4) Waves transfer energy and information, but not matter.

Question 2

SP4a
1) What is frequency and what are its units?
2) What is the amplitude of a wave?
3) What is the wavelength of a wave and what are its units?
4) What is the velocity of a wave and its units?

Answer 2

1) Frequency is the amount of vibrations per second measured in hertz (Hz).
2) The amplitude is the height of a wave above rest point.
3) The wavelength is the distance between two matching points on a wave, measured in metres.
4) The velocity of a wave is the speed of the wave in the direction that it is travelling, measured in metres per second.

Question 3

SP4a
1) What is the wavelength of a longitudinal wave?
2) What is the wavelength of a transverse wave?
3) What is the time period of a wave and its units?

Answer 3

1) The wavelength of a longitudinal wave is the distance from one compression to the next.
2) The wavelength of a transverse wave is the distance from one peak to the next or from one trough to the next.
3) The time period is how long it takes to complete one wave length, measured in seconds.

Question 4

SP4a
1) What is the difference between longitudinal and transverse waves?
SP4b
2) What is the equation relating frequency, wave speed and wavelength (and the symbols)?
3) What is the equation relating time, wave speed and distance?

Answer 4

1) A transverse wave is where the oscillations are perpendicular to the direction of energy transfer. A longitudinal wave is where the oscillations are parallel to the direction of energy transfer. Therefore, the direction of energy transfer is different for these waves.
2) Speed (m/s) = frequency (Hz) x wavelength (m)
v (velocity) = f (frequency) x λ (lambda)
3) Wave speed (m/s) = distance (m) / time (s)

Question 5

SP4b - Core Practical
1) What is the aim of the investigating waves core practical?
2) What is the method to measure the speed of waves in water?

Answer 5

1) The aim is to measure the suitability of equipment to measure the speed, frequency and wavelength of a wave in a solid and a fluid.
2) A. Set up a ripple tank with a straight dipper near one side of the tank. Fasten a ruler to one of the adjacent sides so you can see its markings above the water level.
B. Vary the voltage to the motor until you get waves with a wavelength about half as long as the ripple tank (so you can always see two waves).
C. Count how many waves are formed in 10 seconds and write it down.
D. Look at the waves against the ruler. Use the markings on the ruler to estimate the wavelength of the waves. Use the wavelength and frequency to calculate the speed of the waves.
E. Mark two points on the same edge of the ripple tank as the ruler. Measure the distance between your points. Use the stopwatch to find out how long it takes a wave to go from one mark to the other. Use this information to calculate the speed of the waves.

Question 6

SP4b - Core Practical
1) What can be done to improve the accuracy of the waves core practical?
2) What are 2 examples of errors for the waves core practical, and how can this problem be solved?
3) How do you know how many waves there are based on the shadows created?
4) How can you calculate the frequency of the wave based on the number of waves peaks formed within a certain time?

Answer 6

1) It can be difficult to identify the wave fronts while they are moving. Therefore, the following can be done:
- Use a stroboscope (flashing light) matched to the same frequency of the waves, this will be indicated by the waves appearing to be stationary
- The frequency can be read from the frequency setting of the stroboscope, and the wavelength will be easier to determine while the waves appear still
2) - One error could be the measurement of the wavelength. To improve the accuracy of the wavelength measurement in the ripple tank:
Measure across a number of waves (e.g. 5 of them) and then divide the distance by the number of waves
- Another error could be the measurement of the frequency. To improve the accuracy of the frequency measurement in the ripple tank:
Measure across a longer time period (e.g. a minute) and then divide the number of waves by the time
3) The number of waves is one less than the number of shadows. So if there are 2 shadows, there is 1 wave. If there are 10 shadows, there are 9 waves, etc.
4) Divide the number of wave peaks by the time they were formed in to find the frequency of the wave.

Question 7

SP4b - Core Practical
1) What are the independent, dependent and 3 control variables for the waves core practical on water?
2) What are the independent, dependent and 3 control variables for the waves core practical in a solid?

Answer 7

1) Independent variable: frequency, f
Dependent variable: wavelength, λ
Control variables:
- Same depth of water in the tank
- Same position of the cork when the person starts timing
- Same equipment used
2) Independent variable: frequency, f
Dependent variable: wavelength, λ
Control variables:
- Same metal rod
- Same hammer
- The device for measuring the frequency must be the same distance away

Question 8

SP4b - Core Practical
1) What is the method for the waves core practical in a solid?
2) How long is the wavelength in this practical?
3) What are safety considerations for the investigating waves core practical?

Answer 8

1) A. Suspend a metal rod horizontally using clamp stands and rubber bands.
B. Hit one end of the rod with a hammer. Hold a smartphone with a frequency app near the rod and note down the peak frequency.
C. Measure the length of the rod and write it down. The wavelength will be twice the length of the rod.
D. Use the frequency and wavelength to calculate the speed of sound in the rod.
2) The wavelength is twice the length of the rod
3) - Care should be taken when working with water and electricity in close proximity
- Use a rubber string instead of a metal wire, in case it snaps under tension

Question 9

SP4c
1) What is refraction?
2) What is the normal?
3) How does the direction of a wave change when it goes from one material to another, in terms of density?

Answer 9

1) Refraction is the change in direction of a wave when it goes through a different medium.
2) The normal is an imaginary line perpendicular (at a right angle) to the boundaries of two materials.
3) When a wave is passing into a denser material, it slows down and bends towards the normal.
When a wave is passing into a less dense material, it speeds up and bends away from the normal.

Question 10

SP4c
1) What are the effects of the refraction of light?
2) How can a change in wave speed cause a change in direction?

Answer 10

1) Refraction can cause optical illusions as the light waves appear to come from a different position to their actual source.
2) When waves travel through a medium of a different density, this causes a wave to either speed up (less dense) and bend away from the normal, or slow down (more dense) and towards the normal. This causes the waves to change direction.

Question 11

SP4d
1) What happens when a wave is reflected?
2) What happens when a wave is refracted?
3) What happens when a wave is transmitted?
4) What happens when a wave is absorbed?

Answer 11

1) A wave hits a boundary between two media and does not pass through, but instead stays in the original medium.
2) The wave passes enters into the new material but changes the direction in which it is travelling.
3) The wave passes through a material and is not absorbed or reflected.
4) The energy that the wave is carrying is transferred into the material.

Question 12

SP4d
Describe how changes in velocity, frequency and wavelength are related when sound waves go from one medium to another.

Answer 12

Sound waves travel at different speeds in different materials. The velocity of a wave is equal to the frequency multiplied by the wavelength. Therefore, if the velocity changes, wavelength must also change. However, the frequency does not change, because waves would have to be created or destroyed at the boundary, but that’s not possible.

Question 13

SP4e
1) List the parts of the human ear, in the order in which they transmit vibrations.
2) List the functions of the parts of the human ear, in the order that they transmit vibrations

Answer 13

1) The ear canal, the eardrum, the tiny bones in the ear, the cochlea, the auditory nerve
2) 1. Sound waves enter the ear canal.
2. Sound waves make the eardrum, which is a thin membrane, to vibrate.
3. Sound waves are passed onto tiny bones which amplify the sound waves (make them bigger)
4. Vibrations are passed onto the liquid inside the cochlea
5. Tiny hairs inside the cochlea detect these vibrations and create electrical signals called impulses.
6. Impulses travel along neurons in the auditory nerve to reach the brain.

Question 14

SP4e
1) How are sound waves in air converted to vibrations in solids, and back into air again?
2) What is the range of human hearing?
3) Why can the human ear only detect certain frequencies?

Answer 14

1) A sound wave is produced. The particles in the air vibrate back and forth as the sound waves travel through the air to the solid. When the sound wave hits the solid, the air particles hitting the solid cause it to vibrate. The particles in the solid vibrate and transmit the wave through the solid. When the vibrations hit the other end of the solid, the vibrations cause the surrounding air particles to vibrate, generating a sound wave which is heard on the other side.
2) Humans can hear sounds between 20 Hz and 20,000 Hz.
3) The cochlea can only detect certain frequencies, as the hairs in the human cochlea have different lengths and vibrate a different frequencies of sound.

Question 15

SP4e
1) What is the cochlea and how does it work?
2) What is the base of the cochlea?
3) What is the apex of the cochlea?
4) What are 3 factors that limit the range of human hearing?

Answer 15

1) The cochlea is a coiled tube containing a liquid. It detects the different frequencies of sound reaching the ear.
This is how it works: It contains a membrane with tiny hairs that vibrate at different frequencies of sound.
2) The base the end of the cochlea that is is the narrow, thick and stiff.
3) The apex is the end of the cochlea that is wide, thin and flexible.
4) The structure of the inner ear, the shape of the eardrum, and the size of the eardrum.

Question 16

SP4f
1) What is the formula to calculate the depth of water from information about time and wave velocity?
2) How is the depth of water calculated using sonar, from information about time and wave velocity?
3) What is ultrasound?

Answer 16

1) Distance (m) = speed (m/s) x time (s)
2) The time between sending and detecting the pulse is the time it takes for the pulse to travel to an object and back again. So to find the distance to the object reflecting the waves, the time must be halved. Then, the time is multiplied by the wave velocity.
3) Ultrasound are sounds made by waves with frequencies higher than 20,000 Hz. These sounds are too high for humans to hear.

Question 17

SP4f
1) How is ultrasound used in foetal scanning?
2) How does ultrasound in sonar work?
3) What is one way that submarines could make themselves more difficult to detect?

Answer 17

1) The ultrasound is used to create an image of the unborn baby. Pulses of ultrasound are directed towards foetus. They are particularly reflected at each boundary, for example the boundary between fluid in the womb and the foetus. The reflections are detected and their timings and distributions are used to produce a video image of inside the womb.
2) Ultrasound can be used to detect objects in deep water and to measure water depth. A loudspeaker on the ship emits a pulse of ultrasound. A special microphone on the ship detects the echo, and the sonar equipment measures the time between the sound being sent out and the echo returning, and uses this time to calculate the distance between the object and the ship.
3) They could be coated with something that absorbs ultrasound waves. This would mean that fewer or no waves are reflected, so it would be harder for a boat to find the submarine.

Question 18

SP4g
1) What is infrasound?
2) What are 3 uses of infrasound?
3) What are seismic waves?
4) What is the shadow zone?

Answer 18

1) Infrasound is sounds with a frequency less than 20 Hz. These sounds are too low for humans to hear.
2) Infrasound can be used to investigate the internal structure of the earth’s core, track animals, and monitor volcanoes.
3) Seismic waves are vibrations caused by earthquakes. They are a type of infrasound wave.
4) The S wave or P wave shadow zone is the area of the Earth’s surface where S waves or P waves are not detected following an earthquake due to refraction.

Question 19

SP4g
1) What are P-waves and S-waves?
2) What are the properties of P-waves and S-waves?
3) How can seismic waves can help us to investigate the Earth’s core?

Answer 19

1) Longitudinal P waves and transverse S waves are the ways that seismic waves released by earthquakes can travel through the earth.
2) S-waves can only travel through solids, not liquid. P-waves can travel through both solids and liquids, but is faster in a solid than in a liquid.
3) S-waves are not detected on the opposite side of the Earth – this suggests that the mantle has solid properties, but the outer core must be liquid. P-waves are detected on the opposite side of the Earth. Refractions between layers cause two shadow zones, where no p-waves are detected – the size and positions of these shadow zones indicate there is a solid inner core.

Question 20

SP4g
1) What are the layers of the earth, and are they solid or liquid?
2) Explain how the fact that certain waves are not detected on the other side of the earth indicates that the Earth’s core is made from liquid
3) State happens to S-waves and P-waves when they cross from the mantle into the core (in terms of wave speed)
4) Explain why a graph of the earth showing seismic waves may stop at a certain point

Answer 20

1) The crust is the outer layer and is a solid. This mantle (section under the crust of the earth) is a solid. The outer core (section under the mantle) is a liquid. The inner core is a solid.
2) As there are no S-waves detected on the other side of the earth, something must be stopping them. S-waves cannot pass through a liquid so it is likely that part of the interior of the earth is in a liquid state.
3) When seismic waves, like S-waves and P-waves, cross from the mantle into the core, their speeds change. P-waves speed up, while S-waves either slow down significantly or can’t pass through the liquid outer core at all, resulting in a shadow zone. This difference in behavior helps scientists to understand the structure of the Earth’s interior.
4) The depth of the graph (eg. 6000km) is half the depth of the earth

Question 21

SP4b
Describe a method to measure sound waves in air

Answer 21

1) Find a large empty area, such as a field or large court.
2) Choose two spots on opposite ends of the area where each person will stand.
3) Measure the distance between the two spots using a tape measure. Alternatively, you can count off measured steps between the two spots.
4) Have your friend take the blocks and stand at one spot, holding them up high.
5) Take the stopwatch and stand at the other spot. Make sure you have a clear view of the blocks.
6) Signal your friend to bang the two blocks together hard.
7) Start the stopwatch as soon as you see the blocks hit each other.
8) Press stop as soon as you hear the sound from the blocks.
9) Calculate the speed of the sound by dividing the distance between you and your friend by the elapsed time. To get a more accurate measurement, repeat the above steps a few times and then take an average of the results.

Question 22

SP4b
1) What is the formula for percentage change?
2) Explain why the times that the students may measure during the speed of sound in air practical are different, and explain how students might improve the experiment.

Answer 22

1) ((final value - initial value) / initial value) x 100
2) The times of the students may vary because reaction time will be different for each of the students or the students may be at different distances from the object producing the sound (Eg. Pistol)
To improve the experiment a much longer distance/ use an electronic timer, which reduces the impact of the reaction time/ gives a more manageable time to measure. Additionally, all the students can stand the same distance from object producing the sound (Eg. Pistol).

Question 23

SP4b - Core Practical
1) Describe how hitting the rod causes a sound wave to travel along the inside of the rod.
2) Explain how hitting the rod causes sound waves to travel through the air
3) Why can the method used for measuring the speed of sound in air not be used to measure the speed of sound in a metal?

Answer 23

1) The particles at the end vibrate more about fixed positions. This causes neighbouring particles to vibrate more.
2) The rod makes the air particles vibrate in the same direction that the wave travels, parallel to the direction of energy transfer.
3) The speed of sound in a metal is much higher than in air, and the time between a sound being made and its echo reaching the end of a rod is too short to be measured using a stopwatch.

Question 24

SP4b - Core Practical
1) Describe how hitting the rod causes a sound wave to travel along the inside of the rod.
2) Suggest an alternative way to measure the wavelength accurately that does not involve a strobe light

Answer 24

1) The particles at the end vibrate more about fixed positions. This causes neighbouring particles to vibrate more.
2) Take an image with a camera/video/phone. Then use a meter rule in image as scale bar, and measure wavelength and calculate value from scale bar.

Question 25

Describe how the student should measure the angle of refraction, when investigating refraction in a rectangular glass block

Answer 25

Trace or mark where the light ray went into and of through the glass block . Then remove block, joint the entry and exit points of the ray of light. Finally, use a protractor to measure the angle between the refracted ray and the normal.