Topic 4 - Waves

Question 1

What are waves?

Answer 1

• Waves are one way in which energy may be transferred between stores.
• Both mechanical and electromagnetic waves will transfer energy but not matter.

Question 2

What is frequency?

Answer 2

The number of waves passing a point each second.

Question 3

What is the wave length?

Answer 3

• The distance covered by a full cycle of the wave.

- Usually measured from peak to peak, or trough to trough.

Question 4

What is the rest position?

Answer 4

The undisturbed position of particles or fields when they are not vibrating.

Question 5

What is the Displacement?

Answer 5

The distance that a certain point in the medium has moved from its rest position.

Question 6

What is the Peak?

Answer 6

The number of waves passing a point each second. The highest point above the rest position.

Question 7

What is the Trough?

Answer 7

The lowest point below the rest position.

Question 8

What is the Amplitude?

Answer 8

The maximum displacement of a point of a wave from its rest position.

Question 9

What is the time period?

Answer 9

• The time taken for a full cycle of the wave.

- Usually measured from peak to peak, or trough to trough

Question 10

What is the difference between transverse and longitudinal waves?

Answer 10

• In longitudinal waves, the vibrations are parallel to the direction of wave travel.
• In transverse waves, the vibrations are at right angles to the direction of wave travel.

Question 11

List examples of longitudinal. (3)

Answer 11

• sound waves
• ultrasound waves
• seismic P-waves

Question 12

List examples of transverse waves. (5)

Answer 12

• ripples on the surface of water
• vibrations in a guitar string
• a Mexican wave in a sports stadium
• electromagnetic waves – eg light waves, microwaves, radio waves
• seismic S-waves

Question 13

How do you work out wave speed?(2)

Answer 13

Wave speed (m/s) = frequency (Hz) x wavelength (m)

Or

Wave speed = distance (m) / time (s)

Question 14

How do you measure the velocity of sound in air? (4)

Answer 14

1) Set up the oscilloscope so the detected waves each microphone are shown as separate waves.
2) Start with both microphones next to the speaker, then slowly move one away until the two waves are aligned on the display, but have moved exactly one wavelength apart.
3) Measure the distance between the microphones to find one wavelength.
4) Use the formula wave speed = frequency (look on signal generator) x wavelength

Question 15

How do you measure the speed of water ripples? (5)

Answer 15

1) Using a signal generator attached to the dipper of a ripple tank you can create water waves at a set frequency.
2) Dim the lights and turn on the strobe light - you’ll see wave pattern made by the shadows of the wave crests on the screen below the tank.
3) Alter the frequency of the strobe light until the wave pattern on the screen appears to ‘freeze’ and stop moving. This happens when the frequency of the waves and the strobe light are equal - the waves appear not to move because they are being lit at the same point in their cycle each time.
4) The distance between each shadow line is equal to one wavelength. Measure the distance between lines that are 10 wavelengths apart, then find the average wavelength
5) Use wave speed = frequency x wavelength to calculate the speed of the waves.

Question 16

What is reflection?

Answer 16

This is when the incoming ray is neither absorbed or transmitted, but instead is sent back away from the second material.

Question 17

How are waves refracted at the boundary in terms of change of direction and speed?

Answer 17

• When a wave crosses a boundary between materials, it changes speed.
• If the wave hits the boundary at an angle, the change of speed will cause a change in direction - refraction.
• If the wave hits the boundary along the normal, it will change speed but will not change direction.

Question 18

How do sound waves travel through solid?

Answer 18

• They cause particles in the solid to vibrate.
• These vibrations are passed through the surrounding medium as a series of compressions (waves squashed together) and rarefactions (spaces between wave).

Question 19

How does the human ear work? (6)

Answer 19

• Sound waves that reach your eardrum cause it to vibrate.
• These vibrations are passed on to tiny bones in your ear called ossicles, through the semicircular canals and to the cochlea which get sent to your brain and volumes, depending on their frequency and intensity.
• The cochlea turns these vibrations into electrical signals. The brain interprets the signals as sounds of different pitches and volumes, depending on their frequency and intensity.
• Human hearing is limited by the size and shape of our eardrum, and the structure of all the parts within the ear that vibrate to transmit the sound wave.

Question 20

What is ultrasound?

Answer 20

Sound with frequencies greater than 20,000 Hz

Question 21

What is infrasound?

Answer 21

Sound with frequencies less than 20 Hz

Question 22

Uses of ultrasound (3)

Answer 22

• Medical imaging (foetal scanning)
• Industrial image (finding flaws in materials) - if there is a flaw such as a crack in the object, the waves will be reflected sooner.
• Sonar for boats and submarines

Question 23

Uses of infrasound

Answer 23

• Track volcanic eruptions

- Communication between elephants and whales

Question 24

What are the two main seismic waves?

Answer 24

P-waves and S-waves

Question 25

Properties of P-waves (4)

Answer 25

• Longitudinal
• Can travel through solids and liquids
• Travel faster than S-waves
• Can travel through the Earth’s core

Question 26

Properties of S-waves (4)

Answer 26

• Transverse
• Can only travel through solids
• Slower than P-waves
• Can’t pass through liquid outer core

Question 27

What affects the way different substance reflect, absorb, transmit or refract?

Answer 27

Different substances reflect, refract, transmit or absorb waves in ways that vary with wavelength.

Question 28

An investigation to determine the suitability of apparatus to measure the speed, frequency and wavelength of waves in a fluid. (4)

Answer 28

1) Set up a ripple, motor, a plane wave generator and a stroboscope.
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 s marked with 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 and the wave positions marked with pencil marks on the paper.
4) Calculate the wave speed.

Question 29

What is absorption?

Answer 29

• This is when the wave transfers energy to the second material’s energy stores.
• Often, the energy is transferred to a thermal energy store leading to heating.

Question 30

What is transmission?

Answer 30

This is when the waves carries on travelling through the new material which often leads to refraction, this can be used on communication as well as in lenses of glasses and camera.

Question 31

What is refraction?

Answer 31

• Refraction is the change in direction of a wave at a interface (boundary) between two materials.
• All wave types can be refracted.

Question 32

How can you find the speed of waves in solids?

Answer 32

+By measuring the frequency of the sound waves produced when you hit the object [eg. a rod with a hammer]. - Hitting the rod causes waves to be produced along the rod - these waves make the rod vibrate and produce sound waves in the air around the rod.
1] Measure and record the length of a metal rod, eg. a brass rod.
2] Set up the apparatus with a clamp and a rod attached to elastic bands [also have a hammer and microphone]
3] Tap the road with the hammer - write down the peak frequency displayed by the computer.
4] Repeat three times to get an average peak frequency
5] Calculate speed of the wave using wave speed = frequency x wavelength [where wavelength is = twice the length of the rod]