4.4.1 - Wave Motion

Question 1

What are waves?

Answer 1

A ‘disturbance’ that transfers energy from one place to another without any net transfer of matter.

Question 2

What is the difference between a mechanical and an electromagnetic wave?

Answer 2

Mechanical: Waves which need a substance for their transmission.

Electromagnetic: Waves which do not need a substance for their transmission.

Question 3

Define the term ‘progressive wave’.

Answer 3

Waves that transfer energy away from a source.

Question 4

Describe the events that take place in a progressive wave.

Answer 4

• Wave disturbance is initiated at a particular point.
• This sets the particles of the substance at that point, into vibration.
• This then causes neighbouring particles to vibrate in the same way and so the wave progresses through the substance.

Question 5

How do particles oscillate in a progressive wave?

Answer 5

All particles oscillate vertically, but do not move forwards or backwards.

Question 6

Define the term ‘longitudinal wave’.

Answer 6

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

Question 7

What two features form in a longitudinal wave, due to changes in pressure?

Answer 7

• Compression

- Rarefaction

Question 8

Give two examples of longitudinal waves.

Answer 8

• Sound Waves

- Primary Seismic Waves (P Waves)

Question 9

Define the term ‘transverse wave’.

Answer 9

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

Question 10

Give three examples of transverse waves.

Answer 10

• Electromagnetic Waves
• Secondary Seismic Waves (S Waves)
• Water Waves

Question 11

What is the purpose of an oscilloscope?

Answer 11

Displays a voltage-time signal and can be used as a voltmeter to display and measure the output from a microphone/signal generator.

Question 12

What does the time-varying voltage trace on an oscilloscope represent?

Answer 12

Displacement against time for the longitudinal sound wave.

Question 13

What do you need to know to determine the frequency of a wave that is displayed on an oscilloscope?

Answer 13

The setting of the time base on the oscilloscope, (this is the time taken for the luminous dot produced by the cathode ray tube to move a horizontal distance of 1cm across the oscilloscope screen).

Question 14

How do you calculate time period?

Answer 14

period (s) = distance between peaks (cm) * time base setting (ms cm^-1)

Question 15

What does each horizontal distance on the oscilloscope screen represent?

Answer 15

A unit of time.

Question 16

What does the time base control on the oscilloscope vary?

Answer 16

The seconds or milliseconds per division.

Question 17

How can you reduce the uncertainty in the frequency measurement?

Answer 17

By altering the time base such that one full wave has the widest possible range in the x direction.

Question 18

When is it useful for the time base to be turned off (and for the spot to no longer move across the screen)?

Answer 18

When you are just looking at the intensity of the sound waves.

Question 19

What does each vertical division on the oscilloscope represent, and how is it controlled?

Answer 19

• A unit of voltage

- The sensitivity control (Y-gain) varies the volts per division

Question 20

How do you calculate frequency?

Answer 20

f = 1/T

frequency (Hz) = 1 / period (s)

Question 21

What is the wave equation?

Answer 21

v = λ / T

wave speed (ms^-1) = wavelength (m) / period (s)

Question 22

What is the wave equation when substituting f for 1 / T?

Answer 22

v = f * λ

velocity (ms^-1) = frequency (Hz) * wavelength (m)

Question 23

What is reflection?

Answer 23

When waves rebound from a barrier, changing direction, but remaining in the same medium.

Question 24

Which waves can be reflected?

Answer 24

All wave types.

Question 25

What are wavefronts?

Answer 25

Lines of constant phase (such as crests). They are drawn for each successive wave, with the distance between wavefronts representing wavelength.

Question 26

Rays are drawn at what angle to wavefronts?

Answer 26

90 degrees.

Question 27

What does not change after a wave has been reflected.?

Answer 27

Wavelength.

Question 28

What is refraction?

Answer 28

When waves change direction when they travel from one medium to another due to a difference in the wave speed in each medium.

Question 29

Which two things can be observed when refraction occurs?

Answer 29

• The wave will change its speed

- There may be a change in direction

Question 30

When does refraction occur?

Answer 30

When a wave moves from one material to another, and each material has a different density.

Question 31

What is diffraction?

Answer 31

When a wave spreads out after passing around an obstacle or through a gap.

Question 32

When does the greatest diffraction take place?

Answer 32

When the wavelength of the wave being diffracted is the same size as the gap they are travelling through.

Question 33

What is interference?

Answer 33

The addition of two or more waves (superposition) that results in a new wave pattern.

Question 34

How can refraction and reflection be investigated in a ripple tank?

Answer 34

1) Motorised straight-edged bar produces plane (straight) waves while a smaller dipper produces circular waves.
2) When a light is shone from above through the waves produced, the bright bands/curves of light seen on the screen below the tank show the wave crests - this makes it possible to measure the wavelength of the water waves, and investigate the angles of reflection and refraction.
3) Reflection at plane and curved surfaces can be investigated, and the angles of incidence and reflection measured with respect to the normal.

4) - A glass sheet is used to decrease the water depth and so produce a region with a different wave speed.
- The water level can also be adjusted.

5) - If the separation of the wavefronts decreases this shows they are travelling more slowly.
- If the wavefronts are at a non-zero angle when they cross the barrier, the waves also change direction.

Question 35

How can diffraction and interference be studied in a ripple tank?

Answer 35

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Question 36

What is intensity (of a progressive wave)?

Answer 36

The rate at which energy is transferred from one location to another as the wave travels through space, perpendicular to the direction of wave travel.

Question 37

How is intensity calculated?

Answer 37

I = P / A

intensity (W m^-2) = power output of the source (W or J s^-1) / area over which the radiation falls (m^2)

Question 38

What happens to amplitude of a wave as it spreads out from a source?

Answer 38

It decreases.

Question 39

What is the energy of a wave proportional to?

Answer 39

The square of its amplitude.

Question 40

What is the relationship between intensity and amplitude of a wave?

Answer 40

I ∝ A^2

Question 41

Describe two differences between transverse waves and longitudinal waves.

Answer 41

Transverse Waves

• Particles in the medium vibrate in directions perpendicular to the direction of travel of the wave.
• Crest and trough are formed in medium.

Longitudinal Waves

• Particles in the medium vibrate in directions parallel to the direction of travel of the wave.
• Compression and rarefaction occur in the medium.