Flashcards in 4.4.1 - Wave Motion Deck (41):
What are waves?
A 'disturbance' that transfers energy from one place to another without any net transfer of matter.
What is the difference between a mechanical and an electromagnetic wave?
Mechanical: Waves which need a substance for their transmission.
Electromagnetic: Waves which do not need a substance for their transmission.
Define the term 'progressive wave'.
Waves that transfer energy away from a source.
Describe the events that take place in a progressive wave.
- 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.
How do particles oscillate in a progressive wave?
All particles oscillate vertically, but do not move forwards or backwards.
Define the term 'longitudinal wave'.
A wave in which the oscillations are parallel to the direction of wave propagation.
What two features form in a longitudinal wave, due to changes in pressure?
Give two examples of longitudinal waves.
- Sound Waves
- Primary Seismic Waves (P Waves)
Define the term 'transverse wave'.
A wave in which the oscillations are perpendicular to the direction of wave propagation.
Give three examples of transverse waves.
- Electromagnetic Waves
- Secondary Seismic Waves (S Waves)
- Water Waves
What is the purpose of an oscilloscope?
Displays a voltage-time signal and can be used as a voltmeter to display and measure the output from a microphone/signal generator.
What does the time-varying voltage trace on an oscilloscope represent?
Displacement against time for the longitudinal sound wave.
What do you need to know to determine the frequency of a wave that is displayed on an oscilloscope?
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).
How do you calculate time period?
period (s) = distance between peaks (cm) * time base setting (ms cm^-1)
What does each horizontal distance on the oscilloscope screen represent?
A unit of time.
What does the time base control on the oscilloscope vary?
The seconds or milliseconds per division.
How can you reduce the uncertainty in the frequency measurement?
By altering the time base such that one full wave has the widest possible range in the x direction.
When is it useful for the time base to be turned off (and for the spot to no longer move across the screen)?
When you are just looking at the intensity of the sound waves.
What does each vertical division on the oscilloscope represent, and how is it controlled?
- A unit of voltage
- The sensitivity control (Y-gain) varies the volts per division
How do you calculate frequency?
f = 1/T
frequency (Hz) = 1 / period (s)
What is the wave equation?
v = λ / T
wave speed (ms^-1) = wavelength (m) / period (s)
What is the wave equation when substituting f for 1 / T?
v = f * λ
velocity (ms^-1) = frequency (Hz) * wavelength (m)
What is reflection?
When waves rebound from a barrier, changing direction, but remaining in the same medium.
Which waves can be reflected?
All wave types.
What are wavefronts?
Lines of constant phase (such as crests). They are drawn for each successive wave, with the distance between wavefronts representing wavelength.
Rays are drawn at what angle to wavefronts?
What does not change after a wave has been reflected.?
What is refraction?
When waves change direction when they travel from one medium to another due to a difference in the wave speed in each medium.
Which two things can be observed when refraction occurs?
- The wave will change its speed
- There may be a change in direction
When does refraction occur?
When a wave moves from one material to another, and each material has a different density.
What is diffraction?
When a wave spreads out after passing around an obstacle or through a gap.
When does the greatest diffraction take place?
When the wavelength of the wave being diffracted is the same size as the gap they are travelling through.
What is interference?
The addition of two or more waves (superposition) that results in a new wave pattern.
How can refraction and reflection be investigated in a ripple tank?
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.
How can diffraction and interference be studied in a ripple tank?
What is intensity (of a progressive wave)?
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.
How is intensity calculated?
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)
What happens to amplitude of a wave as it spreads out from a source?
What is the energy of a wave proportional to?
The square of its amplitude.
What is the relationship between intensity and amplitude of a wave?
I ∝ A^2