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AS Physics > Waves (topic 4) > Flashcards

Waves (topic 4) Flashcards

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1
Q

Mechanical waves (definition)

A

Waves that are produced by vibrations though a substance

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2
Q

Electromagnetic waves (definition)

A

Oscillating electric and magnetic fields that progress through space without the need for a substance

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3
Q

How do mechanical waves travel?

A

Particles in the medium vibrate in a way which makes nearby particles also vibrate

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4
Q

How do electromagnetic waves travel?

A

The vibrating electric field creates a vibrating magnetic field, which generates a vibrating electric field further on, and this continues.

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5
Q

Longitudinal waves (definition)

A

Waves where the direction of vibration of the particles is PARALLEL to the direction of wave travel

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6
Q

3 examples of longitudinal waves

A

Sound waves
Primary seismic waves
Compression waves (on a slinky)

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7
Q

How do longitudinal waves move through a slinky? (2)

A

“Forward” movement creates a compression wave
“Reverse” movement creates a corresponding rarefaction (expansion) wave

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8
Q

Transverse wave (definition)

A

Waves where the direction of travel is perpendicular to the direction of wave travel

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9
Q

3 examples of transverse waves

A

Electromagnetic waves
Secondary seismic waves
Waves on a string/wire

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10
Q

How do transverse waves move through a rope?

A

The sideways movements (left and right) travel along the rope, pulling each unaffected part sideways as the part before it moves sideways.

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11
Q

How does a plane-polarised transverse wave behave?

A

Vibrations remain in one plane only

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12
Q

How does a polaroid filter work?

A

It only allows though light which vibrates according to the alignment of its molecules

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13
Q

What happens when two polaroid filters are “crossed”?

A

The alignment of molecules in one filter is 90* to the alignment of molecules in the other filter, so no light can pass through both filters

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14
Q

What happens if two polaroid filters are held together and one is rotated?

A

Light intensity through both decreases as the molecules within the filter get closer to 90* alignment, then increases again as they pass 90* and go towards 180* where maximum light intensity is allowed.

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15
Q

How do polaroid sunglasses work?

A

Reduce the glare of light reflected by water/glass by polarising it

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16
Q

Which plane is polarised of an electromagnetic wave?

A

The plane in which the electric field oscillates

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17
Q

Displacement (definition in terms of waves)

A

Distance of a vibrating particle from its equilibrium position

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18
Q

Amplitude (definition)

A

The maximum displacement of a vibrating particle from its equilibrium position

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19
Q

Wavelength (definition)

A

Distance between the same points on two adjacent waves

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20
Q

Wave cycle (definition)

A

From maximum displacement to next maximum displacement

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21
Q

Wave period (definition)

A

Time for one complete wave to pass a fixed point

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22
Q

Wave frequency (definition)

A

Number of waves passing a point per second (Hz)

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23
Q

Period of wave equation

A

period = 1/f

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24
Q

Wavespeed equation

A

c = distance travelled in one cycle/time taken for one cycle

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25
Q

Wave phase (definition)

A

Fraction of a cycle it has completed since the start of a cycle

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26
Q

Phase difference (definition)

A

Fraction of a cycle between the vibrations of two particles vibrating at the same frequency

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27
Q

Convert between degrees and radians

A

1 cycle = 360* = 2pi radians

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28
Q

Calculate phase difference in radians

A

radians = 2pi(d) / wavelength

where d = distance along a wave

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29
Q

What can all waves do? (3)

A

Reflect
Refract
Diffract

30
Q

Wavefronts (definition)

A

Waves observed in a ripple tank

31
Q

What happens when straight waves are directed at a certain angle to a hard flat surface?

A

The reflected wave travels away at the same angle to the normal as the incident ray travelled towards the surface

32
Q

What happens when waves pass a boundary between two mediums?

A

Wave speed changes, which changes the direction of the wave (refraction)

33
Q

What happens when waves reach an obstacle/gap between two obstacles?

A

They diffract, spreading out

34
Q

What can affect diffraction, and how? (2)

A

Narrower gap = waves spread out more

Longer wavelength = waves spread out more

35
Q

How does the size of a satellite dish affect its use?

A

Bigger dishes can receive stronger signal as more radio waves are reflected onto the arial. However, they must be aligned more carefully because they reflect the wave to a smaller focus

36
Q

What is the principle of superposition?

A

When two waves meet, the total displacement is equal to the sum of individual displacements at that point

37
Q

Superposition - points of note

A

Supercrest - two crests reinforce each other

Supertrough - two troughs reinforce each other

Minimum - two waves cancel out but they have different amplitudes

38
Q

Stationary wave formation

A

Two progressive waves on the same medium (e.g. string) cancel each other out at each point where they are 180* out of phase (nodes)

39
Q

Interference (definition)

A

Cancellation and reinforcement happen at fixed positions because waves are passing through each other at a constant frequency and phase difference

40
Q

Why do coherent sources of waves produce an interference pattern when they overlap?

A

They vibrate at the same frequency with a constant phase difference

41
Q

What would happen is two sources of waves with random phase differences were passing each other?

A

Points of cancellation and reinforcement would move at random, so there would be no interference pattern

42
Q

Method for microwave test of superposition (4)

A

1) receiver in the path of a transmitter of 3cm microwaves. Prove that signal strength gets weaker as receiver moved away

2) Place a metal plate between transmitter and receiver to show no waves can pass through

3) Use two plates to make a narrow slit and show that receiver can detect diffracted waves. Wider slit = less diffraction

4) Use a third plate to create two diffraction points, then use receiver to show cancellation and reinforcement patterns.

43
Q

How is a stationary wave produced by plucking a string?

A

Making the middle vibrate creates two progressive waves which travel towards each end and reflect back

44
Q

What is the simplest stationary wave pattern on a string? (2)

A

First harmonic/fundamental node of vibration

45
Q

Node (definition)

A

Point of cancellation on a stationary wave

46
Q

Antinode (definition)

A

Supercrest (maximum amplitude) on a stationary wave

47
Q

What must the distance between the nodes be for the first harmonic to appear?

A

wavelength/2

48
Q

How do stationary waves form their pattern?

A

When the progressive waves are in phase, they reinforce each other to produce the maximum displacement. 1/4 phase later, the waves now cancel each other out entirely. This repeats, creating the “loops” between nodes.

49
Q

What facts are always true for a stationary wave pattern? (2)

A

The amplitude of a vibrating particle varies from position 0 to maximum amplitude at an antinode

The phase difference between two particles is 0 is they are separated by an even number of nodes, or 180* if they are separated by an odd number of nodes.

50
Q

How are stationary waves and progressive waves different in frequency?

A

S - all particles APART FROM NODES vibrate at the same frequency

P - all particles vibrate at the same frequency

51
Q

How are stationary waves and progressive waves different in amplitude?

A

S - amplitude varies from zero at the nodes to maximum at the antinodes

P - amplitude is the same for all particles

52
Q

How are stationary waves and progressive waves different in phase difference?

A

S - phase difference = m(pi), where m is the number of nodes between the two particles

P - phase difference = 2(pi)d/wavelegnth

53
Q

How does sound in a pipe create stationary waves?

A

Sound resonates at certain frequencies in an air-filled tube/pipe. In a pipe closed at one end, resonant frequencies occur when there is an antinode at the open end and a node at the closed end

54
Q

How can microwaves be used to test for stationary waves?

A

Microwaves from transmitter directed at a metal plate and reflect. Detector moved between the two to find nodes.

55
Q

Why do stationary wave patterns occur at quantized frequencies?

A

There must be a node at either end, so the “loops” occur at a ratio with the fundamental frequency (e.g. f, 2f, 3f, etc)

56
Q

How is a stationary wave created by a vibrator?

A

One progressive wave (crest) reflects at the boundary and returns as a trough, then reflects again as a crest, combining with the next crest the vibrator produces.

57
Q

What condition must be met for a vibrator to create a stationary wave?

A

The time taken for the wave to travel to the boundary and back must be equal to the time taken for a whole number of cycles of the vibrator

58
Q

Equation for length of a vibrating string to form a stationary wave

A

L = (n x wavelengths)/2
where n is the number of “loops”

59
Q

What can you do to a string to increase f, and what does this do?

A

Raising the tension OR shortening the length of the string
will INCREASE the pitch as f increases

60
Q

What is mu?

A

Mass per unit length m/L

61
Q

When is a guitar considered “tuned”?

A

When the tuning fork (vibrates at a single frequency) has the same frequency as the first harmonic of the string

62
Q

How does an oscilloscope work? (3)

A
  • There’s an electron gun at one end of the electron tube, surrounded by necessary circuits.
  • It fires electrons in a beam towards a fluorescent screen at the other end.
  • Light is emitted from the spot on the screen where the beam hits.
63
Q

What affects where the spot of light is on an oscilloscope screen?

A

There are two deflecting plates which change the position of the spot using the p.d. applied across them. The displacement of the spot is proportional to the supplied p.d.

64
Q

How do the X-plates of an oscilloscope work? (2)

A
  • The X-plates are connected the the time base circuit, which makes the spot move at a constant speed left to right across the screen, then back again (faster because it is the “reset” not tracing the wave)
65
Q

How do the Y-plates of an oscilloscope work?

A

The p.d. to be displayed is connected to the Y-plates via the Y-input, making the spot move up and down relative to the p.d. applied as it moves across the screen. This traces the waveform at the Y-input.

66
Q

How is the x-scale of an oscilloscope calibrated?

A

The speed of the spot moving left to right across the screen is constant and proportional to the p.d. applied to them. Because distance is constant and we control the speed, the time is what is represented on the x-scale, usually in milliseconds or microseconds per cm.

67
Q

How is the y-scale of an oscilloscope calibrated?

A

Because the vertical displacement is proportional to the p.d. through the y-input, it can be calibrated in volts per cm.

68
Q

Other names for y-scale on oscilloscope (2)

A

Y-sensitivity
Y-gain

69
Q

How to measure peak p.d. from an oscilloscope (3)

A
  • measure waveform height peak to trough
  • divide this value by 2 to find amplitude of the waveform
  • multiply by the Y-gain calibration to find the true peak p.d.
70
Q

How to measure frequency from an oscilloscope (3)

A
  • Measure the time period for one full cycle of the waveform using the x-scale
  • multiply by the time base control to find the true time period
  • f = 1/t
71
Q

Method for measuring ultrasound speed using an oscilloscope (4)

A
  • use the time base circuit of the oscilloscope to trigger an ultrasonic transmitter, then have the ultrasonic receiver connected to the Y-input
  • measure real distance between transmitter and receiver
  • measure horizontal distance from the leasing edge of the pulse (from y-gain) to the start of the spot’s sweep across the screen
  • use x-scale to find the time taken for the ultrasound to travel from transmitter to receiver, then use s = d/t

AS Physics (13 decks)

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