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Theme C — Wave Behaviour

C.2 Wave Model — IB Physics HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 11 of 37 1 quiz

📚 IB Physics HL Hub / Topical Revision
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pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Lesson summary

C.2 Wave Model — IB PHYSICS HL notes: Define the properties of travelling waves, distinguish transverse from longitudinal and mechanical from electromagnetic waves, and use v = fλ.Travelling wave — oscillations travel away from the source, transferring energy and not matterPartic…

  • Displacement
  • Wavelength λ
  • Amplitude A
  • Frequency f
  • What the symbols mean
  • Definition

Part 1 of 9Wave properties

Travelling wave
  • Travelling wave oscillations travel away from the source, transferring energy and not matter
  • Particles oscillate about fixed points in mechanical waves
Displacement
  • Displacement the distance of a point on the wave from its equilibrium position, a vector in metres
Wavelength λ
  • Wavelength λ the distance between the same point on two consecutive waves, such as two crests
  • The shortest distance between points in phase
Amplitude A
  • Amplitude A the maximum displacement from the equilibrium position in metres
Period T
  • Period T the time for one complete oscillation to pass a fixed point in seconds
Frequency f
  • Frequency f the number of complete oscillations passing a fixed point per second in hertz
  • f = 1/T
    What the symbols mean: T = period, the time for one complete wave (s) · f = frequency (Hz)

Part 2 of 9Wave speed

Definition
  • Definition wave speed v is the distance the wave travels per unit time in m s−1
Wave equation
  • Wave equation v = fλ = λ/T
    What the symbols mean: v = wave speed (m s−1) · f = frequency (Hz) · λ = wavelength (m)
  • In one period the wave advances one wavelength
Constant speed
  • Constant speed a longer wavelength means a lower frequency and a shorter wavelength a higher frequency
Source and medium
  • Source and medium the frequency is set by the source and the speed by the medium

Part 3 of 9Reading wave graphs

Reading wave graphs
Displacement–distance graph
  • Displacement–distance graph gives the wavelength and the amplitude
Displacement–time graph
  • Displacement–time graph gives the period and the amplitude
Units
  • Units convert cm, ms, μs and nm to SI units before using the equations

Part 4 of 9Transverse waves

Transverse waves
Definition
  • Definition the oscillations are perpendicular to the direction of motion and energy transfer
Features
  • Features peaks (crests) and troughs, with each particle vibrating up and down
Examples
  • Examples electromagnetic waves, waves on a guitar string and water surface waves
Particle motion
  • Particle motion a particle moves upwards when a crest is approaching it

Part 5 of 9Longitudinal waves

Definition
  • Definition the oscillations are parallel to the direction of motion and energy transfer
Compression
  • Compression a region of high pressure where particles are closer together
Rarefaction
  • Rarefaction a region of low pressure where particles are further apart
Examples
  • Examples sound and ultrasound
Energy transfer
  • Energy transfer each compression makes nearby particles vibrate with more energy, producing the next compression further along
Medium
  • Medium they are mechanical waves and cannot travel through a vacuum

Part 6 of 9Sound waves

1Source
  • Source an oscillating source changes the density of the surrounding medium and sends out compressions and rarefactions
2Audible range
  • Audible range humans hear frequencies from 20 Hz to 20 kHz
3Pitch
  • Pitch related to frequency, so a high pitch is a high frequency and short wavelength
4Volume
  • Volume related to amplitude, so a large amplitude is loud
5Speed in air
  • Speed in air about 340 m s−1 at room temperature and greater at higher temperature because the particles have more kinetic energy
6Speed in different states
  • Speed in different states fastest in solids because closely packed particles pass on oscillations quickly, slowest in gases

Part 7 of 9Electromagnetic waves

Electromagnetic waves
Nature
  • Nature combined oscillation of electric and magnetic fields perpendicular to each other and to the direction of energy transfer
Transverse
  • Transverse they can travel through a vacuum
Speed
  • Speed all travel at c = 3.00 × 108 m s−1 in a vacuum
Spectrum
  • Spectrum continuous in frequency
  • Shorter wavelength means higher frequency and greater energy
Order of decreasing wavelength
  • Order of decreasing wavelength radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays
Visible light
  • Visible light wavelengths from about 700 nm (red) to 400 nm (violet)
Frequency
  • Frequency f = c/λ so the longest wavelength in a band gives the lowest frequency

Part 8 of 9Mechanical and electromagnetic waves

FeatureMechanicalElectromagnetic
MediumRequiredNot required
TypeTransverse or longitudinalTransverse only
SourceOscillating particles in a mediumOscillating charged particles
SpeedDepends on the medium, much less than cc in a vacuum
ExamplesSound, water waves, seismic wavesRadio waves, UV rays, X-rays

Part 9 of 9More from the specification

More from the specification
Wave properties
  • Wavelength, frequency, amplitude and period are measured from displacement graphs, and v = fλ.
Transverse and longitudinal
  • Transverse oscillations are perpendicular to the direction of propagate;
    • sound waves are longitudinal with compressions and rarefactions.
Wavefronts and rays
  • Rays show the direction of travel and are perpendicular to wavefronts.
Intensity
  • Proportional to amplitude squared and to power per unit area.
Sounds
  • Sound requires a medium to propagate, unlike electromagnetic waves.

Key terms

TermMeaning
WavelengthThe shortest distance between two points on a wave that are in phase.
Transverse waveA wave in which the oscillations are perpendicular to the direction of energy transfer.
Longitudinal waveA wave in which the oscillations are parallel to the direction of energy transfer.
CompressionA region of a longitudinal wave where the pressure is above normal.
RarefactionA region of a longitudinal wave where the pressure is below normal.
Mechanical waveA wave that needs a material medium.
Electromagnetic waveA transverse wave of oscillating electric and magnetic fields that can travel through a vacuum.

Avoid these mistakes

1
Saying all mechanical waves are longitudinal — water and seismic waves can be transverse.
2
Saying the wave carries the particles along — particles only oscillate about fixed positions.
3
Forgetting to convert units such as ms and nm before calculating.
4
Mixing up period and wavelength on graphs — the x-axis shows time for period and distance for wavelength.
5
Saying sound travels in a vacuum — it needs a medium.

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