Physics Formulae/Waves Formulae

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Lead Article: Tables of Physics Formulae


This article is a summary of the laws, principles, defining quantities, and useful formulae in the analysis of Waves.


General Fundamental Quantites[edit]

For transverse directions, the remaining cartesian unit vectors i and j can be used.


Quantity (Common Name/s) (Common) Symbol/s SI Units Dimension
Number of Wave Cycles dimensionless dimensionless
(Transverse) Displacement m [L]
(Transverse) Displacement Amplitude m [L]
(Transverse) Velocity Amplitude m s-1 [L][T]-1
(Transverse) Acceleration Amplitude m s-2 [L][T]-2
(Longnitudinal) Displacement m [L]
Period s [T]
Wavelength m [L]
Phase Angle rad dimensionless

General Derived Quantites[edit]

The most general definition of (instantaneous) frequency is:



For a monochromatic (one frequency) waveform the change reduces to the linear gradient:



but common pratice is to set N = 1 cycle, then setting t = T = time period for 1 cycle gives the more useful definition:



Quantity (Common Name/s) (Common) Symbol/s Defining Equation SI Units Dimension
(Transverse) Velocity m s-1 [L][T]-1
(Transverse) Acceleration m s-2 [L][T]-2
Path Length Difference m [L]
(Longnitudinal) Velocity m s-1 [L][T]-1
Frequency Hz = s-1 [T]-1
Angular Frequency/ Pulsatance Hz = s-1 [T]-1
Time Delay, Time Lag/Lead s [T]
Scalar Wavenumber Two definitions are used:

In the formalism which follows, only the first

definition is used.

m-1 [L]-1
Vector Wavenumber Again two definitions are possible:

In the formalism which follows, only the first

definition is used.

m-1 [L]-1
Phase Differance rad dimensionless
Phase (No standard symbol, is used

only here for clarity of equivalances )

rad dimensionless
Wave Energy E J [M] [L]2 [T]-2
Wave Power P W = J s-1 [M] [L]2 [T]-3
Wave Intensity I W m-2 [M] [T]-3
Wave Intensity (per unit Solid Angle) I


Often reduces to

W m-2 sr-1 [M] [T]-3

Phase

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point. Physically;

wave popagation in +x direction


wave popagation in -x direction

Phase angle can lag if:

or lead if:


Relation between quantities of space, time, and angle analogues used to describe the phase is summarized simply:


Standing Waves[edit]

Harmonic Number
Harmonic Series

Propagating Waves[edit]

Wave Equation


Any wavefunction of the form



satisfies the hyperbolic PDE:



Principle of Superposition for Waves



General Mechanical Wave Results[edit]

Average Wave Power
Intensity


Sound Waves[edit]

Sound Intensity and Level


Quantity (Common Name/s) (Common) Symbol/s
Sound Level


Sound Beats and Standing Waves


pipe, two open ends
Pipe, one open end for n odd
Acoustic Beat Frequency


Sonic Doppler Effect


Sonic Doppler Effect

Mach Cone Angle

(Supersonic Shockwave, Sonic boom)


Sound Wavefunctions


Acoustic Pressure Amplitude
Sound Displacement Function
Sound pressure-variation function

Superposition, Interferance/Diffraction[edit]

Resonance
Phase and Interference


Constructive Interference


Destructive Interference


n is any integer;

Phase Velocities in Various Media[edit]

The general equation for the phase velocity of any wave is (equivalent to the simple "speed-distance-time" relation, using wave quantities):



The general equation for the group velocity of any wave is:



A common misconception occurs between phase velocity and group velocity (analogous to centres of mass and gravity). They happen to be equal in non-dispersive media.

In dispersive media the phase velocity is not necessarily the same as the group velocity. The phase velocity varies with frequency.

The phase velocity is the rate at which the phase of the wave propagates in space.

The group velocity is the rate at which the wave envelope, i.e. the changes in amplitude, propagates. The wave envelope is the profile of the wave amplitudes; all transverse displacements are bound by the envelope profile.

Intuitively the wave envelope is the "global profile" of the wave, which "contains" changing "local profiles inside the global profile". Each propagates at generally different speeds determined by the important function below called the Dispersion Relation , given in explicit form and implicit form respectively.




The use of ω(k) for explicit form is standard, since the phase velocity ω/k and the group velocity dω/dk usually have convenient representations by this function.


For more specific media through which waves propagate, phase velocities are tabulated below. All cases are idealized, and the media are non-dispersive, so the group and phase velocity are equal.


Taut String
Solid Rods
Fluids
Gases


The generalization for these formulae is for any type of stress or pressure p, volume mass density ρ, tension force F, linear mass density μ for a given medium:


Pulsatances of Common Osscilators[edit]

Pulatances (angular frequencies) for simple osscilating systems, the linear and angular Simple Harmonic Oscillator (SHO) and Damped Harmonic Oscillator (DHO) are summarized in the table below. They are often useful shortcuts in calculations.


= Spring constant (not wavenumber).


Linear
Linear DHO
Angular SHO
Low Amplitude Simple Pendulum
Low Amplitude Physical Pendulum

Sinusiodal Waves[edit]

Equation of a Sinusiodal Wave is



Recall that wave propagation is in direction for .


Sinusiodal waves are important since any waveform can be created by applying the principle of superposition to sinusoidal waves of varying frequencies, amplitudes and phases. The physical concept is easily manipulated by application of Fourier Transforms.

Wave Energy[edit]

Quantity (Common Name/s) (Common) Symbol/s
potential harmonic energy
kinetic harmonic energy
total harmonic energy
damped mechanical energy


General Wavefunctions[edit]

Sinusiodal Solutions to the Wave Equation[edit]

The following may be duduced by applying the principle of superposition to two sinusiodal waves, using trigonometric identities. Most often the angle addition and sum-to-product formulae are useful; in more advanced work complex numbers and Fourier series and transforms are often used.

Wavefunction Nomenclature Superposition Resultant
Standing Wave

Beats


Coherant Interferance


Note: When adding two wavefunctions togther the following trigonometric identity proves very useful:


Non-Solutions to the Wave Equation[edit]

Exponentially Damped Waveform
Solitary Wave

Common Waveforms

Triangular
Square
Saw-Tooth

External links[edit]

{{Physics resources}}