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Wave Dynamics Chapter 1. Waves

Recommended Article : 【Physics】 Physics Table of Contents


1. Definition of Waves

2. Types of Waves

3. Representation of Waves

4. Wave Functions



1. Definition of Waves

⑴ Wave : Energy transmission without the movement of a medium (substance)

① Standing waves can be seen as exceptions.

② Thermal conduction fits the definition, but it is not a wave.

⑵ (Distinct Concept) Vibration : Periodic occurrence at a specific location. Waves propagate to different places.

⑶ (Distinct Concept) Pulse : A disturbance that passes through only once.

⑷ Wave Source : The point where a wave originates.

⑸ Medium : Substance that transmits waves.



2. Types of Waves

⑴ Classification based on the presence of a medium

① Elastic waves : Mechanical waves. Require a medium.

○ External force causes deformation → Deformation induces stress → Stress causes deformation of adjacent molecules.

○ Examples: Water waves, sound waves, seismic waves.

② Particle waves : Do not require a medium.

○ Electromagnetic waves (light) : Gauge invariance, involvement of scalar, vector, tensor.

○ Gravitational waves : Gauge invariance, involvement of scalar, vector, tensor.

○ Matter waves : Probability amplitude density, Fermions → Spinors (approximately in the context of relativity)

⑵ Classification based on the direction of vibration

① Longitudinal waves (e.g., P-waves in earthquakes, sound waves, longitudinal waves in slinky)

○ Definition: Waves in which the direction of propagation aligns with the direction of vibration of the medium.

○ Vibration direction and energy transfer direction are parallel.

○ Involvement of compressional forces.

② Transverse waves (e.g., S-waves in earthquakes, electromagnetic waves, transverse waves in slinky)

○ Definition: Waves in which the direction of propagation is perpendicular to the direction of vibration of the medium.

○ Vibration direction and energy transfer direction are perpendicular.

○ Involvement of shear forces.

③ While water waves are classified as transverse waves, they also have a longitudinal component due to the circular motion of water particles.

⑶ Classification based on waveform

① Wavefront : A line or surface connecting points of equal phase, like a crest or trough.

② Plane waves : Wavefront forms a straight line.

③ Spherical waves : Wavefront forms a circle.



3. Representation of Waves

⑴ Crest : The point of maximum displacement in a wave.

⑵ Trough : The point of minimum displacement in a wave.

⑶ Amplitude : Maximum displacement of the medium during vibration.

⑷ Wavelength : Distance between two neighboring points in phase, such as crest to crest or trough to trough.

⑸ Period : Time T for a repeating pattern of a wave.

⑹ Frequency : Number of pattern repetitions per unit time, reciprocal of period (f).

⑺ Wave Velocity

⑻ Antinode : Point of maximum amplitude.

⑼ Node : Point of minimum amplitude.

⑽ Example

Figure 1. Determination of wavelength, frequency, and velocity of a wave example

① (a) represents the situation at t = 0, and (b) represents the situation at t = 1 s.

② From (a), we can determine that λ = 4 m.

③ From (a) and (b), we can determine that v = 1 m/s.

④ Frequency can be determined using the formula.



4. Wave Functions

⑴ Definition : Language used to describe progressive waves and periodic waves.

⑵ General Expressions

① Wavenumber :

② Angular frequency :

⑶ Wave Velocity : Speed at which a wave propagates.

① Phase velocity : Tracing points with the same phase.

○ ω/k > 0 : vp > 0, indicating forward propagation.

○ ω/k < 0 : vp < 0, indicating backward propagation.

Example 1. String Waves

Proof 1. Dimensional analysis

○ [μ] = ML-1

○ [T] = MLT-2

○ [v] = LT-1

○ [(T / μ)0.5 ] = [v] ⇔ v ∝ (T / μ)0.5

Proof 2. Derived from the wave equation

Figure 2. Derivation of velocity of string waves through the wave equation Wave Equation

Proof 3. Derived through thought experiment : Utilizing the centripetal force formula

Figure 3. Derivation of velocity using the centripetal force formula

Example 2. Slinky Waves : Faster with higher spring constant

Example 3. Speed of Water Waves : Detailed explanation in Oceanography

○ General formula for wave speed

○ Speed of deep-water waves

○ Speed of shallow-water waves

Example 4. Speed of Sound

○ Solid > Liquid > Gas, in order of increasing speed

○ Faster at higher air temperatures

Example 5. Speed of Electromagnetic Waves : Same in vacuum for all electromagnetic waves

⑷ Displacement-Related Physical Quantities

① Displacement velocity

② Displacement acceleration

③ Curvature

④ Others

⑸ Energy

① Tension and linear density remain constant when amplitude is small.

② Kinetic energy : Utilized by

③ Potential energy : Utilized by

④ Total energy

○ Elastic waves’ total energy is proportional to the square of frequency.

○ Quantum particles’ total energy is proportional to frequency, i.e., E = hf.

○ Photons, as quantum particles, have total energy proportional to frequency.

⑤ Energy transmission rate



Input : 2019.05.03 17:14

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