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Waves - Transverse and Longitudinal Waves

Grade 11CBSEPhysics

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

A wave is a disturbance that travels through a medium, transporting energy and momentum without the actual physical transfer of matter.

In a Transverse Wave, the particles of the medium oscillate perpendicular to the direction of wave propagation. These waves consist of 'crests' (highest points) and 'troughs' (lowest points). Examples include waves on a plucked string or electromagnetic waves.

In a Longitudinal Wave, the particles of the medium oscillate parallel to the direction of wave propagation. These waves consist of 'compressions' (regions of high pressure/density) and 'rarefactions' (regions of low pressure/density). Examples include sound waves in air.

Wavelength (λ\lambda) is the distance between two consecutive points in the same phase, such as two consecutive crests or compressions.

Frequency (ν\nu or ff) is the number of oscillations per unit time, while the Time Period (TT) is the time taken for one complete oscillation, related by T=1/νT = 1/\nu.

The displacement of a sinusoidal wave traveling along the positive xx-axis is given by the function y(x,t)=Asin(kxωt+ϕ)y(x, t) = A \sin(kx - \omega t + \phi), where AA is the amplitude, kk is the angular wave number, and ω\omega is the angular frequency.

Transverse waves can only propagate through solids and on the surface of liquids because they require shear strength. Longitudinal waves can propagate through solids, liquids, and gases.

📐Formulae

v=νλv = \nu \lambda

T=1νT = \frac{1}{\nu}

ω=2πν=2πT\omega = 2\pi\nu = \frac{2\pi}{T}

k=2πλk = \frac{2\pi}{\lambda}

v=ωkv = \frac{\omega}{k}

y(x,t)=Asin(kxωt+ϕ)y(x, t) = A \sin(kx - \omega t + \phi)

💡Examples

Problem 1:

A harmonic wave is described by the equation y(x,t)=0.03sin(40x2t)y(x, t) = 0.03 \sin(40x - 2t), where xx and yy are in meters and tt is in seconds. Determine the amplitude, wavelength, and speed of the wave.

Solution:

Comparing with the standard equation y(x,t)=Asin(kxωt)y(x, t) = A \sin(kx - \omega t), we get:

  1. Amplitude A=0.03 mA = 0.03\text{ m}.
  2. Wave number k=40 rad/mk = 40\text{ rad/m}. Since k=2πλk = \frac{2\pi}{\lambda}, we have λ=2π40=π200.157 m\lambda = \frac{2\pi}{40} = \frac{\pi}{20} \approx 0.157\text{ m}.
  3. Angular frequency ω=2 rad/s\omega = 2\text{ rad/s}. Wave speed v=ωk=240=0.05 m/sv = \frac{\omega}{k} = \frac{2}{40} = 0.05\text{ m/s}.

Explanation:

The parameters are extracted directly by comparing the given wave function with the general wave displacement formula.

Problem 2:

Calculate the frequency of a sound wave (longitudinal) traveling in air if its wavelength is 0.5 m0.5\text{ m} and the speed of sound is 340 m/s340\text{ m/s}.

Solution:

Given v=340 m/sv = 340\text{ m/s} and λ=0.5 m\lambda = 0.5\text{ m}. Using the relation v=νλv = \nu \lambda, we find: ν=vλ\nu = \frac{v}{\lambda} ν=3400.5=680 Hz\nu = \frac{340}{0.5} = 680\text{ Hz}

Explanation:

The frequency is the ratio of wave speed to wavelength, representing how many compressions pass a fixed point per second.

Transverse and Longitudinal Waves - Revision Notes & Key Formulas | CBSE Class 11 Physics