AQA A Level Physics复习笔记3.2.3 Harmonics

Harmonics

 

  • Stationary waves can have different wave patterns, known as harmonics
    • These depend on the frequency of the vibration and the situation in which they are created

     

  • These harmonics can be observed on a string with two fixed ends
  • As the frequency is increased, more harmonics begin to appear

Harmonics on a String

  • When a stationary wave, such as a vibrating string, is fixed at both ends, the simplest wave pattern is a single loop made up of two nodes and an antinode
    • This is called the first harmonic

     

  • The particular frequencies (i.e. resonant frequencies) of stationary waves formed depend on the length of the string L and the wave speed v
  • The frequencies can be calculated from the string length and wave equation
  • For a string of length L, the wavelength of the lowest harmonic is 2L
    • This is because there is only one loop of the stationary wave, which is a half wavelength

     

  • Therefore, the frequency is equal to:

3.2.3-First-Harmonic-Equation

 

  • The second harmonic has three nodes and two antinodes
  • The wavelength is L and the frequency is equal to:

3.2.3-Second-Harmonic-Equation

  • The third harmonic has four nodes and three antinodes
  • The wavelength is 2L / 3 and the frequency is equal to:

 

3.2.3-Third-Harmonic-Equation

 

  • The nth harmonic has n antinodes and n + 1 nodes
  • The wavelengths and frequencies of the first three harmonics can be summarised as follows:

Fixed-end-wavelengths-and-harmonics-1-e1616147493267Fixed-end-wavelengths-and-harmonics-2

Diagram showing the first three modes of vibration of a stretched string with corresponding frequencies

 

 

Worked Example

A stationary wave made from a string vibrating in the third harmonic has a frequency of 150 Hz.Calculate the frequency of the fifth harmonic

Step 1: Calculate the frequency of the first harmonic

f3 = 3 f1

f1 = f3 ÷ 3 = 150 ÷ 3 = 50 Hz

Step 2: Calculate the frequency of the fifth harmonic

f5 = 5 f1

f5 = 5 × 50 = 250 Hz

Exam Tip

Make sure to match the correct wavelength with the harmonic asked for in the question:

  • The first harmonic (or n = 1) is the lowest frequency with half or quarter of a wavelength
  • The second harmonic (or n = 2) is a full wavelength

Frequency of the First Harmonic

  • The speed of a wave travelling along a string with two fixed ends is given by:

3.2.3-Velocity-Equation

  • Where:
    • T = tension in the string (N)
    • μ = mass per unit length of the string (kg m–1)

     

  • For the first harmonic of a stationary wave of length L, the wavelength, λ = 2L
  • Therefore, according to the wave equation, the speed of the stationary wave is:

v = fλ = f × 2L

  • Combining these two equations leads to the frequency of the first harmonic:

3.2.3-Frequency-of-First-Harmonic

  • Where:
    • f = frequency (Hz)
    • L = the length of the string (m)

     

Worked Example

A guitar string of mass 3.2 g and length 90 cm is fixed onto a guitar. The string is tightened to a tension of 65 N between two bridges at a distance of 75 cm.Guitar-String-First-Harmonic-QuestionCalculate the frequency of the first harmonic produced when the string is plucked.

Guitar-String-First-Harmonic-Answer

Harmonics-Worked-Example

 

 

转载自savemyexams

 

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