IB DP Physics: SL复习笔记4.3.2 Amplitude & Intensity

Amplitude & Intensity

Intensity

  • The intensity of a wave is defined as follows:

Power per unit area

  • Intensity is measured in W m–2
  • Power is defined as:

The rate of energy transfer

  • Therefore, intensity can also be defined as:

The rate of energy transfer per unit area

  • For spherical waves being emitted by a point source equally in all directions, the intensity follows an inverse square law with distance from the point source7117-intensity-of-a-spherical-wave

Intensity decreases by the inverse square law

Intensity Variation with Amplitude

  • By definition, the intensity of a wave (its power per unit area) is proportional to the energy transferred by the wave
  • The intensity of a wave at a particular point is related to the amplitude of the wave at that point
  • The energy transferred by a wave is proportional to the square of the amplitude
  • Therefore, the intensity of a wave is proportional to the square of the amplitude
  • Where:
    • I = intensity of the wave in W m–2
    • A = amplitude of the wave in metres (m)

Worked Example

A person stands 10 m away from a loudspeaker. The sound produced by the loudspeaker is very loud, so the person moves 20 m away from it.

State the effect of this change on the intensity and the amplitude of the sound waves heard by the person.

Step 1: Write down the known quantities

    • Original distance, r1 = 10 m
    • New distance, r2 = 20 m

Step 2: Write down the relationship between the intensity of a wave and the distance from the point source producing the wave

Wave-Intensity-and-Distance

Step 2: State the new intensity

    • Since the distance doubles (r2 = 2r1), the intensity is reduced by a factor four

Amplitude-and-Intensity-Worked-Example-Intensity-Variation

Step 3: Write down the relationship between the intensity of a wave and its amplitude

Intensity-and-Amplitude

Step 4: State the new amplitude

    • Since the intensity is reduced by a factor four, the amplitude decreases by half

Amplitude-and-Intensity-Worked-Example-Amplitude-Variation

 

 

 

 

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