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SPM Physics

6.4.1 Electromagnetic Waves

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  1. Electromagnetic waves come from a whole variety of sources. They differ greatly in their wavelengths and in their effects, but they have certain fundamental properties in common.
  2. The common properties of electromagnetic waves are as below:
    1. They are transverse waves:
      1. Electromagnetic waves are transverse wave.
      2. The moving wave effect is produced by oscillating electric and magnetic fields oscillate at right angles to the direction of travel and at right angles to each other
    2. They can travel in Vacuum:
      Electromagnetic wave can travel in vacuum

    3. They travel with the same speed in vacuum:
      1. In vacuum, all the electromagnetic waves travel at the same speed commonly known as the speed of light (3 × 108m/s).
      2. In other medium, different electromagnetic wave may travel at different speed.
    4. They can be polarised:
      1. A transverse wave can be polarized.
      2. Plane polarized light will be produced when light travels through a polarizing material like polaroid.
      3. Polaroid is a type of material that only allows light waves of one plane to pass through.
    5. They are neutral:
      Electromagnetic waves are electrically neutral.

 

6.3.3 Applications of Sound Wave

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Sonar


Q & A

Explain how sonar is used to measure the depth of a sea. for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions.
  1. In the sonar system, the broadcasting equipment emits ultrasonic pulses.
  2. The pulses are reflected back from the base of the sea or object in the sea and create an echo.
  3. The echo is traced by a hydrophone.
  4. The time interval t between transmitting the pulses and receiving them again is recorded by electronic equipment.
  5. If the ultrasonic velocity is v, then depth of the sea d is given by d = ½ vt

Disintegration of Kidney Stone

High intensity ultrasound wave is used to break kidney stone in a patient's body.

Ultrasound Scanning

  1. Ultrasound is used for the scanning of foetus in thge womb.
  2. Ultrasound is used in ultrasonography for visualizing body structures including tendons, muscles, joints, vessels and internal organs.


Cleaning

Jewelers use ultrasound to clean rings and watches.

 

6.3.2 Loudness and Pitch

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  1. Loudness of a sound depends on the amplitude of the sound wave.
  2. The greater the amplitude of a sound wave, the louder the sound is.
  3. The pitch of a sound is high or low of the sound.
  4. The pitch of sound is determined by its frequency. The higher the frequency, the higher the pitch.

 

6.3.1 Sound Wave

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Characteristic of Sound Wave

  1. Sound wave is a series of compression and rarefaction of layers of air molecules repeatedly through space.
  2. The forward and backward vibration of the air molecules in the direction of motion of a sound wave shows that sound is a longitudinal wave.

Propagation of Sound Wave

  1. Sound wave is a mechanical wave that requires a medium for its propagation. Therefore sound wave cannot propagate in vacuum.
  2. The medium for propagation can be solid, liquid or gas.
  3. Sound waves propagate fastest in solid and slowest in gas.

Type of Sound Wave

  1. Human ear is capable of hearing sound with frequency in the range of 20Hz – 20,000Hz, and the sound wave with frequency in this range is called an audio/Sonic wave.
  2. Sound wave with frequency lower than 20Hz is called an Infrasonic Wave.
  3. Sound wave with frequency higher than 20,000Hz is called an Ultrasonic wave.

 Infrasonic Audio/Sonic  Ultrasonic 
 <20Hz  20Hz – 20000Hz  >20 kHz


Speed of Sound Wave

  1. Speed of sound wave in solids is greater than in liquids, which is greater than in gases.
  2. Speed of sound in air is not affected by pressure, but is affected by the temperature.
  3. As temperature increases, speed of sound in air (and other gases) is also increases.
  4. Sound usually travels more slowly with greater altitude, due to reduced temperature.
  5. Speed of sound can be calculated by the equation 

 

6.2.5.4 Interference of Sound Wave

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Experiment

  1. Figure below shows the apparatus set up to investigate the interference of sound wave.
  2. Two distanced speakers are connected to the same audio generator.
  3. A microphone is move in front of the 2 speakers from  left to right.


Result:
Alternating loud and soft sounds are detected as the microphone is moved from left to right.

Formula

Wavelength of the sound wave is given by the equation


a = Distance between two loudspeakers
x = Distance between two successive loud sound.
D = Distance between the straight line path and the two loudspeakers.


Q & A

Explain briefly why alternating loud and soft sound is detected in front of the speakers?
  1. The alternating loud and soft sounds are caused by interference of the sound wave.
  2. The loud sound corresponds to the occurrence of constructive interference.
  3. The soft sound corresponds to the occurrence of the destructive interference.

Q & A

Explain why the experiment is not suitable to be done in a lab.

Sound wave will be reflected by the wall and causes interference of reflected waves.

Q & A

How do we make sure that the sources of the sound wave are coherent?

The two loud speakers are sources of two coherent sound waves as they are connected to the same audio signal generator.