MCQ Detailed View

The cracking or snapping sound heard when dry hair is combed with a hard rubber comb is caused by small electric sparks, a phenomenon resulting from electrostatic discharge.

When a rubber comb is passed through dry hair, friction occurs between the comb and the hair strands. This friction transfers electrons from one material to the other, creating an electrostatic charge imbalance. The comb often becomes negatively charged, while the hair becomes positively charged due to the loss of electrons. This process is called triboelectric charging.

As the comb continues to move, the increasing charge difference between the comb, hair, and surrounding air builds up electrostatic potential. When this potential exceeds the insulating capacity of the surrounding air, tiny electric discharges or sparks occur. These micro-sparks ionize the air molecules, producing a popping or cracking sound that we can hear. Sometimes, this effect is visible in a dark room as small flashes of light.

The other options are incorrect:

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  Hair barging against the charged comb: This refers to mechanical friction only and does not explain the cracking sound.

  
  


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  Rubbing of the comb with hair: Friction causes charging but does not directly produce the sound; the sparks do.

  
  


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  None of the above: This is wrong because the phenomenon is well understood scientifically.

  
  

This effect is more noticeable when:

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  The air is dry, reducing moisture that could carry away charges.

  
  


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  The comb and hair are insulating materials, preventing charge leakage.

  
  


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  Combing speed is fast, generating more friction and higher charge buildup.

  
  

Electrostatic discharges like this occur in many everyday situations, such as:

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  Feeling a shock after walking on a carpet and touching metal.

  
  


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  Sparks when removing clothes made of synthetic fabrics.

  
  

Thus, the cracking sound while combing dry hair with a hard rubber comb is due to tiny electric sparks caused by electrostatic discharge, a fundamental principle of static electricity in physics.