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CBSE Class 7 Answered

<div>when we blow the flame of a candle it goes off. but sometimes to make a fire we have to blow air on it. why does this happen</div> <div>&nbsp;</div>
Asked by mumbai.sanjit | 26 Jun, 2016, 04:53: PM
Expert Answer
  • Pressure difference which is created by the blowing carbon dioxide near the fire plays a vital role towards making of the fire. 
  • The blowing carbon dioxide has greater velocity than the atmospheric air
  • This causes to lower the pressure of air near the fire,
  • Applying Bernoulli’s theorem which states that pressure is inversely proportional to the velocity.
  • The pressure difference causes nearby atmospheric air to rush towards the fire, thereby supplying the oxygen mixed in that atmospheric air.
  • Oxygen is very good supporter of combustion, but it should be noted that oxygen is not a combustible gas itself.
  • With large amount of oxygen mixed in atmospheric air being blown towards fire, the fire glows exceedingly.
  • That is why to make small amount of fire slight blowing of air is done.
  • But if the blown air has a comparitively larger velocity, then it completely displaces the air and heat blanket from the point of fire, which is required by the fire to sustain itself.
  • Thus if the air is blown with a higher velocity, the fire goes off.
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