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Question

A laboratory thermometer A is kept 7cm away on the side of the flame while a similar thermometer B is kept 7cm above the flame of a candle. Which of the thermometers A or B will show a greater rise in temperature? Give a reason for your answer.


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Solution

Step 1: Given data

  1. Both the thermometer A and B are held 7cm away from the candle.
  2. Thermometer A is kept 7cm away on the side of the flame of the candle.
  3. Thermometer B is kept 7cm above the flame of a candle.

Step 2: To find

Which thermometer will show a greater rise in temperature

Step 3: Explanation

  1. The heat can not flow by conduction because there is no contact between the flame and thermometer.
  2. So, it Will flow either by convection or by radiation.
  3. Now, heat can flow by convection only at the upper portion of the candle because the air near the candle gets heated and hot air rises up.
  4. Radiative heat transfer can take place in any direction.
  5. As the thermometer A is kept 7cm away on the side of the flame, thus heat can flow to the bulb of the thermometer only by radiation.
  6. As the thermometer B is kept 7cm away above the flame, thus heat can flow to the bulb of the thermometer by radiation as well as convection.
  7. Therefore, more heat will flow to the bulb of the thermometer B, thus the rise of the temperature will be high.

Hence, the thermometers B will show a greater rise in temperature.


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