Verified Solution • 2025–26 Session moderate

QPages 34-37: Q55: HC Verma: Concepts of Physics - Volume 2 Kinetic Theory of Gases Pages 34-37: Q55 Solution: A barometer correctly reads the atmospheric pressure as 76 cm…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often mistakenly assume the mercury column height equals the atmospheric pressure or try to add the vapour pressure to the reading. Remember that the vapour pressure exerts an upward force on the mercury surface, so the column height must decrease by the amount of the saturation vapour pressure ($h_{min} = P_{atm} - P_{vap}$).

A barometer correctly reads the atmospheric pressure as 76 cm of mercury. Water droplets are slowly introduced into the barometer tube by a dropper. The height of the mercury column first decreases and then becomes constant. If the saturation vapour pressure at the atmospheric temperature is 0.80 cm of mercury, find the height of the mercury column when it reaches its minimum value.

Step-by-Step Verified Solution

**Answer:** Minimum height = 75.2 cm **Step-by-Step Explanation:** Initial barometer reading = 76 cm Hg. Water droplets introduced; saturation vapour pressure = 0.80 cm Hg. The minimum height of the mercury column occurs when the space above is saturated with water vapour:
hmin=76−0.80=75.2 cmh_{min} = 76 - 0.80 = 75.2 \text{ cm}

All Questions in this Chapter

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