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Question: Answered & Verified by Expert
A mercury thermometer is constructed as shown in the diagram. The capillary tube has a diameter of 0.004 cm, and the bulb has a diameter of 0.250 cm. Neglecting the expansion of the glass, find the change in height of the mercury column with a temperature change of 30.0 °C.

PhysicsThermal Properties of MatterNEET
Options:
  • A 3.55 cm
  • B 2.60 cm
  • C 4.50 cm
  • D 3.33 cm
Solution:
2026 Upvotes Verified Answer
The correct answer is: 3.55 cm
For an easy-to-read thermometer, the column should rise by a few centimetres.
We use the definition of the coefficient of expansion of the glass, the volume of liquid in the capillary will be ΔV=AΔh, where A is the cross-sectional area of the capillary. Let Vi represent the volume of the bulb.
ΔV=ViγΔT
Δh=ViAγΔT=43πRbulb3πRcap2γΔT
Δh=430.125 cm30.002 cm21.82×10-4/C30.0C=3.55 cm
This is a practical thermometer. Glass expands so little as compared to mercury that only the third digit of the answer would be affected by including the expansion of the glass in our analysis.

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