Question

Ordinary mercury-in-glass thermometer works on the principle of:

• A. Volumetric expansion of mercury with increase in temperature
• B. Increase of vapour pressure with increase in temperature
• C. Increase of electrical resistance with increase in temperature
• D. Decrease of vapour pressure with increase in temperature

Hint:

The volumetric expansion rule for liquids can be mathematically modeled as: \( V_t = V_0(1 + \beta \Delta T) \). Here, \(\beta\) represents the coefficient of volume expansion, which forms the core principle of basic liquid-in-glass thermometry.

Answer 1

The Correct Option is A

Concept: Thermometers function by translating thermal energy variations into a readable change in a physical property known as a thermometric property. For fluid-filled capillary systems, changes in temperature lead to changes in atomic spacing, manifesting as structural volumetric expansion or contraction.

Step 1: Explaining the physics of a mercury-in-glass thermometer.
Mercury is a liquid metal with a highly linear coefficient of thermal expansion. When the bulb of the thermometer absorbs thermal energy, the kinetic energy of the mercury atoms rises, causing the liquid to expand. Because glass expands at a much smaller rate than mercury, the expanding mercury is forced to rise up the narrow calibrated capillary tube. The height of the liquid column is directly proportional to the volumetric expansion, which corresponds to the system temperature.

Step 2: Disproving alternative options.

• Options (B) and (D): Vapour pressure thermometers rely on vapour pressure curves, not mercury-in-glass configurations.

• Option (C): Resistance change is the principle behind Resistance Temperature Detectors (RTDs) and thermistors, not liquid thermal expansion devices.