Question

Which of the following statements is true for a thermometer?
a) Coefficient of cubical expansion of liquid must be greater than that of bulb material
b) Coefficient of cubical expansion of liquid may be equal to that of bulb material
c) Coefficient of cubical expansion of liquid must be less than that of bulb material
d) none of the above

Answer 1

Hint : During the heat shift from a body to the thermostat, the bulb and the liquid in the thermometer extend. The extension of the liquid must be higher than the bulb because the liquid has to expand in the capillary tube. Hence, the cubical expansion coefficient of the liquid must be higher than that of the bulb element.

Complete step-by-step solution:
Temperature is the estimation of the coldness or hotness of a body on any chosen rule. It is estimated using a thermometer. Temperature is a fundamental physical quantity and a scalar quantity.
A thermometer scale is obtained by choosing two temperatures described as fixed points; the lower fixed position and the upper fixed position. The steam temperature is utilized since impurities do not influence its temperature but will increase the boiling point of water. The series between these two positions is then split into equal divisions.
In the thermometer, the liquid extends up a capillary tube when the bulb is ignited. The liquid utilized in this thermometer should maintain the following characteristics for the thermometer to be helpful: Be readily apparent, extend and shrink regularly, have a broad spectrum of temperature, i.e., significant boiling point and lesser freezing point, be sensible to slight temperature differences, should not moisten the glass.
When fired, the liquid level in the glass tube first decreases and then starts increasing. This primary drop in the level is because the glass was burned first and extended. Later the liquid took the heat energy and extended hence the increase in the level.
So, the coefficient of cubical expansion of liquid must be greater than that of bulb material.
Option (a) is correct.

Note: The rate of expansion in liquids is higher than in solids because the particles are lightly far distant. When the temperature rises, the liquid molecules attain more energy raising their rate of motion. The weak linkages between these molecules are moreover decreased. The molecules thus extend and hold more space.