Question

Thermometer construction is based on the principle of:
A) Zeroth Law of Thermodynamics
B) First Law of Thermodynamics
C) Second Law of Thermodynamics
D) Third Law of Thermodynamics

Answer 1

Thermometer is used to measure the temperature of a body using the property that metals expands when heat is provided. Mercury is used inside the thermometer and the thermometer is attached to the body whose temperature is required to be measured.

Explanation:
Zeroth law of thermodynamics: Zeroth law of thermodynamics states that when two thermodynamics states are in equilibrium with a third one then those two will be in equilibrium with each other.

First law of thermodynamics: First law states that the total energy of any isolated system is constant. Energy can be transformed from one form to another but they can neither be created nor be destroyed.

Second law of thermodynamics: Second law states that entropy of an isolated system can never decrease. It remains constant if all the processes are reversible.

Third law of thermodynamics: Entropy of any system approaches to some constant value as its temperature approaches to absolute zero.

Here, notice how a thermometer is constructed. The mercury is kept inside the bulb of the thermometer and a very tiny channel is kept to allow the mercury to expand. Now, to measure the temperature of a body the bulb of the thermometer is kept in touch with the body. They exchange heat to come to equilibrium. Now, inside the bulb mercury also comes in equilibrium of the bulb by expanding its volume. So, using zeroth law we can say the temperature of mercury is the same as that of the body because both of them are in equilibrium with the bulb.

Correct answer: Thermometer construction is based on the principle of (a) Zeroth Law of Thermodynamics.

Note: This question is indeed a very simple question. Here the reasoning is so intuitive that sometimes it might not strike into the mind of a student as the answer. So, here the main striking point is that although we use mercury to measure the temperature, we don’t actually touch mercury with the body. And using this argument the answer comes immediately.