# NCERT Class 11 Physics Solutions: Chapter 11 – Thermal Properties of Matter-Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 11.4:

(a) The triple-point of water is a standard fixed point in modern thermometry. Why? What is wrong in taking the melting point of ice and the boiling point of water as standard fixed points (as was originally done in the Celsius scale) ?

(b) There were two fixed points in the original Celsius scale as mentioned above which were assigned the number and respectively. On the absolute scale, one of the fixed points is the triple point of water, which on the Kelvin absolute scale is assigned the number What is the other fixed point on this (Kelvin) scale?

(c) The absolute temperature (Kelvin scale) is related to the temperature on the Celsius scale by

Why do we have in this relation, and not ?

(d) What is the temperature of the triple point of water on an absolute scale whose unit interval size is equal to that of the Fahrenheit scale?

Explanation:

The triple point of water has a unique value of At particular values of volume and pressure, the triple point of water is always The melting point of ice and boiling point of water do not have particular values because these points depend on pressure and temperature.

Expalantion:

The absolute is the other fixed point on the Kelvin absolute scale.

Expalantion:

The temperature is the triple point of water. It is not the melting point of ice. The temperature on Celsius scale is the melting point of ice. Its corresponding value on Kelvin scale is .

Hence, absolute temperature (Kelvin scale) is related to temperature , on Celsius scale as:

Expalantion:

Let TF be the temperature on Fahrenheit scale and TK be the temperature on absolute scale. Both the temperatures can be related as:

Let be the temperature on Fahrenheit scale and be the temperature on absolute scale. Both the temperatures can be related as:

It is given that:

Subtracting equation from equation , we get:

Triple point of water

∴ Triple point of water on absolute scale

Question 11.5:

Two ideal gas thermometers use oxygen and hydrogen respectively. The following observations are made:

(a) What is the absolute temperature of normal melting point of Sulphur as read by thermometers ?

(b) What do you think is the reason behind the slight difference in answers of thermometers (The thermometers are not faulty) . What further procedure is needed in the experiment to reduce the discrepancy between the two readings?

Explanation:

Triple point of water,

At this temperature, pressure in thermometer

Let be the normal melting point of sulphur.

At this temperature, pressure in thermometer

According to Charles՚ law, we have the relation:

Therefore, the absolute temperature of the normal melting point of Sulphur as read by thermometer

At triple point the pressure in thermometer

At temperature , the pressure in thermometer

According to Charles՚ law, we can write the relation:

Therefore, the absolute temperature of the normal melting point of Sulphur as read by thermometer B is

Explanation:

The oxygen and hydrogen gas present in thermometers respectively are not perfect ideal gases. Hence, there is a slight difference between the readings of thermometers

To reduce the discrepancy between the two readings, the experiment should be carried under low pressure conditions. At low pressure, these gases behave as perfect ideal gases.

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