Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Allout Thermometer- Demonstrate Charle's law

Introduction

This experiment describes the principle of gas thermometer. It is based on the increase in pressure as the temperature of the gas is raised.

Apparatus

An empty bottle of `Allout' or any liquid mosquito repellant, straw, glass tumbler, coffee heater, standard thermometer.

Construction: Take a used `Allout' bottle and remove wick. Fill it up to say half with water and put a straw in it. The lower end of the straw will be well inside water. Seal the space between the neck and straw completely. Now blow into the straw to forcefully bubble some more air in the straw to the bottle. When you release, some water will come up in the straw due to increased pressure of air. Your thermometer is ready for calibration. We are giving you the thermometer at this stage.

Procedure

The next task is calibration. Use a standard thermometer to measure the room temperature. Look at the level of water in the straw. Heat water in a glass tumbler. Measure its temperature using standard thermometer. Put the `Allout' bottle in the tumbler and wait for sometime. See the level of water in the straw. This way you can calibrate the thermometer.

Discussion

The volume of air (and vapour) in the bottle is almost fixed. As the heat is given to this air, its pressure increases. This increased pressure forces the water to go up in the straw, the increase in pressure being equal to \(\Delta h\rho g\). This way a temperature-water level calibration can be done. You can also use this to demonstrate Charle's law (\(P\propto T\)) qualitatively.