Concepts of Physics

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

Boiling Water in Paper Cup

Objective

To study about specific heat and convection.

Introduction

We have to supply a certain amount of heat to raise the temperature of a substance by 1 degree celsius. This amount of heat is called specific heat capacity of the substance.

Apparatus

paper cup, candle, matchbox

Procedure

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You can boil water in a cup made of paper. Make a paper cup and fill it with water. Place this cup on the flame carefully. You will be surprised that paper does not burn. Slowly water temperature increases and it may start boiling. This demo explain convection of heat.

Discussion

Water absorbs most of the heat and prevent rise in temperature of the paper to its ignition point. The heat transfer within the water is through convection. Water molecules at the bottom of the cup absorb heat from the paper and rises up. The molecules from the surrounding colder area occupies the empty space created due to rise of molecules. This process continues and prevent the paper to become too hot. Also, specific heat capacity of the water is quite high. It absorbs a large amount of heat before its temperature increases significantly.

Note

Variant: Take two balloons. Fill one of them with air and other with water. Bring them close to the flame. The balloon with air burst immediately but the balloon with water does not burst.