Concepts of Physics

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

See Convection Current in Air

Procedure

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Take a kerosene lamp with some kerosene and wick in place or a candle. Take a cardboard box, like a shoe box. Place it in a way that the opening side is vertical and is towards you. Cut two holes on the top cover of the box so that the glass covers of the lamp can be fitted in these. Put a lamp inside the box below one of the holes. One of the glass covers should go through the hole and fit with the lamp. Let me call this cover 1. Put another glass cover in the other hole. I will call it cover 2. Close any gap remaining between the glass covers and the cardboard top cover.

Light the lamp, put it in place and close the box. Light an incense stick. Where is the smoke going? It goes up. That is the natural tendency of smoke. Put the stick near the glass cover 1. The smoke goes up. Now put the stick near the glass cover 2. Here is the real climax. The smoke is pulled down into the glass cover 2 and it come out from the glass cover 1.

Why is the smoke dragged into the glass cover against its natural tendency to go up? This is because the burning wick of the lamp produced hot gases which rose up and went out through the glass cover 1. To fill the void, air should rush in. The only path available for fresh air is through the glass cover 2. So convection current is set up where air from outside goes into cover-2, goes into the box, and then to the wick area. The hot gases formed there go up and come out of cover 1. When the stick is placed near the cover 2, smoke is dragged by the air current that already exist there. The flow path of smoke is just the path of convection current.