Take a sheet of paper. Draw a spiral on it. Cut the paper along the spiral. Suspend the paper above a burning candle. Observe what happens. Provide an explanation for your observation.
When a spiral-cut paper is suspended above a burning candle, the paper spiral begins to rotate.
This happens because the air above the candle flame gets heated. Hot air expands, becomes lighter, and rises upward.
As the hot air rises, it creates an upward current of air. This rising hot air pushes against the paper spiral, causing it to rotate.
The rotation demonstrates convection currents in air. The spiral shape of the paper allows it to catch the rising air more effectively, making the rotation visible.
This activity shows that hot air rises due to convection, similar to how smoke rises from an incense stick.
Explanation
The chapter explains convection in section 7.2: 'As the air in the cup warms up, it expands and occupies more space. As a result, it becomes lighter and rises up.' Also, 'You must have observed that when an incense stick (agarbatti) is burnt, smoke rises up. Smoke is a mixture of hot gases and tiny solid particles that are released when something burns. As it is warmer than the surrounding air, it rises up.' The spiral paper rotates because of the rising hot air current created by convection.
Solution Steps
Air above the candle flame gets heated
Hot air expands, becomes lighter, and rises upward
Rising hot air creates an upward current of air
The upward air current pushes against the paper spiral, causing rotation
The spiral shape catches the rising air effectively, making rotation visible
This demonstrates convection currents in air