Hot air rises

Pema points out the smoke rising from the burning firewood. Why does smoke go up?

Activity 7.2 hangs two identical paper cups upside down from threads of equal length at the two ends of a wooden stick, balanced so the stick is level. A burning candle is placed below one cup.

The cup above the candle rises. The air around the flame heats up. Warm air expands and takes up more space, so it becomes lighter, and lighter air rises. The rising warm air pushes up into the cup and lifts that end of the stick.

You can see the same expansion with a partly blown-up balloon left in the Sun: the air inside heats up, expands, and the balloon gets bigger.

This is why smoke from a burning incense stick rises. Smoke is a mixture of hot gases and tiny solid particles released when something burns. It is warmer than the air around it, so it goes up.

That settles a practical question too. A smoke detector should be fitted on the ceiling, because the smoke it has to detect travels upwards.