The following figure shows a rectangular box with two tubes P and Q at the top. A burning candle R is placed under the tube P. The candle burns brightly.
(i) How is the air in the box circulating?
(ii) What will happen if the opening Q is closed?
(i) Air is entering into the box from tube Q and as air reaches near the candle its temperature increase and air become lighter and leaves out from tube P.
(ii) If opening Q is closed then circulation of air stops and there is no way for air to enter. Air inside the box will leave out from tube P and after some time candle stops because for burning it requires air which is not circulating in the box now.