Thursday, 26 June 2014 14:28

I2-45: CONVECTION - HIGH/LOW CANDLES IN CYLINDER

• ID Code: I2-45
• Purpose: Brainteaser regarding convection.
• Description: Two candles, one on the desk and one raised about 30 cm, are lit and then covered by a tube about 50 cm high and 10 cm in diameter. The tube is sealed at the top by a dark plastic cover so that no air may flow into the tube as the experiment progresses. Which candle will go out first, or will they go out simultaneously?

The answer can be seen in the sequence of photographs above. The photograph at the left was taken after the candles were lit but before the tube had been put in place over the candles. The left center picture was taken immediately after the tube was placed over the candles. The center photograph was taken after about one minute. The right center photograph was taken after about 1.5 minutes, and the photograph at the right was taken after about 2.5 minutes, showing the order in which the candles became extinguished.

• Availability: Available
• Loc codes: I2
• Question of the Week:

Two candles, shown in the photograph at the left below, are lit and then covered by a plastic tube (sealed at the top), as seen in the photograph at the right below. In the apparatus as pictured the tube is about 50 cm tall and 10 cm in diameter, and the upper candle is about 30 cm from the bottom.

After some period of time, as the oxygen in the tube is consumed by the candle flames, the candles will both cease to burn. The question involves the order in which the candles will go out.

Which of the following statements is true?

• (a) The top candle will go out first, then the bottom candle.
• (b) The bottom candle will go out first, then the top candle.
• (c) Both candles will go out at the same time.

The answer is (a): The top candle will go out, then the bottom candle, as can be seen in the sequence of  four photographs below.

 t=0 t=1 minute t=1.5 minutes t=2.5 minutes

The photograph at the far left was taken immediately after the plastic tube was placed over the candles; both are buning steadily. The photograph at the left center, taken about 1 minute after the tube was placed over the candles, shows the two candles still happily burning. The photograph at the right center was taken after about 1.5 minutes, just after the upper candle went out. The photograph at the far right was taken after 2.5 minutes, just after the bottom candle went out.

As the candles burn, the hot gases composing the products of combustion will be less dense than the cooler original air, and will rise to the top of the tube. The upper candle will therefore be extinguished sooner than the lower one.

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