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Question of the Week

  • I2-42: FALLING CANDLE

    I2-42
    Demonstrate how a flame burns in the absence of normal convection.
    A candle, attached to the lid of a one-gallon jug, is lit and the lid screwed onto the upside-down jug. Throw the upside-down jug into the air and catch it or hold the upside-down jug high and drop it and catch it as it falls. While it is falling, the system inside the jar is in a weightless environment, so convection currents cease. In normal burning, the hot air rises by convection, allowing cooler air containing more oxygen to continuously feed the fire. Without these convection currents the candle should immediately go out, BUT IT DOES NOT.
    I2

    A candle mounted on the lid of a gallon jug is lit, and the lid quickly affixed to the jug. In this configuration the candle will remain lit for over one minute before the oxygen in the jug is sufficiently used up by the combustion process and the flame is extinguished.

    Now suppose that the candle flame is lit and the lid again quickly affixed to the jug. However, the bottle is now dropped about six feet starting from the orientation shown in the photograph below.

    i2 42

    What will happen? In particular, by the time the jug falls six feet the candle flame will:

    • (a) burn more brightly.(b) remain at about the same brightness.
    • (c) burn less brightly.
    • (d) go out.

     

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

    I2-45
    Brainteaser regarding convection.

    Two candles, one at the level of the table and one raised approximately 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 to prevent air from flowing into the tube as the experiment progresses.

    Engagement Suggestion:
    • Encourage students to predict which candle will go out first, and why.
    • As the demonstration will typically take 2-4 minutes, discuss other related matters and then check in on the demonstration from time to time; ask students if it is behaving as they expected.
    • When both candles have gone out, ask students to discuss what they saw.
    Background:

    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.
    I2

    ii2-45ai2-45bi2-45ci2-45d

     

     

    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.

     

    i2 45 i2 45a

     

    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.