Here's a followup to last week's candle puzzle.

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.

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.

After May 15th, click Read More for the answer.

 The answer is (c): the candle flame will burn with slightly less brightness than it had before it was dropped, as seen in this slow-motion mpeg video.

 

This is a question that is often misunderstood. It has been long understood that a when a candle flame burns, the hot combustion products rise, pulling fresh air and oxygen into the flame by the process of convection. These convection currents then continue to feed the flame indefinitely; if they did not exist the flame would go out due to lack of oxygen.

This should be precisely the condition in the falling jug. When the jug is falling the air in the jug becomes weightless (The jug becomes a local inertial frame of reference), so convection currents cease, fresh oxygen should not reach the flame, and the candle should go out virtually instantaneously. However, although most of this argument is true and logical (the jug does become a local inertial frame.) the candle flame does not go out. The way in which oxygen gets to the flame changes dramatically, but the flame continues to burn. In this situation the flame gets its oxygen by diffusion, as long as there is sufficient oxygen in the air and it can reach the flame by diffusion. The flame will stay lit indefinitely, but its character changes - it is blue in color and becomes spherical in shape except for the part adjacent to the candle. This can be seen in the photograph below of a candle flame taken in a situation of microgravity - inside a falling inertial flame - in particular, the space station. If you look carefully at the flame as the bottle drops (frame by frame) you can actually see the shape of the flame beginning to change: it becomes slightly smaller and shorter.

This experiment has been interpreted incorrectly because the flame often goes out when the container stops, due to the rapid acceleration it undergoes; this is seen in this slow-motion video, which has a few extra frames and includes the actual catch. This video shows the entire sequence in real time.