I thing most of you know this paradox, but here a quick introduction: You place a paraffin candle in water so that it barely floats and light it. Now the candle keeps burning and doesn't sink, more exactly it also rises, paradox(?)…
Now I already know the basics about this problem, those are that the water cools the outer walls of the candle and therefore prevent them from melting which leads to an decrease in weight over time, but not in buoyancy.
But are there any small, but important details I should know? And what different types of wax but paraffin, does this have affect on the result?
Thanks in advance!
Best Answer
The water on the outside keeps the outside layer of wax from melting so it cannot burn away. So the burning candle slowly hollows out the middle of the candle reducing its weight, but not its apparent volume.
Buoyant force is the upward force the water exerts on the candle. It will be equal to the weight of the water displaced by the candle. And since the candle is floating, it balances the gravitational force on the candle.
As the weight of the candle is reduced, the buoyant force overtakes the weight slightly pushing the candle up reducing the amount of displaced water (and consequently the buoyant force) until it reaches equilibrium. As the candle burns this continues.
My guess is that the bottom of the candle will move up, while the flame will slowly descend under the surface (inside the walls of the candle).
EDIT: Looks like someone wrote a (surprisingly long) paper on this.