You can comfortably hold your fingers close beside a candle flame, but not very close above the flame. Why?
In a still room, smoke from a candle will sometimes rise only so far, not reaching the ceiling. Explain why.
Is hot air less dense than cold?
Is the explaination have to do with the density of the smoke, which changes as it goes upward cooling? Is there a vagure correlation to a submarine here?
The combustion products mixed with the air heated by passing through the flame area have a density lower than the ambient air in the room and rise. Therefore the gas is much hotter above the flame where these hot gases are heading.
As You approach the ceiling, the hot gases can not go through the ceiling (hopefully) and start to spread out, mixing with the room air and cooling off. In fluid mechanics this might be modeled by putting a virtual candle above the ceiling blowing smoke down like a mirror. As the two flows met, they would not proceed but would spread out sideways.
The reason why you can comfortably hold your fingers close beside a candle flame, but not very close above the flame is due to convection. Convection is the transfer of heat through the movement of a fluid, such as air.
When you hold your fingers close beside the candle flame, the heat from the flame warms the air directly above it. The heated air then rises due to its lower density compared to the surrounding cooler air. As the heated air rises, it creates a flow of cooler air from the surroundings towards the flame to replace the rising hot air. This flow of air, known as convection currents, helps to dissipate the heat and protect your fingers from getting burnt.
On the other hand, when you hold your fingers very close above the flame, there is not enough space for the convection currents to form and carry away the heat. As a result, the heat remains concentrated near the flame, making it uncomfortable or even painful to keep your fingers in that position.
As for the smoke from a candle not reaching the ceiling in a still room, it can be explained by the same principle of convection. When a candle burns, it produces hot gases and smoke. The hot gases and smoke rise due to convection, as the hotter gases have lower density than the surrounding air. However, if the room is still and there is no air movement, the rising hot gases and smoke may not reach the ceiling before they cool down and disperse.
Additionally, the ceiling of a room is usually cooler than the area closer to the flame, which can also hinder the smoke's rise. As the smoke rises, it cools down and loses buoyancy, causing it to mix with the cooler air near the ceiling instead of reaching it.
To further facilitate the smoke to rise and reach the ceiling, you can create air movement in the room by opening a window or turning on a fan. This will help to disperse the rising hot gases and smoke more effectively, allowing them to reach the ceiling.