why does a flame always point upwards
Its obvious from the above illustration that gravity has a role to play. However, one can't help but wonder why a flame would go upwards where there is strong gravitation pull like on earth and remain spherical where there is no gravity like on a space station (
). The answer is buoyancy. When you start a fire, fuel combines with oxygen to release heat, light, carbon dioxide, water vapor, soot etc. The heat energy generated in this process heats up the air around the flame which in turn reduces its density** (a good analogy is water vapor which is hotter and less denser as against water which is colder and heavier). This hot air around the flame then starts rising up and the surrounding colder heavier air rushes down into its place accelerating the hot air upwards which in turn causes the flame to shoot up (a good analogy here is a wooden log dipped in water. the.
Basically the buoyancy shoots the flame up. It is this hot air rushing upwards that also causes fire to flicker. The following image shows what happens to the air around a flame. However, in a micro gravity environment, there is no reason for the heavier air to fall down and rush into the place of the lighter air. So the flame just remains a spherical blob as you would imagine. So there you have it. A combination and gravity and gravity induced buoyancy causes the hot air to rise up causing the flames to point upwards here on earth. **Just in case you are curious as to why hot air is less denser than cold air in the first place - the heat gives the molecules in the gas higher (kinetic) energy.
So these molecules can move faster and further apart because they now have the energy needed to overcome their binding forces. The molecules being farther apart means that the material is less dense. You must have observed how a candle burns when lights go off. We know that to produce fire, we need a fuel and oxygen to support combustion. Here the fuel is the paraffin (candle wax) which gives out light and heat when candle is lit. There is also a wick which is made of some kind of absorbent material. The heat generated when the wick is lit turns the solid wax into liquid wax. As the wick is made of absorbent substance, it absorbs the liquid wax causing the liquid wax to be drawn up the wick. Due to heat, the liquid wax now turns into vapor.
This is how the candle burns. Also as wax burns completely there is less candle wax after burning than before. The flame color varies because of the temperature difference. The hottest part of the flame is blue in color and cooler parts are in orange or yellow color. The blue zone is rich in oxygen compared to cooler zones. Paraffin wax is composed mainly of carbon atoms and hydrogen atoms. The carbon and hydrogen atoms break free in the blue zone. Hydrogen atoms react with oxygen to form water vapor and some of the carbon atoms form carbon dioxide. Carbon atoms in the orange or yellow zone with less oxygen forms soot. When the candle burns, the vapor reacts with oxygen to give out carbon dioxide, water vapor along with heat and light.
The flame of the candle heats up the air around the flame. Heated air being light rises up. To take its place, cold air rushes in. This cold air in turn gets heated up and rises. The incoming air comes from all the directions from the sides as well the burning material. This activity of less dense air (hot air) rising up and more dense air (cold air) taking its place, happens in a continuous cycle causing the elongated shape of flame. The flame points upwards on earth due to gravity. Gravity causes the warm air rise upwards to a lower pressure. The same candle flame in the space where there is no gravity (micro gravity region), doesn t point up but is in the form of a sphere. Why eating asparagus makes urine smell? What is Myocardial Infarction (MI) or Heart Attack?
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