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Playing and doing experiment with flame was one of our favorite childhood interests. But I've always been wondering how a flame takes its shape and behaves like a very light object. If we blow gently it moves, if we put obstruction its shape gets changed. It's very much like a normal object, isn't it? How do all these happen?

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    $\begingroup$ Maybe the object is a gas! $\endgroup$ Nov 21, 2018 at 13:08

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It's very much like a normal object, isn't it?

That's because it is a normal object. Smoke.

When you turn on the element on an electric stove, it begins to glow red. That's because objects give off light when they heat up. As you increase the temperature the color changes and gets "more white" so we speak of "red hot" and "white hot".

But it is clear in this example we are talking about some underlying object, "the stove is red hot". We don't say "the red hot".

But in the case of fire, we do sometimes confuse it for an object on its own. We speak of "the flame". But what you're actually looking at is smoke that has been heated "yellow hot". That is all, you are looking at hot smoke.

When you consider it that way, I don't think you'll be surprised to see that it...

  • behaves like a very light object (smoke is light)
  • if we blow gently it moves (smoke moves when you blow it)
  • if we put obstruction its shape gets changed (smoke flows around obstructions)

how a flame takes its shape

Now this is a little different. As the smoke rises it is giving off energy - the light you see is one of those forms of energy. So as it moves away from the source, it is constantly cooling. It's also mixing with surrounding cold air, which is being drawn in by convection. It's complex, but basically this causes the smoke to suddenly cool at a characteristic distance from the source, at which point, poof, it looks like smoke. It's the pattern of convection that determines the classic teardrop shape.

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