kenstauffer

Why does a match go out when you blow on it?

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I already know the answer to this, but it did perplex me for a long time. It's an interesting question to test your skills in applying physics to a particular problem. So....

What causes a match to go out when you blow on it?

(NOTE: The reason I was long perplexed by this simple case was that oxygen is what drives combustion for all chemical reactions which we normally call "fire". So my dilemma was how could blowing oxygen on a combusting material ever cause it to go out.)

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I already know the answer to this, but it did perplex me for a long time. It's an interesting question to test your skills in applying physics to a particular problem. So....

What causes a match to go out when you blow on it?

(NOTE: The reason I was long perplexed by this simple case was that oxygen is what drives combustion for all chemical reactions which we normally call "fire". So my dilemma was how could blowing oxygen on a combusting material ever cause it to go out.)

I'm no expert, but the most sensible answer that occurs to me is that the temperature of the match is decreased (i.e., the heat is removed) from the area of the match that is under combustion. Perhaps this could be verified by blowing air at the same temperature as the flammability point of the match. Would the flame go out? I don't think it would because the temperature at which the wood burns would not be lowered.

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(NOTE: The reason I was long perplexed by this simple case was that oxygen is what drives combustion for all chemical reactions which we normally call "fire". So my dilemma was how could blowing oxygen on a combusting material ever cause it to go out.)

I haven't haven't been brushing up on my chemistry (and biology) lately, but I don't believe oxygen is what you're actually "bowing out" on the flame. It's carbon dioxide--which is 'harmful' to combustion.

The respiratory system acts a sort of filter: the inhaled oxygen stays in (and is used by) the body, and carbon dioxide is what's exhaled.

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I haven't haven't been brushing up on my chemistry (and biology) lately, but I don't believe oxygen is what you're actually "bowing out" on the flame. It's carbon dioxide--which is 'harmful' to combustion.

The respiratory system acts a sort of filter: the inhaled oxygen stays in (and is used by) the body, and carbon dioxide is what's exhaled.

Keep in mind that the match also goes out if a lot of wind is blown on it with the help of a fan.

Thus CO2 is a factor when we blow on the match but it isn't when air is blown on it through some other means. The latter case thus cannot be explained by CO2.

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You should also note that while there is an increased amount of CO2 in the air you breath out, there is still a significant amount of O2. Otherwise the 'Rescue Breathing' part of CPR wouldn't be very effective.

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Perhaps it is because you are overloading the system with one element of combustion. You are smothering the process. One could ask the same question as: Why does a car engine stall if it gets too much gasoline? Just as more fuel doesn't equal more combustion, more air wouldn't either.

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There you have it. To concretize the distinguishing cases, consider what happens to a flame in a raging-hot blast furnace when you force in masses of air, vs. apply the same amount of air to a burning twig.

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Keep in mind that the match also goes out if a lot of wind is blown on it with the help of a fan.

Thus CO2 is a factor when we blow on the match but it isn't when air is blown on it through some other means. The latter case thus cannot be explained by CO2.

The match also goes out if you move it quickly in the air with your hand.

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Its pretty sad that only one (the third) out of the four experts' explanations at this web page (Dept. of Energy) is accurate. Heat is not being blown away from the flame; what's happening is the flame -- a region of burning gas generating heat -- is being blown away from the candle and dispersed. With the flame gone, the candle/wick are not dense/massive enough to retain enough heat to drive the fuel in the wax into the air and to ignite that fuel-air mixture.

I suspect the trick candles that re-ignite after you blow them out have a dense wick or some other component that stays hot enough to restrat the combustion process.

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It's also interesting to note that when you blow on the embers of a hot piece of wood, you are feeding the fire. It glows all the brighter.

So, while a candle can't feed the fire fast enough when blown on, wood is able to.

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Its pretty sad that only one (the third) out of the four experts' explanations at this web page (Dept. of Energy) is accurate. Heat is not being blown away from the flame; what's happening is the flame -- a region of burning gas generating heat -- is being blown away from the candle and dispersed. With the flame gone, the candle/wick are not dense/massive enough to retain enough heat to drive the fuel in the wax into the air and to ignite that fuel-air mixture.

I suspect the trick candles that re-ignite after you blow them out have a dense wick or some other component that stays hot enough to restrat the combustion process.

Your answer is confusing. You state that heat is NOT being blown away, yet the FLAME is. Well, the flame is just an optical presentation of heat within the visible range of wavelengths, just as infrared is radiation from heat at invisible wavelengths.

You state "with the flame gone... [there is not] enough heat to drive the fuel in the wax into the air." Well, that's just another way of saying that the ignition temperature of the candle has not been reached because the heat was removed.

A flame is not just a "region of burning gas generating heat." It is the heat generated by the reaction from the burning process. If you were to heat the entire candle up to the ignition temperature in an oven, the entire candle would burst into flames.

As for the re-igniting candles, I would suspect that the wick contains material that has a very low ignition temperature so that the flame reignites before sufficient heat is removed.

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It's also interesting to note that when you blow on the embers of a hot piece of wood, you are feeding the fire.  It glows all the brighter.

So, while a candle can't feed the fire fast enough when blown on, wood is able to.

I believe that has to do with the surface area being burned. The embers have a larger surface area to volume ratio than the piece of wood. As a result, the embers are actually burning at a higher temperature than the wood. When you blow on the embers, you don't have enough volume of air to cool it and you are actually feeding the fire with oxygen.

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The answer to this question has to do with the dynamics of the flame surface. The flame surface is where the oxygen diffuses into the *core* of the flame, where the most energetic combustion is taking place. If left alone, the shape of the flame surface is a function of the concentrations of combustible and oxidizer. However, when you blow on the flame surface, you are disrupting the *shape* of the surface through which oxygen diffuses into the core of the flame. Blowing hard enough makes the flame "lift" off of the match, taking with it the path through which oxygen diffuses, and thus ending combustion.

As for wood, the diffusive influx of oxygen occurs on a "hard" surface (the surface of the wood, or coal), therefore blowing on it does not disrupt the *surface* through which oxygen diffuses, it only increases or decreases the concentration available for combustion.

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The answer to this question has to do with the dynamics of the flame surface.  The flame surface is where the oxygen diffuses into the *core* of the flame, where the most energetic combustion is taking place.  If left alone, the shape of the flame surface is a function of the concentrations of combustible and oxidizer.  However, when you blow on the flame surface, you are disrupting the *shape* of the surface through which oxygen diffuses into the core of the flame.  Blowing hard enough makes the flame "lift" off of the match, taking with it the path through which oxygen diffuses, and thus ending combustion.

As for wood, the diffusive influx of oxygen occurs on a "hard" surface (the surface of the wood, or coal), therefore blowing on it does not disrupt the *surface* through which oxygen diffuses, it only increases or decreases the concentration available for combustion.

<{POST_SNAPBACK}>

Then why does shaking a match cause the flame to go out?

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All you've done is change the reference frame of the problem: the speed of the wind relative to the match is still non-zero and still great enough to disrupt the flame surface. Notice that if you blow lightly, the match doesn't go out, and if you move the match only slightly, again it doesn't blow out. Just restate the problem in this way: picture yourself a tiny little man standing on the head of the match, is there any difference between you moving towards the wind, or the wind moving towards you?

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All you've done is change the reference frame of the problem: the speed of the wind relative to the match is still non-zero and still great enough to disrupt the flame surface.  Notice that if you blow lightly, the match doesn't go out, and if you move the match only slightly, again it doesn't blow out.  Just restate the problem in this way: picture yourself a tiny little man standing on the head of the match, is there any difference between you moving towards the wind, or the wind moving towards you?

<{POST_SNAPBACK}>

Let me try to rephrase the question. You said previously,

As for wood, the diffusive influx of oxygen occurs on a "hard" surface (the surface of the wood, or coal), therefore blowing on it does not disrupt the *surface* through which oxygen diffuses, it only increases or decreases the concentration available for combustion.

Is not a match made of wood? According to what you state, wood does not get its surface disrupted by blowing on it. Then why does the flame go out?

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Actually, matches are made of phosphorus. The combustion process of this material is different than that of wood or coal. The essential difference is that coal and wood retain some of the heat from the combustion process. While blowing out of a flame from a wood-burning process follows the same principle as that of blowing out a match flame, the wood and coal retain enough heat for the flame to be restarted easily. I should've been clearer in my first post.

I didn't mean to imply that wood- and coal-burning flames cannot be put out by the same principle, but that they are more resilient against blowout because the heat they retain allows them to restart the combustion process on the fly, while a match retains zero heat, thus you would have to restrike it to add heat by friction.

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Actually, matches are made of phosphorus.  The combustion process of this material is different than that of wood or coal.  The essential difference is that coal and wood retain some of the heat from the combustion process. While blowing out of a flame from a wood-burning process follows the same principle as that of blowing out a match flame, the wood and coal retain enough heat for the flame to be restarted easily.  I should've been clearer in my first post.

I didn't mean to imply that wood- and coal-burning flames cannot be put out by the same principle, but that they are more resilient against blowout because the heat they retain allows them to restart the combustion process on the fly, while a match retains zero heat, thus you would have to restrike it to add heat by friction.

<{POST_SNAPBACK}>

The head of the match contains phosphorus. For a short history of matches, see Matches. It is interesting to note how altruism was used by President Taft to convince the match company to give up its patent "for the good of mankind."

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I like that one.

<{POST_SNAPBACK}>

I think they were blown away by the argument.

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