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Does a fire heat up a room hotter than it? or cool it down?

Imagine you heated a room to more than the temperature of a fire and then lit a fire in that room, would the fire heat up the room further or cool it down? I thought of this question when i noticed that you blow on hot food to cool it, and blowing on ice cream melts it.

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Categories: Domestic Science.

Tags: thermodynamics, fire, heat, icecream.

 

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0urob0ross_ says:

The fire would heat the room.  The fire is releasing chemical energy, from the fuel, into the room.  The energy is released as heat.  The room would therefore heat up further, as there is more heat-energy present in the room when the fire is lit.  To cool the room, you would need some mechanism to remove heat energy from the room, and a fire does not do this.

Blowing on hot food to cool it down works mainly by convection.  The hot food heats the air around it, and a hot air-layer is actually a pretty good insulator, serving to keep the food hot.  Admittedly, the hotter air does rise and takes some of the heat with it, so the food will cool down.  Blowing all the hot air away speeds this process up.  Using cooler air will increase the effect, but breath is cool enough compared to food we find 'hot'.  Similarly with cold food, you are blowing away a cold layer of air, and the air you blow from your lungs will be at body temperature - higher than a typical ambient room temperature.  So, blowing warm air to displace cold air causes the ice cream to melt faster.

To go back to the room question, it's instructive to think of an air conditioning system located entirely in the room.  It wouldn't cool the room, because the heat taken out of the room at one end of the conditioner would be put back into the room at the other end.  The net result would be no change.  This (and the convection reasons above) is why AC units need to have the heat-exchanger end located outside, where the heat can be disposed of without it going back into the room.

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Tags: thermodynamics, fire, heat, icecream.

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posted on 2009-08-29 22:06:58 | Report abuse

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Vilayvann says:

I agree with the ice-cream and the hot food part. However, I've never realized that there's no change in the room when the air conditioner is on. You mean, the hot air is converted to the cooler air?

I now wonder about the refridgerator. Does it take the heat energy from the food and then convert into WORK so that the food is cool?

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Tags: thermodynamics, fire, heat, icecream.

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posted on 2010-11-12 03:08:41 | Report abuse

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Georg says:

The fire would heat the room.

 

This is not the whole truth!.

1st

we needed to know what is meant by "fire".

( What do You mean with temperature of that fire?

Any flame or solid burning substance has very different

temperatures in different places)

2nd

what substace is burning there?

Example:

lets assume a hydrogen/oxygen  flame.

this flame has about 2300 °C in the hottest parts.

If we heat the room to that temperature, we will see,

that the "flame" does not heat the room to a higher

temperature, the flame will vanish!

2300 °C is the equilibrium temperature of a hydrogen flame,

at that temperature the forward reaction (burning) is as fast

as the back reaction (decomposition of water).

Such equilibrium temperatures exist for all "fires",

so the general effect of preheating the room is that

the "fire" does not heat any more.

Georg

 

 

 

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Tags: thermodynamics, fire, heat, icecream.

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posted on 2010-12-23 11:00:41 | Report abuse


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Jon-Richfield says:

I agree with everything OO said, but would like to add a point about blowing on hot food of most kinds to cool it. The cool air you blow not only displaces the insulating layer of hot air, but also replaces the water-saturated hot air with cooler, drier air that becomes even less water-saturated as it warms up on contact with the hot food. This increases the net rate of evaporation from the moist, hot food, which increases the rate of cooling; in suitable circumstances it can increase it quite drastically.

This effect would be less important in dealing with say, chocolate, which is mainly fat, or hard sugar, but most kinds of food, such as meat, soup, or baked potato, contain quite a lot of free water.

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posted on 2010-11-12 12:45:29 | Report abuse


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