Comments on: Latent heat http://www.sindark.com/2009/08/04/latent-heat/ Temporarily Torontonian Fri, 10 Feb 2012 16:08:33 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: . http://www.sindark.com/2009/08/04/latent-heat/#comment-100296 . Wed, 03 Nov 2010 15:36:58 +0000 http://www.sindark.com/?p=6110#comment-100296 Example 2: melting ice. The latent heat of melting of ice is 6 kJ/mol, or 333 kJ per kg, a quantity I have never been able to memorise... until now! Using the same trick as above, we can convert this into an equivalent temperature rise, by dividing by the heat capacity. The answer is "the latent heat of melting of ice 'is' 80 degrees C". I don't think I'll forget that number! It really brings home why mountaineers spend so much time melting snow. <a href="http://withouthotair.blogspot.com/2010/10/making-numbers-stick-desalination.html" rel="nofollow">The energy to melt the snow is roughly the same as the energy to bring the melted snow up to boiling point!</a> Example 3: vaporizing water. We can apply the same trick to the heat required to vaporize water (2258 kJ/kg). The answer is (2258 kJ/kg) / (4.2 kJ/kg/C) in C = 538 C. This number violates the "should be between 1 and 200" rule, so it is not super-memorable, but it is quite striking, isn't it - whereas near-boiling water is 373 degrees above absolute zero, the energy required to actually boil it is equivalent to another 538 degrees of temperature rise! Maybe the best way to obey the "1-200" rule is to reexpress this heat once more, comparing it to the energy required to bring the water from 0 to 100 C. It is bigger by a factor of 5.4. So "the time for the kettle to boil itself dry is about 5 times the time taken to bring it to the boil". Here ends the lesson. Example 2: melting ice. The latent heat of melting of ice is 6 kJ/mol, or 333 kJ per kg, a quantity I have never been able to memorise… until now! Using the same trick as above, we can convert this into an equivalent temperature rise, by dividing by the heat capacity. The answer is “the latent heat of melting of ice ‘is’ 80 degrees C”.
I don’t think I’ll forget that number! It really brings home why mountaineers spend so much time melting snow. The energy to melt the snow is roughly the same as the energy to bring the melted snow up to boiling point!

Example 3: vaporizing water. We can apply the same trick to the heat required to vaporize water (2258 kJ/kg). The answer is (2258 kJ/kg) / (4.2 kJ/kg/C) in C = 538 C. This number violates the “should be between 1 and 200″ rule, so it is not super-memorable, but it is quite striking, isn’t it – whereas near-boiling water is 373 degrees above absolute zero, the energy required to actually boil it is equivalent to another 538 degrees of temperature rise! Maybe the best way to obey the “1-200″ rule is to reexpress this heat once more, comparing it to the energy required to bring the water from 0 to 100 C. It is bigger by a factor of 5.4. So “the time for the kettle to boil itself dry is about 5 times the time taken to bring it to the boil”.
Here ends the lesson.

]]> By: Latent heat and storms http://www.sindark.com/2009/08/04/latent-heat/#comment-86415 Latent heat and storms Mon, 08 Feb 2010 15:14:23 +0000 http://www.sindark.com/?p=6110#comment-86415 [...] When energy is used to heat something up, the temperature does not increase smoothly as the energy is put in. Most significantly, this is because causing matter to change states takes energy in itself, above and beyond the energy that goes into warming. Imagine a big block of ice at 0°C. A lot of energy has to go into it before it becomes a pool of water at 0°C. The same is true for turning 100°C water into 100°C steam. Latent heat has been discussed here before. [...] [...] When energy is used to heat something up, the temperature does not increase smoothly as the energy is put in. Most significantly, this is because causing matter to change states takes energy in itself, above and beyond the energy that goes into warming. Imagine a big block of ice at 0°C. A lot of energy has to go into it before it becomes a pool of water at 0°C. The same is true for turning 100°C water into 100°C steam. Latent heat has been discussed here before. [...]

]]> By: Matt http://www.sindark.com/2009/08/04/latent-heat/#comment-80494 Matt Wed, 05 Aug 2009 17:06:56 +0000 http://www.sindark.com/?p=6110#comment-80494 Well, it's not the <i>same</i> and I can't find the equation to see if it's more or less. Basically, there are polynomial equations for latent heat where you can plug in the temperature and it spits out a value. The answers over small temperature ranges never vary greatly, and so my feeling is that the latent heat of vaporization at, say, body temperature is slightly more than at 100ºC. It can be helpful to view enthalpy (heat due to temperature) as kinetic energy, and latent heat as potential energy that you get back when the water (or whatever) re-condenses. Well, it’s not the same and I can’t find the equation to see if it’s more or less. Basically, there are polynomial equations for latent heat where you can plug in the temperature and it spits out a value. The answers over small temperature ranges never vary greatly, and so my feeling is that the latent heat of vaporization at, say, body temperature is slightly more than at 100ºC.

It can be helpful to view enthalpy (heat due to temperature) as kinetic energy, and latent heat as potential energy that you get back when the water (or whatever) re-condenses.

]]> By: Milan http://www.sindark.com/2009/08/04/latent-heat/#comment-80492 Milan Wed, 05 Aug 2009 12:06:56 +0000 http://www.sindark.com/?p=6110#comment-80492 Does it take as much energy to evaporate one gram of non-boiling water as to convert one gram of hundred degree water to water vapour? Does it take as much energy to evaporate one gram of non-boiling water as to convert one gram of hundred degree water to water vapour?

]]> By: Matt http://www.sindark.com/2009/08/04/latent-heat/#comment-80480 Matt Tue, 04 Aug 2009 19:52:05 +0000 http://www.sindark.com/?p=6110#comment-80480 Indeed, latent heat is a very useful thing to understand. It is the reason sweat is cooling, that it takes longer for a pot to boil dry than it does to boil, etc. Indeed, latent heat is a very useful thing to understand. It is the reason sweat is cooling, that it takes longer for a pot to boil dry than it does to boil, etc.

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