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Difference between revisions of "40d:Specific heat"
m (wot's a kelvingram?) |
(There is no such thing as a "degree Kelvin" - the unit is simply known as Kelvin because it is an absolute unit of measure. See http://en.wikipedia.org/wiki/Kelvin) |
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As <b>Dwarf Fortress</b> does not have a concept of insulation, <b>SPEC_HEAT</b> can be used to &ldquot;cheat&rdquot; by causing a material which <u>should</u> be quite insulative to actually take more energy to heat up, or lose more energy to cool down, than more normal materials. The result is a material which, if used in clothing, will keep a dwarf form freezing longer than normal cloth or leather would. --> | As <b>Dwarf Fortress</b> does not have a concept of insulation, <b>SPEC_HEAT</b> can be used to &ldquot;cheat&rdquot; by causing a material which <u>should</u> be quite insulative to actually take more energy to heat up, or lose more energy to cool down, than more normal materials. The result is a material which, if used in clothing, will keep a dwarf form freezing longer than normal cloth or leather would. --> | ||
| − | <b>SPEC_HEAT</b> appears to be in units of <b>J kg<sup>-1 </sup>K<sup>-1</sup></b> at <b>STP</b>; i.e. <b>1000</b> times the normally reported <b>c<sub>p</sub></b> in Joules per gram per | + | <b>SPEC_HEAT</b> appears to be in units of <b>J kg<sup>-1 </sup>K<sup>-1</sup></b> at <b>STP</b>; i.e. <b>1000</b> times the normally reported <b>c<sub>p</sub></b> in Joules per gram per Kelvin, or exactly the more rarely reported <b>c<sub>p</sub></b> in Joules per kilogram per Kelvin. |
| − | It is not understood why the game uses the <b>°C</b> or <b> | + | It is not understood why the game uses the <b>°C</b> or <b>K</b> scale here, while using the <b>°DF</b> = <b>°F + 9968</b> scale elsewhere. |
=== Sample specific heats as used by Dwarf Fortress === | === Sample specific heats as used by Dwarf Fortress === | ||
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[http://en.wikipedia.org/wiki/Specific_heat_capacity#Table_of_specific_heat_capacities] and [http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html] have tables with other materials’ specific heats. | [http://en.wikipedia.org/wiki/Specific_heat_capacity#Table_of_specific_heat_capacities] and [http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html] have tables with other materials’ specific heats. | ||
| − | References may report specific heat in Joules per gram per degree Celsius. Because Celcius and Kelvin use the same scale, differing only in their zero points, this is exactly the same as reporting in Joules per gram per | + | References may report specific heat in Joules per gram per degree Celsius. Because Celcius and Kelvin use the same scale, differing only in their zero points, this is exactly the same as reporting in Joules per gram per Kelvin. <b>J/g°C</b> is equal to <b>J/gK</b> |
Sometimes references will give specific heat in calories per gram kelvin, or calories per kilogram kelvin. To convert calories per gram kelvin to <b>DF</b> units, multiply by <b>4184</b>. To convert calories per kilogram kelvin to <b>DF</b> units, multiply by <b>4.184</b>. | Sometimes references will give specific heat in calories per gram kelvin, or calories per kilogram kelvin. To convert calories per gram kelvin to <b>DF</b> units, multiply by <b>4184</b>. To convert calories per kilogram kelvin to <b>DF</b> units, multiply by <b>4.184</b>. | ||
| − | Older references may give specific heat in B.T.U. per pound per degree Fahrenheit. 1 B.T.U. per pound per degree Fahrenheit is by definition equal to 1 calorie per gram per | + | Older references may give specific heat in B.T.U. per pound per degree Fahrenheit. 1 B.T.U. per pound per degree Fahrenheit is by definition equal to 1 calorie per gram per Kelvin. <b>Btu/lb°F</b> is equal to <b>cal/gK</b>. So just multiply by <b>4184</b> to convert to <b>DF</b> units. |
| − | Sometimes references will report specific heat of liquids or gases in Joules (or calories) per [http://en.wikipedia.org/wiki/Mole_(unit) mole] per | + | Sometimes references will report specific heat of liquids or gases in Joules (or calories) per [http://en.wikipedia.org/wiki/Mole_(unit) mole] per Kelvin. The symbol for this is <b>C<sup>p</sup></b> (with a capital C). This is less useful for our purposes, as you will need to know the molecular weight (in grams per mole) of the compound to convert it. |
:An example: the molar heat capacity of ethyl alcohol is 113 Joules per mole per degree Centigrade. The molecular weight of ethyl alcohol is 46.07 grams per mole. Divide 113 J/mol°C by 46.07 g/mol, resulting in 2.45 J/g°C. Multiply by 1000 g/kg, resulting in 2450 J/kg°C. That is our desired result in <b>DF</b> units. | :An example: the molar heat capacity of ethyl alcohol is 113 Joules per mole per degree Centigrade. The molecular weight of ethyl alcohol is 46.07 grams per mole. Divide 113 J/mol°C by 46.07 g/mol, resulting in 2.45 J/g°C. Multiply by 1000 g/kg, resulting in 2450 J/kg°C. That is our desired result in <b>DF</b> units. | ||
Note that specific <u>latent</u> heat is a different concept; do not use those values for <b>SPEC_HEAT</b>. | Note that specific <u>latent</u> heat is a different concept; do not use those values for <b>SPEC_HEAT</b>. | ||
Revision as of 15:21, 16 May 2008
The SPEC_HEAT token
The SPEC_HEAT token controls a material's specific heat capacity.
Specific heat is not a temperature. It is, more or less, the amount of energy it takes to raise the temperature of the material from one temperature to another. Gold, for instance, has a very low specific heat, so it takes very little energy to raise its temperature by one degree. Water, with a very high specific heat, takes 32 times as much energy to raise its temperature by one degree. For more information, see [1] [2] [3].
SPEC_HEAT appears to be in units of J kg-1 K-1 at STP; i.e. 1000 times the normally reported cp in Joules per gram per Kelvin, or exactly the more rarely reported cp in Joules per kilogram per Kelvin.
It is not understood why the game uses the °C or K scale here, while using the °DF = °F + 9968 scale elsewhere.
Sample specific heats as used by Dwarf Fortress
| Material | cp in J kg-1 K-1 | DF uses |
|---|---|---|
| Adamantine | n/a | 7500 |
| Aluminum | 897 | 900 |
| Copper | 385 | 385 |
| Carbon (Graphite) [*] | 710 | 409 |
| Carbon (Diamond) [*] | 519 | 409 |
| Gold | 129.1 | 129 |
| Iron, cast | ≈500 | 450 |
| Iron, pig | >500 | 500 |
| Iron, pure | 443 | 450 |
| Iron, wrought | 480-500 | 450 |
| Nickel | 444 | 444 |
| Zinc | 388 | 390 |
| Alcohol, ethyl, anhydrous | 2440 | not in raws |
| Alcohol, ethyl, 100 proof | 3856 | not in raws |
| Beef, carcass, not frozen | 2850 | not in raws |
| Charcoal | ≈ 1000 | not in raws |
| Cheese, Cheddar, aged 60 weeks | 3012 | not in raws |
| Coke | 850 | not in raws |
| Glass, crystal | ≈ 500 | not in raws |
| Glass, silica | 790 | not in raws |
| Sand | 835 | not in raws |
| Silk (silkworm?) | ≈ 1380 | not in raws |
| Silk (hornet) | ≈ 1500 | not in raws |
| Silk (spider, viscid) | ≈ 1400 | not in raws |
| Water, liquid, at STP | 4218 | not in raws |
| Water, solid, at STP | 2114 | not in raws |
| Wood [†] | 1700 to 2900 | 420, hardcoded |
[*] Note the inconsistency in the Carbon values.
[†] The specific heat of wood varies somewhat between different species. However, the variance is dominated by the moisture content of the wood. Moreover, wet wood behaves very differently below freezing. See [4]
The default value for SPEC_HEAT is 409, except for STONE types, which get 800. However, default values are not established for the various creature_product_SPEC_HEAT tokens.
[5] and [6] have tables with other materials’ specific heats.
References may report specific heat in Joules per gram per degree Celsius. Because Celcius and Kelvin use the same scale, differing only in their zero points, this is exactly the same as reporting in Joules per gram per Kelvin. J/g°C is equal to J/gK
Sometimes references will give specific heat in calories per gram kelvin, or calories per kilogram kelvin. To convert calories per gram kelvin to DF units, multiply by 4184. To convert calories per kilogram kelvin to DF units, multiply by 4.184.
Older references may give specific heat in B.T.U. per pound per degree Fahrenheit. 1 B.T.U. per pound per degree Fahrenheit is by definition equal to 1 calorie per gram per Kelvin. Btu/lb°F is equal to cal/gK. So just multiply by 4184 to convert to DF units.
Sometimes references will report specific heat of liquids or gases in Joules (or calories) per mole per Kelvin. The symbol for this is Cp (with a capital C). This is less useful for our purposes, as you will need to know the molecular weight (in grams per mole) of the compound to convert it.
- An example: the molar heat capacity of ethyl alcohol is 113 Joules per mole per degree Centigrade. The molecular weight of ethyl alcohol is 46.07 grams per mole. Divide 113 J/mol°C by 46.07 g/mol, resulting in 2.45 J/g°C. Multiply by 1000 g/kg, resulting in 2450 J/kg°C. That is our desired result in DF units.
Note that specific latent heat is a different concept; do not use those values for SPEC_HEAT.