I think density is the key here, but what do I know .
Assuming the three bars have the same volume but are obviously different densitys. Then the Iron will contain more energy than the copper and aluminium at the same temperature. equilibrium, i.e. heat transfer will stop when your hand reaches the same temperature as the bar. If this temperature is the same for all three bars then the iron will have transfered more energy and done more damage.
Will the equilibrium temperature be the same?
Regards
John.
0
Straight_ManGeeky, in my own wayNaples, FLIcrontian
edited August 2004
Pythagoras:
Cast iron dutch ovens and cast iron style pans are used because they spread heat more evenly throughout the metal than other metals. But, they hold heat also, in the thick metal. They do not shed heat when air blows across thier outer surfaces, as fast as other metals. Downside, though, is that they OXIDIZE. Oxide layer keeps them from accepting heat as well as getiing rid of it (shedding it). so, for cooking and wood stoves that are kept relatively rust free or are OILED with cooking oil to protect them from oxidizing they are good.
You are talking potential, yes, iron if not ozide corroded can ABSORB (and, unfortunately, also HOLD) more heat than other metals, it also melts at higher temp. But, for computers you need the ability to also release heat into air or into a Peltier surface, or into water or coolant. Second, iron tends to spread heat evenly and not hyper FAST. It is too dense and too ionically stable relatively, for what is needed. Copper actully XMITs or spreads heat well, but it also oxidizes easily. And the oxidization coating acts as insulator as far as both accepting and shedding heat. The side in contact with the HS compound will not oxidize, but the rest will in damp air.
Density can HELP rule how much in KCals (or MCals) can be absorbed by a given cubic, yes. But release to air flowing by is another story indeed. If it cannot flow heat fast, it tends to not release heat well. Ironically, what can be induced to accept electric flow quickly and have the charge dropped to zero quickly also is in theory the best for quick heat dispersion and shedding of surface heat into a colder surface that is not metal itself.
Increasing density can be inversely related to how well the metal or semimetallic compound sheds heat at high speed. A GRANITE rock in the desert can get quite hot, and burn soft skin, or if hot enough desert you CAN fry an egg on a hot rock, but after a cool night the same rock might still be warm-- it will not be 45 F in the rock (as the air can be in Northern New Mexico) come morning if it was 100 F outside in sun for most of previous day (and that temp for most days daytime time ranges for previous month), it will be more like 50-55 F, ie warmer than the air temp around it. It holds heat too well for a high flow-thought-and-out-of-needed cooling solution. But Granite and obsidian are incredibly dense. So is glass-- glass does not accept lots of heat fast, either (ceramics can be made that DO, but I am thinking clear glass). None shed heat well in a relatively short time.
Am I hijacking the thread??? Not really, I am trying to tie it back to cooling our toys to promote info feed in it and interest in it. Cooling thermodynamics is hugely interesting to me, if we can get some practical ideas flowing such that we get better cooling solutions. Lets add PRACTICE to this thread. So far, it could make a very good intro to the science of cooling our tech toys.
Missleman, I think your confusing issues here
"Copper conducts heat better than aluminum does; that is, copper moves heat faster than aluminum does. This myth that aluminum radiates heat better than copper probably got started by someone who noticed that an aluminum heatsink cools down faster than a copper one does."
You have two separate things going on, conductivity ("copper moves heat faster") and emissivity ("aluminum radiates heat better"). For conductivity I get these numbers (metric units);
This seems to say that copper (oxidized) would still be better for fins on a HS. For an illustration of emissivity, think of a black marble/granite surface (or basically any black surface). Inside with no heat source, the surface will get really cool, lower than the air temp even. This is due to a high emissivity.
You also have to worry about specific heats;
Aluminum = 915
Copper = 390
Iron = 450
This means the aluminum can hold more heat than copper on a per kg basis (which is why AL will burn you the worst with the same weight, the same size bar? maybe/ maybe not), but then the same size sink in AL will weigh less.
In convection cooling (actually just conduction and radiation combined), I would think a polished copper base and a rough or oxidized copper fin arrangement would still be the best. You get both high conductivity, high emissivity, but moderate specific heat (which may actually be good since you need some Delta T to get efficient convection cooling and copper weighs more than AL).
just some quick thoughts after looking at a thermo text.
Comments
Welcome to short-media!
Assuming the three bars have the same volume but are obviously different densitys. Then the Iron will contain more energy than the copper and aluminium at the same temperature. equilibrium, i.e. heat transfer will stop when your hand reaches the same temperature as the bar. If this temperature is the same for all three bars then the iron will have transfered more energy and done more damage.
Will the equilibrium temperature be the same?
Regards
John.
Cast iron dutch ovens and cast iron style pans are used because they spread heat more evenly throughout the metal than other metals. But, they hold heat also, in the thick metal. They do not shed heat when air blows across thier outer surfaces, as fast as other metals. Downside, though, is that they OXIDIZE. Oxide layer keeps them from accepting heat as well as getiing rid of it (shedding it). so, for cooking and wood stoves that are kept relatively rust free or are OILED with cooking oil to protect them from oxidizing they are good.
You are talking potential, yes, iron if not ozide corroded can ABSORB (and, unfortunately, also HOLD) more heat than other metals, it also melts at higher temp. But, for computers you need the ability to also release heat into air or into a Peltier surface, or into water or coolant. Second, iron tends to spread heat evenly and not hyper FAST. It is too dense and too ionically stable relatively, for what is needed. Copper actully XMITs or spreads heat well, but it also oxidizes easily. And the oxidization coating acts as insulator as far as both accepting and shedding heat. The side in contact with the HS compound will not oxidize, but the rest will in damp air.
Density can HELP rule how much in KCals (or MCals) can be absorbed by a given cubic, yes. But release to air flowing by is another story indeed. If it cannot flow heat fast, it tends to not release heat well. Ironically, what can be induced to accept electric flow quickly and have the charge dropped to zero quickly also is in theory the best for quick heat dispersion and shedding of surface heat into a colder surface that is not metal itself.
Increasing density can be inversely related to how well the metal or semimetallic compound sheds heat at high speed. A GRANITE rock in the desert can get quite hot, and burn soft skin, or if hot enough desert you CAN fry an egg on a hot rock, but after a cool night the same rock might still be warm-- it will not be 45 F in the rock (as the air can be in Northern New Mexico) come morning if it was 100 F outside in sun for most of previous day (and that temp for most days daytime time ranges for previous month), it will be more like 50-55 F, ie warmer than the air temp around it. It holds heat too well for a high flow-thought-and-out-of-needed cooling solution. But Granite and obsidian are incredibly dense. So is glass-- glass does not accept lots of heat fast, either (ceramics can be made that DO, but I am thinking clear glass). None shed heat well in a relatively short time.
Am I hijacking the thread??? Not really, I am trying to tie it back to cooling our toys to promote info feed in it and interest in it. Cooling thermodynamics is hugely interesting to me, if we can get some practical ideas flowing such that we get better cooling solutions. Lets add PRACTICE to this thread. So far, it could make a very good intro to the science of cooling our tech toys.
They have a rude but good argument going a few posts down.
Now for the goodies thermal conductivity chart
http://www.aml.arizona.edu/classes/mse222/1997_diamond/thermal.htm
"Copper conducts heat better than aluminum does; that is, copper moves heat faster than aluminum does. This myth that aluminum radiates heat better than copper probably got started by someone who noticed that an aluminum heatsink cools down faster than a copper one does."
You have two separate things going on, conductivity ("copper moves heat faster") and emissivity ("aluminum radiates heat better"). For conductivity I get these numbers (metric units);
Aluminum = 206
Copper = 380
Iron = 70
Now for emissivity;
Aluminum = 0.25
copper, polished = 0.02
Copper, oxidized = 0.7
Iron, polished = 0.15
Iron, oxidized = 0.7
This seems to say that copper (oxidized) would still be better for fins on a HS. For an illustration of emissivity, think of a black marble/granite surface (or basically any black surface). Inside with no heat source, the surface will get really cool, lower than the air temp even. This is due to a high emissivity.
You also have to worry about specific heats;
Aluminum = 915
Copper = 390
Iron = 450
This means the aluminum can hold more heat than copper on a per kg basis (which is why AL will burn you the worst with the same weight, the same size bar? maybe/ maybe not), but then the same size sink in AL will weigh less.
In convection cooling (actually just conduction and radiation combined), I would think a polished copper base and a rough or oxidized copper fin arrangement would still be the best. You get both high conductivity, high emissivity, but moderate specific heat (which may actually be good since you need some Delta T to get efficient convection cooling and copper weighs more than AL).
just some quick thoughts after looking at a thermo text.