Need help with science homework

[cassandra and sara's daddy]

Kids question was if its cold outside and the window glass is cold is it because of the cold conducting to the glass? Or the heat conducting away from glass to the cold outside.
I went with second answer based on the concept that heat is energy and as such it goes towards the cold( absence of energy)
What say you better educated folks.


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[mtnbkr]

Heat is conducted out.

Chris

[MechAg94]

I thought that was why materials that are good heat conductors often feel colder than those that are not.  They pull the heat out of your skin and conduct it away so it continues to feel cold.

[Hawkmoon]

Both answers above are correct. Heat transfer ALWAYS goes from the warm to the cold. Not to overly complicate the question, but then you get into the three modes of heat transfer: conduction, convection, and radiation.

In the case of window glass, I think the transfer would be by radiation, not conduction. But I'm not certain -- it was a VERY long time ago when I studied that stuff.

[zxcvbob]

Heat is a form of energy, cold is not.  So the right answer is the heat is conducted away.

[Hawkmoon]

Heat is a form of energy, cold is not.  So the right answer is the heat is conducted away.

Yeah.

It's like light. When you open the closet door, does the light rush in or does the dark rush out?

[Nick1911]

Everyone else nailed it.  Heat is conducted from hot to cold.  That is, there is a movement of heat energy in an effort to reach equilibrium.

Because the question asks about the glass itself being cold - the mechanism is conduction.

[MechAg94]

Probably some radiant heat transfer also, but I imagine conduction is paramount. 

[Firethorn]

If it's for a kid they may not be to the point of covering the differences between conduction, convection, and radiation.

As stated, heat's energy so what's making the glass cold is the heat conducting away, whether that's conduction or radiation. 

Also, science problems tend to be simplified, ergo they're probably considering single pane glass, and referring to all heat transfer as 'conducting'.

[230RN]

Ohm's law has many parallels in heat, air, water, etc., conduction.

For water:
Volume of water flowing = pressure difference / pipe resistance

For air:
Volume of air flowing = pressure difference / resistance of trees and houses in the way

For heat:
Amount of heat flow = temperature difference / insulation quality

For electricity:
Quantity of electrons flowing = voltage difference / resistance of conductor

Given, in each case, the approriate units are used.  Note the "etc." above.

Jes' pointin' out the obvious.

[birdman]

I thought that was why materials that are good heat conductors often feel colder than those that are not.  They pull the heat out of your skin and conduct it away so it continues to feel cold.

If you ever get a chance to feel a strip of pitch-based ultra-high thermal conductivity carbon fiber, do so.  We used to call K1100X based laminates "black ice" because they had such high conductivity (about double that of copper) the thermal diffusivity was so fast if you touched the end you weren't hokding to something cold, you'd feel it almost instantly.  Basically, they always felt cold because it would just act like a huge fin sucks your body heat out of your fingers into the air.

I assume a big strip of diamond would feel similar...but a wee bit harder to get.

[Chuck Dye]

Depending on kiddo's level, it may be worthwhile to look at coefficient of thermal conductivity.  The hot chocolate in the ceramic mug versus the enamelware pannikin was my introduction at about six years old.

[KD5NRH]

If you ever get a chance to feel a strip of pitch-based ultra-high thermal conductivity carbon fiber, do so.  We used to call K1100X based laminates "black ice" because they had such high conductivity (about double that of copper) the thermal diffusivity was so fast if you touched the end you weren't hokding to something cold, you'd feel it almost instantly.  Basically, they always felt cold because it would just act like a huge fin sucks your body heat out of your fingers into the air.

Sounds insanely useful as a heatsink.  How hard is it to get one's hands on some of this stuff?  Have any data on the other physical properties?

[230RN]

^ Ditto.  Sounds ideal for Peltier coolers.  I've got a little ~0.5 cu ft. Peltier fridge where the radiator doesn't reject heat fast enough, and it ends up going right back to the interior of the box because the whole junction gets too hot.

Takes an enormous amount of current at 12V, but can't get the box below about ~50°F.  Wouldn't mind 'sperimenting with a better heat sink.

I bought it from somebody for $2.50 just for playing around  purposes.  Wouldn't mind upgrading it to actual usefulness, though.  Yes, I tried an outboard fan.  Right now, I'm just using it to store  a lb of BP in. 

[zxcvbob]

^ Ditto.  Sounds ideal for Peltier coolers.  I've got a little ~0.5 cu ft. Peltier fridge where the radiator doesn't reject heat fast enough, and it ends up going right back to the interior of the box because the whole junction gets too hot.

Takes an enormous amount of current at 12V, but can't get the box below about ~50°F.  Wouldn't mind 'sperimenting with a better heat sink.

I bought it from somebody for $2.50 just for playing around  purposes.  Wouldn't mind upgrading it to actual usefulness, though.  Yes, I tried an outboard fan.  Right now, I'm just using it to store  a lb of BP in. 

I've got one of those that I dug out of a dumpster.  I assume it has a bad power supply (cheap Chinese electrolytic capacitors.)  The Peltier device and heat sink/fan looks useful for some kind of brewing project.

[230RN]

^
Here's a pic of mine FWIW with power supply for scaling, 5-7/8" long dimension. Brand is "Cool It. personal refrigerator."

Maker of power supply is DVE, model DSA-0501-12.  Power supply output is 12V 4.1A.  (I told you it took a humongous amount of power.)  It's a switching power supply and my notes say it oscillates on and off at 3Hz with or without load.

You can run it off a car battery, which was my original intent, but the input jack is polarized.  If this one is like yours and you need to know which is +, let me know, but I have to take the back off to figure that out.

The "BP" stands for.... well, you know.

Had to excavate it out of the back of my reloading closet.... tsk-tsk, the things I have to do for youse guys!

Terry

[KD5NRH]

You can run it off a car battery, which was my original intent, but the input jack is polarized.  If this one is like yours and you need to know which is +, let me know, but I have to take the back off to figure that out.

Peltiers are reversible; hook it up one way (at reduced current if you want) and one side gets cooler while the other gets hotter.  Swap polarity and it does the opposite.  Thus, it's easy to determine the correct polarity experimentally.

[230RN]

Good to remind me of that, thanks.  Been a while since I played with them (it).

I guess the only way to make it more effective (not more efficient) would be to stack them so the second one pulls heat off the first, then go to the heat sink/fins.  Or use a power supply that puts out cold electrons.

I figure I got a good little BP magazine and a little learnin' for only $2.50.

[zxcvbob]

Whenever I get around to playing with it, I'm going to look up the specs on the P device to see what voltage it takes.  I've got a bunch of assorted laptop power supplies in various voltages; one of them ought to work.  Not sure if I'll leave it in the Black and Decker minifridge, or take it apart and use it to cool a carboy-wrapped-in-fiberglass for lagering beer.

It's not very high on my todo list, so it might take a few months before I get to it.

[230RN]

"one of them ought to work."

4.1 Amps, don't forget.

[birdman]

Sounds insanely useful as a heatsink.  How hard is it to get one's hands on some of this stuff?  Have any data on the other physical properties?

It's hard to get it in cloth form, like -really- hard.  You can get a spool of 2k tow and wrap around something covered in non-permeable teflon to make a unidirectional cloth, then impregnate, vacuum compress and go from there.  But at that point, there are many other possibilities that are easier.

But since you asked
http://www.ebay.com/itm/Amoco-Performance-Products-CyTec-K1100-Thornel-Carbon-Fiber-1200-ft-/181058381054

Note, that is $1000 A POUND

[230RN]

^ "Note, that is $1000 A POUND"

If it saves one child Joule, it's worth it.

Terry runs and hides.  Again.

[KD5NRH]

Note, that is $1000 A POUND

At that price, there should be someone on a street corner somewhere with small lots...