Can someone explain this?

[UIC]

---

Members don't see this ad.

If we put a running refrigerator in a small closet, close the closet door and open the refrigerator door, the closet will warm. Anyone know why?

 

[Deeder03]

I gather it has something to do with how gases expand etc. The phrase expanding gases cool, compressed gases warm?

http://www.explainthatstuff.com/refrigerator.html

what do we know about the closet itself?

 

[Isoprop]

Thermodynamics. In simple terms, it takes work to move heat from a cold object to a hot object. That work has to come from outside the system. So if the closet is prefectly isolated, then no energy can go into our closet system, and work can't be done to cool the closet.

 

[rls303]

Thermodynamics. In simple terms, it takes work to move heat from a cold object to a hot object. That work has to come from outside the system. So if the closet is prefectly isolated, then no energy can go into our closet system, and work can't be done to cool the closet.

So is this the type of scenario where W=PV? Where we would need to change the pressure and volume of the closet to see any change in temperature? LOL, sorry if that sounds random

Thanks in advance!

 

[fizzgig]

i don't think so. basically the refridge is a machine, not a box of magic, so the absolute best it could do in theory would be to pull heat from air on one side (the open refridging side) and dump it at its rear. air would equalize, it would do it again. it would just be moving heat around but it can't get RID of it if the closet is isolated from heat transfer.

then we get back to reality and the fridge is way less than 100% efficient. it is doing work - using electricity to power a pump that compresses and allows expansion of a fluid so it can gather heat from spot1 and dump it in spot2. since it does this inefficiently it generates extra heat in the process, which also gets dumped into the closet. hence T rises over time.

 

[rls303]

i don't think so. basically the refridge is a machine, not a box of magic, so the absolute best it could do in theory would be to pull heat from air on one side (the open refridging side) and dump it at its rear. air would equalize, it would do it again. it would just be moving heat around but it can't get RID of it if the closet is isolated from heat transfer.

then we get back to reality and the fridge is way less than 100% efficient. it is doing work - using electricity to power a pump that compresses and allows expansion of a fluid so it can gather heat from spot1 and dump it in spot2. since it does this inefficiently it generates extra heat in the process, which also gets dumped into the closet. hence T rises over time.

So the cold reservoir disappears after some time? and we just have a heat reservoir that exists?

This whole concept of machines is so confusing for me! I did an EK passage today regarding the Carnot theorem, I totally got owned! Is this something very big on the MCAT ( I know everything is relevant, but I have not seen soemthing this hard on any AAMC passages). BTW, it is EK GChem Book Page 139 if any of you are interested it is Passage II.

Thanks!

 

[fizzgig]

the fridge will become less efficient i would think as the closet heats up.

the way a fridge works is a fluid is allowed to expand in the wantcold area, and when it expands it cools. if it expands enough to be cooler than the air in the fridge, it will absorb heat. then it gets moved to the rear of the fridge and compressed a lot. this heats it up. if it is heated up to a temp higher than ambient, it will dump heat to the air in your kitchen. then it moves back to the inside of the fridge and expands, cools, and grabs some more heat. rinse n repeat.

as the temp goes up over time (due to you putting electrical to mechanical work into the system to drive the compressor), since the door is open, i guess the expanding fluid on the inside will probably have little problem collecting heat, but as the temp rises, the compressed/heated fluid at the rear of the fridge is trying to dump heat to an ever hotter ambient air pool. since heat transfer is directly related to the gradient of temperature, you can see over time the fridge will get worse and worse at dumping heat, and if it doesn't get rid of as much heat, it won't get to as low of a temp when it expands, so the whole deal loses efficiency.

that's my general understanding.

 

[ElliottB]

A fridge is just a reverse heat engine. Like was stated earlier, energy/work is required to move heat. This should help....

http://physics.bu.edu/~duffy/py105/Heatengines.html