# Homework Help: I need help

1. Nov 18, 2006

### AzMaphysics

Alright, I'm stuck on a few problems with a physics brain teaser sheet.

1. Suppose you have a very small kitchen with a refrigerator. Can you cool off the kitchen by leaving the refreigerator door open?
2. Which bracing for a screen is strongest: two vertical bars, one horizontal bar crossing vertical bar, one diaganol bar, or a vertical bar.
3.A man is walking on a tight rope with the aid of a balance bar. Is a long or short bar better? Is a light or heavier bar better? Is a straight or bendy bar better?

The last two I think I know, but the first one I'm not exactly sure.

I know these seem dumb but can someone please respond.

Last edited: Nov 18, 2006
2. Nov 18, 2006

### Staff: Mentor

Not dumb at all, and welcome to PF, Arizona!

-1- Do you know how a refigerator works? It's basically a heat pump. And it consumes energy from the wall plug, right? What does that mean?

-2- and -3- you already have.

3. Nov 18, 2006

### AzMaphysics

Well I thought that it would cool down the kitchen until it reached an equilibrium but that seemed almost too obvious of an answer.
For 2, I originally thought the diagonal one would be best because it creates two triangles but then I figured the the crossing one would be more stable.
For 3, I thought a long, heavy, and bendy bar would be best

4. Nov 18, 2006

### Staff: Mentor

Perfect on 2 and 3. What can you say about a "heat pump" on 1?

5. Nov 18, 2006

### AzMaphysics

Ok I think i get why it might not cool down the kitchen but I can't put it into words. Doesn't the heat pump somehow move the thermal energy from the refrigerator into the kitchen. Then if you just opened the refrigerator it wouldn't do anything but i can't explain that part.

6. Nov 18, 2006

### Staff: Mentor

The heat pump part is about how you can make a machine to pump thermal energy from one place to another, but it takes energy input to make that pumping happen. You're real close to understanding the answer to your homework question. Try a wikipedia search on refidgerator or heat pump to finish off your research.

EDIT -- apologies for any misspellings.

7. Nov 18, 2006

### AzMaphysics

Ok so I'm still not sure I have the answer. Is it because originally, before the heat pump starts to work, the area inside the refrigerator and the area outside are at equal temp but once the heat pump begins to work the refrigerator area gets cooler but makes the outside area somewhat warmer. Now here is where the question comes in. You open the refrigerator door, the area on the outside cools back down to the original temp and the area inside warms back up to the orignal temp. I don't i still think i'm wrong.

8. Nov 18, 2006

### Staff: Mentor

Definitely wiki refridgeration:

http://en.wikipedia.org/wiki/Refridgeration

and maybe heat pumps as well. A 100% efficient heat pump will pump heat energy from one place to another. A more real heat pump will take energy in and pump some delta-E from one place to another (like from the inside cooling air in a 'fridge to the hot coils on the exterior back of the 'fridge). A non-100% efficient heat pump will add extra waste heat....

Last edited: Nov 18, 2006
9. Nov 18, 2006

### MaxPolun

If you have a perfect refrigerator, that sounds about right, the question is do you have a real refrigerator, or a magical device for moving heat from one place to another? (I don't know this, it is outside of your description).

10. Nov 18, 2006

### AzMaphysics

Well I read them both, and i'm still not sure. Heat and thermodynamics have always been a weak spot for me and the fact that i'm trying to convert 2007 into all bases up to 13 probably doesn't help. I'm guessing the answer is no but have no definitive proof to explain why. At 11:00 at night (i live in Ma), wikipedia is just plain out confusing.