# Homework Help: I am stuck on 3 questions =/

1. Dec 11, 2005

### Goalcard

1. Heat Transfer
A copper slug whose mass mc is 75g is heated in a laboratory oven to a temperature T of 3120C. The slug is then dropped into a glass beaker containing a mass mw =220g of water. The heat capacity Cb of the beaker is 45 cal/K. The initial temperature Ti of the water and the beaker is 120C. What is the final temperature Tf of the slug, the beaker, and the water when thermal equilibrium is reached?

Would this look about right?
mc(water)+mc(copper calorimeter) +mc (beaker) = 0.

2. How to Make Ice Without a Refrigerator
On a clear moonless night, the sky acts like a blackbody radiator at a temperature Ts=
-230 C. You thermally insulate a container from the ground (by placing a poorly conducting layer of straw beneath it). Then you pour water into the container, forming a thin, uniform layer with mass m=4.5 g, with surface area A = 9.0 cm2 and depth= 5.0 mm. The emissivity is .9, and initial temperature 6.00C. Find the time required for the water to freeze via radiation.

On this one I am super lost, no idea from where to start.

3. The Ideal Gas Law
A cylinder contains 12 L of oxygen at 200C and 15 atm. The temperature is raised to 350C, and the volume is reduced to 8.5 L. What is the final pressure of the gas in atmospheres?

Same as the other one, if you guys/gals can point me to the right direction formula wise that would be great.

2. Dec 12, 2005

### Hermite

I didn't have time to read 1 and 2 but for 3 you should be able to use

intial PV/T = final PV/T

3. Dec 12, 2005

### Goalcard

Does this look about right?
P(12)/200=P(8.5)/350

4. Dec 13, 2005

### Goalcard

yes no maybe so?

5. Dec 13, 2005

### Ouabache

you're on the right track.. How about including your initial pressure $P_i$ and solving for final pressure $P_f$

6. Dec 13, 2005

### Goalcard

final pressure would be 37.059?

7. Dec 13, 2005

### Goalcard

Yeah? any tips on the first two?

8. Dec 13, 2005

### Goalcard

Maybe throw me a formula, that would help me a lot, it's due tomorrow(Wednesday)

9. Dec 14, 2005

### Ouabache

Looks good to me..

On the first two, if I were assigned these questions, instead of guessing which formula to use, I would review material on heat transfer and black body radiation. Here is a starting point ref