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Questions...Not as much

by Security
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Security
#1
Dec3-03, 07:52 PM
P: 59
On a hot day, it is 90 degrees in your friend Eustace's kitchen. Eustace decides to close all the doors and windows so no further heat transfer occurs between the outdoors and his apartment, and then he opens his refrigerator. Will the refrigerator cool down the kitchen? Why or why not? Describe the flow of hear in the room after the refrigerator door is opened. (My Teacher says not to consider the windows for this questions and the additional questions that follow.)

A.) After 5 minutes, would the air in the refrigerator be changing temperature less quickly, more quickly, or at the same rate as it did when the refrigerator's door was first opened? Why? (Btw, all my answers to these questions were wrong and long, so thats why I am not posting the them. My Answer: It would be changing more quickly because it would now be trying to draw the energy out of objects in the kitchen to make it cold.

B.) If the refrigerator is unplugged, what will eventually happen to the flow of heat in the room? My Answer: it would only come from the window, not from refrigerator coils. Rethought Question it might be: 'the hear would flow throught the room until it reaches a thermal equilibrium.'
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chroot
#2
Dec3-03, 07:59 PM
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Originally posted by Security
Will the refrigerator cool down the kitchen? Why or why not?
If no heat flow occurs between his apartment and the outside world, then there is no way the room can get colder. The refrigerator cools down the space inside it and dumps the waste heat to the room. Because it's not 100% efficient, it actually exhausts more heat into the room than it removes from the space inside it.
A.) After 5 minutes, would the air in the refrigerator be changing temperature less quickly, more quickly, or at the same rate as it did when the refrigerator's door was first opened?
Temperature equalization happens exponentially. It starts off with large heat transfer (large temperature changes) and slows down the closer the two temperatures are.
B.) If the refrigerator is unplugged, what will eventually happen to the flow of heat in the room?
You already said the room was insulated: no heat transfer occurs between the room and the outside world. Thus, the room must stay the same temperature.

- Warren
Security
#3
Dec3-03, 08:01 PM
P: 59
So, is less, good?

chroot
#4
Dec3-03, 08:04 PM
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Questions...Not as much

Originally posted by Security
So, is less, good?
I have no idea what this means.

- Warren
Security
#5
Dec3-03, 08:05 PM
P: 59
Are less questions more suitable for the forum?
chroot
#6
Dec3-03, 08:19 PM
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Originally posted by Security
Are less questions more suitable for the forum?
You can ask any sort of question you like -- but it doesn't mean I will necessarily answer every single one! Someone almost certainly will, though.

- Warren
Security
#7
Dec3-03, 08:31 PM
P: 59
In 1783 the Montgolfier brothers of France launched what is possibly the first balloon flight carrying passengers - - a duck, a rooster and a sheep. Their balloon, about 35 feet in diameter and constructed of cloth lined with paper, was launched by filling it with smoke. The flight landed safely some 8 minutes later. Explain the physics of this flight (that is, its ascent, descent and landing) in terms of what you have learned in this unit.

In warm climates where the temperature occasionally drops to 0* C at night, harm to tropical plants can be reduced by watering the plants in the evening rather than earlier in the day. Why would watering the plants - - and specifically watering them in the evening -- help protect them from freezing?

- -

I got these couple of answers right, Im just posting them for reference:

Lake Michigan (the only Great Lake which is entirely within the boudaries of the United States and the largest body of fresh water in the U.S.) is located in a relatively cold climate. it is 494 kilometers long and 190 kilometers at its widest, and reaches a depth of 281 meters.

A. On January 1, 1900, what was probably the temperature at the bottom of Lake Michigan? The temperature would be somewhat higher than 4*C
B.Would you expect the temperature to be different at the bottom of Lake Michigan on August 17, 1925? Why or why not? It would probably be around the same temperature in 1900, because of the size of the Lake and poor ability to conduct heat

Help on these questions:

C. During the winter months, the entire lake is not coverd with ice, but near the shore, ice does from sufficiently thick for ice fishing. Explain.
D. Does the lake need to absorb or lose energy for the ice to melt? How much of an energy change must the lake undergo for each gram of ice melted?
E. Would the process of the ice melting change the temperature of the lake? Why or why not?


The specific heat of mercury is .03 cal/g*C, and its boiling point is 357*C. The specific heat of water is 1 cal/g*C. It takes 65 calories of energy to vaporize one gram of mercury and 540 calories to vaporize 1 gram of water. If both substance begin at room temperature (about 22*C), does it take more cenergy to boil a gram of mercury or a gram of water? Show Your calculations.

Hint from Teacher: ^q=mc^T+mL

What is the ideal efficiency of heat engine with a hot reservoir of 500 K and a heat sink of 300 K? Show your calculations.


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