Thermodynamics B/C

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Justin72835
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Re: Thermodynamics B/C

Postby Justin72835 » February 14th, 2018, 7:05 pm

Alright. A cup of tea cools from 170 degrees Fahrenheit to 145 degrees Fahrenheit in 6 minutes in a room at 72 degrees Fahrenheit. How long will it take for the tea to cool from 100 to 90 degrees Fahrenheit in the same room?
Using Newton's Law of Cooling, we get T = Ts + (T0-Ts)e^kt. Plugging in our data gives us a k value of -4.91E-2. We can set up this equation again with 100 to 90 degrees, and calculate a t of 9.0 minutes.
Great job! Your turn!
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Re: Thermodynamics B/C

Postby Riptide » February 14th, 2018, 7:10 pm

A copper can, of negligible heat capacity, contains 1.000 kg of water just above the freezing point. A similar can contains 1.000 kg of water just below the boiling point. The two cans are brought into thermal contact. Find the change in entropy of the system. (Assume for water cm = 4184 J/K)
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Re: Thermodynamics B/C

Postby Justin72835 » February 15th, 2018, 1:41 pm

A copper can, of negligible heat capacity, contains 1.000 kg of water just above the freezing point. A similar can contains 1.000 kg of water just below the boiling point. The two cans are brought into thermal contact. Find the change in entropy of the system. (Assume for water cm = 4184 J/K)
Too lazy to put work, but since the masses are the same, you can use the equation mcln(Tf/Ti) for both volumes of water to get an answer of 101J/K.
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But in ourselves, that we are underlings."


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Re: Thermodynamics B/C

Postby Riptide » February 15th, 2018, 1:43 pm

A copper can, of negligible heat capacity, contains 1.000 kg of water just above the freezing point. A similar can contains 1.000 kg of water just below the boiling point. The two cans are brought into thermal contact. Find the change in entropy of the system. (Assume for water cm = 4184 J/K)
Too lazy to put work, but since the masses are the same, you can use the equation mcln(Tf/Ti) for both volumes of water to get an answer of 101J/K.
You're on a roll! That's right
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Re: Thermodynamics B/C

Postby Schrodingerscat » February 15th, 2018, 5:35 pm

PS: I added answers to all but part g for my question.

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Re: Thermodynamics B/C

Postby JT880 » February 18th, 2018, 11:34 am

Since nobody is going, I though I'd jump in!

Sorry if this is too easy: If 50.0mL of water at a temperature of 60 degrees Celsius is heated at a steady rate of 5000W, how long will it take to reach its boiling point?
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Re: Thermodynamics B/C

Postby Justin72835 » February 18th, 2018, 11:46 am

Since nobody is going, I though I'd jump in!

Sorry if this is too easy: If 50.0mL of water at a temperature of 60 degrees Celsius is heated at a steady rate of 5000W, how long will it take to reach its boiling point?
1.7 seconds?
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But in ourselves, that we are underlings."


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Re: Thermodynamics B/C

Postby JT880 » February 18th, 2018, 3:33 pm

Since nobody is going, I though I'd jump in!

Sorry if this is too easy: If 50.0mL of water at a temperature of 60 degrees Celsius is heated at a steady rate of 5000W, how long will it take to reach its boiling point?
1.7 seconds?
Correct! Guess I should have made it a bit harder. Your turn!
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Re: Thermodynamics B/C

Postby Justin72835 » February 18th, 2018, 4:09 pm

Correct! Guess I should have made it a bit harder. Your turn!
Here's another slightly remsen-esque question :D

The emissivity of tungsten is 0.350. A tungsten sphere with radius 2.50 cm is suspended within a large evacuated enclosure whose walls are at 300.0 K. What power input is required to maintain the sphere at a temperature of 4500.0 K if heat conduction along the supports is neglected?
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But in ourselves, that we are underlings."


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Re: Thermodynamics B/C

Postby MattChina » February 18th, 2018, 5:06 pm

Correct! Guess I should have made it a bit harder. Your turn!
Here's another slightly remsen-esque question :D

The emissivity of tungsten is 0.350. A tungsten sphere with radius 2.50 cm is suspended within a large evacuated enclosure whose walls are at 300.0 K. What power input is required to maintain the sphere at a temperature of 4500.0 K if heat conduction along the supports is neglected?
5.54*10^8 watts?
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