Thermodynamics B/C

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Re: Thermodynamics B/C

Postby UTF-8 U+6211 U+662F » April 12th, 2018, 3:26 pm

atm, Torr, and mmHg are also units that are used fairly often in the SI system to describe pressure, so all of them should be acceptable. Unfortunately not always the case.

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Re: Thermodynamics B/C

Postby jinhusong » April 15th, 2018, 11:35 am

I was helping Thermodynamics B in Northern California State yesterday. It is my first time take part in this event. Here are some thoughts I hope can help students improve the score.

The beaker, prefer better heat conductivity ones. So glass is better than plastic. thinner is better than thicker. If you think it does not matter, think again, like go to the extreme, assume both your beaker already have very high isolation.

Beaker bottom plat is better so that the outside one have better contact with the table normally better heat conductivity and capacity.

Please study the rules. Many many groups only have charts with 7 points. Rule asked at least 10 points.

If your device exceeds the size limit with the lid, try cut it or take the lid off so your device will not be put into tier 2.

Best wishes and good luck.

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Re: Thermodynamics B/C

Postby Alex-RCHS » April 15th, 2018, 1:31 pm

For those who have gone to lots of competitions, is the ice bonus normally administered so that you can measure the temperature of the water in the beaker immediately after adding the cold water? That's what I imagine it is, but the rules aren't super clear.
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Re: Thermodynamics B/C

Postby TheChiScientist » April 15th, 2018, 4:36 pm

Ikr that part is vague but I have rarely seen this in action cuz no one goes for the IWB.
Still, I feel like that part of the rules depends on your ES cuz some don't allow it.
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Re: Thermodynamics B/C

Postby Vrund » April 16th, 2018, 10:58 pm

Had a question about the way thermo would be ran at nationals? So for the initial temperature of the water would it be measured before it is poured into the internal beaker or after? (cuz I've seen it be measured before at invitationals but would just like to make sure) and then for the ice water bonus do you get to know the initial temperature after the ice water is added?

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Re: Thermodynamics B/C

Postby TheChiScientist » April 17th, 2018, 5:34 am

You would need to find the National ES and ask him yourself. Each ES is different.
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Re: Thermodynamics B/C

Postby Vrund » April 17th, 2018, 8:27 am

TheChiScientist wrote:You would need to find the National ES and ask him yourself. Each ES is different.


Have they released any info of who the National ES is going to be?

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Re: Thermodynamics B/C

Postby TheChiScientist » April 17th, 2018, 8:28 am

Tbh not that I am aware of. I would PM Unome as he is more aware of this kind of stuff.
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Re: Thermodynamics B/C

Postby CookiePie1 » April 17th, 2018, 1:00 pm

Vrund wrote:
TheChiScientist wrote:You would need to find the National ES and ask him yourself. Each ES is different.


Have they released any info of who the National ES is going to be?


Maybe also PM chalker. He is a nat'l supervisor and he's on the physics commitee.
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Re: Thermodynamics B/C

Postby JoeyC » April 17th, 2018, 5:02 pm

HI guys! I just got the BDJH regional test, and looking at the one for C division I just can't figure out how to solve this.
1.)You have a piston filled with 8 moles of a monatomic ideal gas at a temperature of 278 K. It is completely insulated (meaning that there is no heat transfer between the walls of the piston) and it is at rest in a vacuum. The piston has a diameter of 25 cm and has a mass of 4.5 kg. You then apply 50 N of force to the top of the piston.

a. (8 points) Determine the change in temperature of the gas:

Answer:98K

Does anyone know what formula they used, and how they used it; Ideal gas law doesn't work with both volume and temperature changing after compression.
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Re: Thermodynamics B/C

Postby UTF-8 U+6211 U+662F » April 17th, 2018, 6:47 pm

JoeyC wrote:HI guys! I just got the BDJH regional test, and looking at the one for C division I just can't figure out how to solve this.
1.)You have a piston filled with 8 moles of a monatomic ideal gas at a temperature of 278 K. It is completely insulated (meaning that there is no heat transfer between the walls of the piston) and it is at rest in a vacuum. The piston has a diameter of 25 cm and has a mass of 4.5 kg. You then apply 50 N of force to the top of the piston.

a. (8 points) Determine the change in temperature of the gas:

Answer:98K

Does anyone know what formula they used, and how they used it; Ideal gas law doesn't work with both volume and temperature changing after compression.

Hint: Adiabatic processes are polytropic.

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Re: Thermodynamics B/C

Postby Alex-RCHS » April 17th, 2018, 7:21 pm

UTF-8 U+6211 U+662F wrote:
JoeyC wrote:HI guys! I just got the BDJH regional test, and looking at the one for C division I just can't figure out how to solve this.
1.)You have a piston filled with 8 moles of a monatomic ideal gas at a temperature of 278 K. It is completely insulated (meaning that there is no heat transfer between the walls of the piston) and it is at rest in a vacuum. The piston has a diameter of 25 cm and has a mass of 4.5 kg. You then apply 50 N of force to the top of the piston.

a. (8 points) Determine the change in temperature of the gas:

Answer:98K

Does anyone know what formula they used, and how they used it; Ideal gas law doesn't work with both volume and temperature changing after compression.

Hint: Adiabatic processes are polytropic.

Actually though, how do you solve that?
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Re: Thermodynamics B/C

Postby jinhusong » April 18th, 2018, 12:03 pm

PV=nRT and U=3/2nRT

The temperature raising because of both V and work (forceXdistance).

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Re: Thermodynamics B/C

Postby Alex-RCHS » April 18th, 2018, 2:30 pm

jinhusong wrote:PV=nRT and U=3/2nRT

The temperature raising because of both V and work (forceXdistance).

I'm afraid I still don't understand. How do you find U? I could see doing force times distance but I don't see any way to find the distance over which the force acts (the amount the piston descends).

And as for the ideal gas law, you don't know what V would be, right?
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Re: Thermodynamics B/C

Postby jinhusong » April 18th, 2018, 4:36 pm

Alex-RCHS wrote:
jinhusong wrote:PV=nRT and U=3/2nRT

The temperature raising because of both V and work (forceXdistance).

I'm afraid I still don't understand. How do you find U? I could see doing force times distance but I don't see any way to find the distance over which the force acts (the amount the piston descends).

And as for the ideal gas law, you don't know what V would be, right?


I did not check the calculation and units. Here is an idea:

Piston area: A=3.14 * 0.125 * 0.125 = 0.0491 (m*m)

Initial:
P0=4.5X9.8/A=898.2 N/(m*m)

V0=n*R*T0/P0=8 * 8.314459 * 278 / 898.2 = 20.59 (m*m*m)

h0 = V0 / A = 419.3m

U0=3/2 * R * T0 * 8 = 27737

======
After apply force of 50N

P2=(4.5X9.8 + 50)/A= 1916.5


U2 = 3 /2 * R * 8 * T2

U2 = U0 + (4.5X9.8 + 50) * LLLL
LLLL is the distance the piston moved.

First equation
3/2 * 8.31446 * 8 * T2 = 27737 + 94.1 * LLLL


======
P2 X (h0-LLLL)*A=8* R * T2


Second equation
1916.5*(419.3-LLLL)*0.0491= 8 * 8.31446 * T2



(1)
LLLL = 1.060 * T - 294.76


(2)
LLLL = 419.3 - 0.7069 * T


T= 404K


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