Hey, so I've been studying for my physics final coming up and going through all the example problems and I've gotten stuck on a few of them. The topics are pretty broad, so I'll separate them to be clear. Any help would be greatly appreciated!
Ideal Gas Law
- A typical scuba tank, when fully charged, contains 12 L of air at 204 atm. Assume an "empty" tank contains air at 34 atm and is connected to an air compressor at sea level. The air compressor intakes air from the atmosphere, compresses it to high pressure, and then inputs this high-pressure air into the scuba tank. If the (average) flow rate of air from the atmosphere into the intake port of the air compressor is 290 L/min, how long will it take to fully charge the scuba tank? Assume the tank remains at the same temperature as the surrounding air during the filling. So initially I tried to just do PV=constant using the rate as the volume, however, the back of the book said differently, and I'm guessing it's because of the initial 34 atm, but I'm not sure how to account for the changes in pressure inside
1st Law of Thermodynamics
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One and one-half moles of an ideal monatomic gas expand adiabatically, performing 7500 J of work in the process. What is the change in temperature of the gas during this expansion? I think I was confused by this one because the equation I was given for the work of an adiabatic process uses the molar specific heats (nCvΔT) and I didn't know the Cv for a generic monatomic gas
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If a heater supplies 1.8x106 J/h to a room 3.5m x 4.6 m x 3.0 m containing air at 20°C and 1.0 atm, by how much will the temperature rise in one hour, assuming no losses of heat or air mass to the outside? Assume air is an ideal diatomic gas with molecular mass 29. again I tried to use ideal gas law to figure out the number of moles of gas in the room based off the volume and then use that in the Q=nCvΔT, but my number kinds out kind of ridiculous
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A house thermostat is normally set to 22°C, but at night it is turned down to 12°C for 9.0 hours. Estimate how much more heat would be needed (stated as a percentage of daily usage) if the thermostat were not turned down at night. Assume that the outside temperature averages 0°C for the 9.0 hours at night and 8°C for the remainder of the day, and that the heat loss from the house is proportional to the difference in temperature inside and out. To obtain an estimate from the data, you will have to make other simplifying assumptions; state what these are. Okay this one I genuinely have no clue where to even start with this one, any sort of step in the right direction would be awesome
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A cylindrical pipe has inner radius R1 and outer radius R2. The interior of the pipe carries hot water at temperature T1. The temperature outside is T2 (< T1). (a) Show that the rate of heat loss for a length l of pipe is dQ/dt = 2pi*k(T1-T2)l/ln(R2/R1), where k is the thermal conductivity of the pipe. I just don't get where that equation can be derived from, since the original is dQ/dt = kA(T1-T2)/l, like how does the l move from the denominator to the numerator when we derive the second equation?
Second Law of Thermodynamics
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A "Carnot" refrigerator (reverse of a Carnot engine) absorbs heat from the freezer compartment at a temperature of -17°C and exhausts it into the room at 25°C. (a) How much work must be done by the refrigerator to change 0.40 kg of water at 25°C into ice at -17°C? (b) If the compressor output is 180 W, what minimum time is needed to take 0.40 kg of 25°C water and freeze it at 0°C so I started by finding the heat needed to freeze the water with Q=mL and L = 333kJ/kg and then I tried finding the 2 other heats using Q=mcΔT but my answer came out much different than the one in the back of the book.
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(a) An ice cube of mass m at 0°C is placed in a large 20°C room. Heat flows (from the room to the ice cube) such that the ice cube melts and the liquid water warms to 20°C. The room is so large that its temperature remains nearly 20°C at all times. calculate the change in entropy for the (water+room) system due to this process. Will this process occur naturally? (b) A mass m of liquid water at 20°C is placed in a large 20°C room. Heat flows (from the water to the room such that the liquid water cools to 0°C and then freezes into a 0°C ice cube. The room is so large that its temperature remains 20°C at all times. Calculate the change in entropy for the (water+room) system due to this process. Will this process occur naturally? So I tried saying ΔS = ΔQ/T = (mcΔT)/T but my answer came out much smaller than the one in the back. I didn't get to the second part because it's much similar to the first part and I was stuck on that one
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Consider an ideal gas of n moles with molar specific heats Cv and Cp. (a) Starting with the first law, show that when temperature and volume of this gas are changed by a reversible process, its change in entropy is given by dS = nCv(dT/t) + nR(dV/V). (b) Show that the expression in part (a) can be written as dS = nCv(dP/P) + nCp(dV/V). (c) Using the expression from part (b), show that is dS = 0 for the reversible process (that is, the process is adiabatic), then PVγ = constant, where γ = Cp/Cv. Again I have no clue what to do on this one. I start with ΔU = Q + W? I don't know where the other parts come from even
Gravity
- (a) What is the gravitational field at the surface of the Earth due to the Sun? (b) Will this affect your weight significantly? so I tried using the equation (vector field g) = (GMs/r2)(vector field r). However, my answer was much larger than the correct answer. I had Ms* as the mass of the sun and r as the distance from Earth to the Sun. Any ideas*
So yeah, that's all the problems I had issues figuring out. Any help at all would be amazing. Thanks in advance!
Submitted June 08, 2015 at 10:46AM by RussianDusk http://ift.tt/1FDYEgC HomeworkHelp
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