PHYS 201 TEST #4 11/21/12 DR. HOLMES NAME
Do all eight problems. The worth of each problem is marked beside the space for the answer. All answers should be in MKS units unless otherwise indicated. Show your work for partial credit. Work should be under the problem, or clearly labeled on an extra sheet placed underneath the top page of the test.
1) a) What is the gauge pressure at a depth of 230 meters (750 ft) under water:?
in Nt/m²: 2.25 x 106 Nt/m2 in lb/in²: 328 lb/in2 in atmospheres: 22.3 atm.
b) If the ocean on a moon with acceleration due to gravity at its surface of 2.4 m/s2 contained liquid methane (density of 0.423 gm/cm3) instead of water, what would the gauge pressure at the same depth be?
2.33 x 105 Nt/m2 .
2) a) If you have a view plate on a submarine that is 230 meters below the surface (and subject to the gauge pressure in problem 1a above), what is the force due to this pressure on the view plate if the area of the view plate is 310 cm2 (circular plate of radius 10 cm)?
In Nt: 69,750 Nt
In lbs: 15,660 lb.
b) If a moon probe was designed to sink in an ocean of methane to a maximum depth of 230 meters (see problem 1b), how much force should a view plate of area 310 cm2 on the probe be able to support?
In Nt: 7,223 Nt.
In lbs: 1,621 lb.
3) A small artery of inside DIAMETER 0.80 mm and length 3.4 cm carries blood. Assume the beginning and the end of the artery are at the same height.
a) If the pressure drop from the front to the back of the artery is 6 mm Hg, what is the pressure drop expressed in Nt/m2 ?
b) Assuming there is the above pressure drop, and given that the viscosity of the blood is 4 x 10-3 Pl (about four times that of water), what is the volume flow of blood per time through the artery (assume laminar flow) expressed:
in m3/sec: 5.89 x 10-8 m3/sec
in cc/sec: 0.0589 cc/sec
c) Given that the density of blood is 1.05 gm/cc, by how much would the blood pressure decrease due to gravity by going up a distance of 32 cm (from the heart to the head)?
In Nt/m2: 3,293 Nt/m2
In mm of Hg: 24.8 mm of Hg.
For problems 4 & 5, consider a house 60 ft x 30 ft x 8 ft with 1,800 ft2 of floor (and ceiling) space and an outside wall area of 1,440 ft2 (60 x 8 plus 30 x 8 plus 60 x 8 plus 30 x 8).
4) Assume the outside walls have thermal insulation due to sheetrock and fiberglass with a total value of R = 16 ft2*oF*hr/BTU. Assume the average inside temperature is 77oF and the average outside temperature is 50oF. Ignore other sources of heat loss such as through the ceiling, through the floor, via convection and radiation.
What is the average heat loss per time (in
b) If the cost of energy is $0.095 / kW*hr, what will the cost be for a month to replace the energy lost by conduction?
If the inside temperature is reset to 68o, what will the new
average heat loss per time (in
5) For the house above (60 x 30 x 8):
a) What is the volume of air space in this room?
in ft3 : 14,400 ft3; in m3 : 408 m3
Now assume the temperature in the room is set to be 77oF.
b) Calculate the number of moles of air in this house:
c) Is the number of moles of air in the house when the temperature is 77oF [more, the same, or less] than when the temperature is 50oF ?
d) Given that the heat capacity of air (at constant pressure) is (7/2)*R, how much energy would it take to heat up the air in this house from 50°F to 77°F ?
7.26 x 106 Joules = 2.02 kW-hr
e) At $.095 / kW-hr, what would this energy cost?
$0.19 = 19 cents
f) If the air in this house is replace by outside air at the rate of once every 4 hours, how much would it cost to keep this house heated for one month assuming the outside temperature and inside temperature remained constant during the whole month and assuming the outgoing hot air did not warm up the incoming cold air? (Neglect other sources of heat such as warm bodies and losses due to radiation and conduction).
6 a) How much energy does it take to heat up 355 cc's of water (12 fluid ounces) (1 cc = 1 milliliter = 1 gram) from 50oF to boiling (212oF) ?
133,740 Joules = 31,950 calories
b) How much energy does it take to boil 355 cc's of water (if the water is already at 212oF) ?
802,456 Joules = 191,700 calories
c) If a person burns energy at the rate of 235 Watts, how much cool water (at 50o) is needed each hour to keep the person cool by evaporation (assume no heat is lost by conduction or radiation ?
7) Assume a certain person has an average skin temperature of 95°F and a skin area of 0.87 m2.
a) How much energy does this person radiate
per second (in
b) If the air temperature is 50°F, how much heat energy per time does this person receive from the air due to radiation?
c) What is the net power lost or gained by radiation by the person?
124 Watts lost.
8) a) State the First Law of Thermodynamics:
b) Where does this law come from, that is, is it a re-statement of a previous law?
c) State the Second Law of Thermodynamics
d) Where does this law come from?: