PHYS 201      TEST #4         11/20/13         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 25 meters (82 ft) under water:?

in Nt/m²: 2.45 x 105 Nt/m2    in lb/in²: 35.7 lb/in2                in atmospheres:  2.43 atm.

b) If the ocean on a moon with acceleration due to gravity at its surface of 2.1 m/s2 contained liquid methane (density of 0.423 gm/cm3) instead of water, what would the gauge pressure at the same depth be?

2.22 x 104 Nt/m2 .

2) a) If you have a view plate on a submarine that is 25 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 500 cm2 (circular plate of diameter 10 in)?

In Nt:  12,250 Nt

In lbs: 2,750 lb.

b) If a moon probe was designed to sink in an ocean of methane to a maximum depth of 25 meters (see problem 1b), how much force should a view plate of area 500 cm2 on the probe be able to support?

In Nt: 1,110 Nt.

In lbs: 249 lb.

3) A small artery of inside DIAMETER 0.80 mm and length 3.5 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 5 mm Hg, what is the pressure drop expressed in Nt/m2 ?

664.5 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: 4.77 x 10-8 m3/sec

in cc/sec: 0.048 cc/sec

c) Given that the density of blood is 1.05 gm/cc, by how much would the blood pressure increase due to gravity by going down a distance of 80 cm (from the heart to the thigh)?

In Nt/m2: 8,232 Nt/m2

In mm of Hg: 61.9 mm of Hg.

For problems 4 & 5, consider a house 45 ft x 30 ft x 8 ft with 1,350 ft2 of floor (and ceiling) space and an outside wall area of 1,200 ft2 (45 x 8 plus 30 x 8 plus 45 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 = 21 ft2*oF*hr/BTU.  Assume the average inside temperature is 68oF and the average outside temperature is 41oF.  Ignore other sources of heat loss such as through the ceiling, through the floor, via convection and radiation.

a)  What is the average heat loss per time (in Watts) due to conduction through the outside walls?

452 Watts

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?

\$30.92

c)  If the inside temperature is reset to 77o, what will the new average heat loss per time (in Watts) due to conduction through the outside walls be?

603 Watts.

5)  For the house above (45 x 30 x 8):

a) What is the volume of air space in this room?

in ft3 :   10,800 ft3;     in m3 : 306 m3

Now assume the temperature in the room is set to be 68oF.

b) Calculate the number of moles of air in this house:

12,700 moles

c) Is the number of moles of air in the house when the temperature is 41oF [more, the same, or less] than when the temperature is 68oF ?

more.

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 41°F to 68°F ?

5.53 x 106 Joules = 1.54 kW-hr

e) At \$.095 / kW-hr, what would this energy cost?

\$0.15 = 15 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).

\$26.29

6 a) How much energy does it take to heat up 475 cc's of water (16 fluid ounces)  (1 cc = 1 milliliter = 1 gram) from 41oF to boiling (212oF) ?

188,893 Joules = 45,125 calories

b) How much energy does it take to boil 475 cc's of water (if the water is already at 212oF) ?

1,073,709 Joules = 256,500 calories

c) If a person burns energy at the rate of 165 Watts, how much cool water (at 41o) is needed each hour to keep the person cool by evaporation (assume no heat is lost by conduction or radiation ?

223.5 cc/hr.

7) Assume a certain person has an average skin temperature of 92°F and a skin area of 1.24 m2.

a) How much energy does this person radiate per second (in Watts)?

619 Watts

b) If the air temperature is 77°F, how much heat energy per time does this person receive from the air due to radiation?

554.5 Watts

c) What is the net power lost or gained by radiation by the person?

64.5 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?: