PHYS 201 TEST
#4 11/24/10 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 8 feet (2.44 meters) under water:?
in Nt/m²: 2.39 x 104 Nt/m2 in lb/in²: 3.48 lb/in2 in atmospheres: .237 atm.
b) If the pool contained alcohol (density of
0.82 gm/cm3) instead of water, what would the gauge pressure at the
same depth be?
1.96 x 104 Nt/m2 .
2) a) If you apply a force of 76 lbs n a
cylinder of area 0.14 in2, what is the gauge pressure on the fluid
behind the cylinder?
In lb/in2: 542.8 lb/in2
In Nt/m2: 3.73 x 106
Nt/m2.
b) If the fluid under the above pressure
pushes on another cylinder of area 18 in2, what will be the force on
this cylinder?
In lbs: 9,771 lb.
In Nt: 43,527 Nt.
3) A small artery of inside DIAMETER 1.8 mm
and length 3.6 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 30 mm Hg, what is the pressure drop expressed in Nt/m2
?
3,987 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: 7.13 x 10-6
m3/sec
in cc/sec: 7.13 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 35 cm (from the heart to the head)?
In Nt/m2: 3,601 Nt/m2
In mm of Hg: 27.1 mm of Hg.
4) A house with 1,200 ft2 of
floor (and ceiling) space has thermal insulation due to sheetrock and
fiberglass with a total value of R = 27 ft2*oF*hr/BTU. Assume the average inside temperature is 68oF
and the average outside temperature is 14oF. Ignore other sources of heat loss such as
through the walls, through the floor, via convection and radiation.
a)
What is the average heat loss per time (in
704 Watts
b) If the cost of energy is $0.09 / kW*hr,
what will the cost be for a month to replace the energy lost by conduction?
$45.61
c)
If the inside temperature is reset to 77o, what will the new
average heat loss per time (in
821 Watts.
5) a) Assume a house has 1,200 ft2
of floor (and ceiling) space (40 ft x 30 ft), all of it with 8 ft ceilings.
What is the volume of air space in this room?
in ft3 : 9,600 ft3;
in m3 : 271.8 m3
Now assume the temperature in the room is
set to be 68oF.
b) Calculate the number of moles of air in
this house:
11,288 moles
c) Is the number of moles of air in the room
when the temperature is 14oF [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 14°F to 68°F ?
9.84 x 106 Joules = 2.733
kW-hr
e) At $.09 / kW-hr, what would this energy
cost?
$0.246 = 24.6 cents
f) If the air in this house is replace by
outside air at the rate of once every 3 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).
$59.03
6 a) How much energy does it take to heat up
300 cc's of water (10 fluid ounces) (1
cc = 1 milliliter = 1 gram) from 50oF to boiling (212oF)
?
113,022 Joules = 27,000 calories
b) How much energy does it take to boil 300
cc's of water (if the water is already at 212oF) ?
678,132 Joules = 162,000 calories
c) If a person burns energy at the rate of
180 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 ?
245.7 cc/hr.
7) Assume a certain person has an average
skin temperature of 95°F and a skin area of 0.78 m2.
a) How much energy does this person radiate
per second (in
398 Watts
b) If the air temperature is 14°F, how much
heat energy per time does this person receive from the air due to radiation?
211.6 Watts
c) What is the net power lost or gained by radiation
by the person?
186.4 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?: