Wednesday, June
8, 2011
Announcements:
- Our final exam is scheduled for next Tuesday from 8:00
- 11:00 am. I will hold an optional review session for the final
exam next Monday during our regular class time and also at 12:00 pm on
Monday in Phillips 215.
- Dmitry will hold an extended SI session on Monday from
5:00 - 7:00 pm in Phillips 220.
- I will hold a 30-minute review session for the Lab Exam
today immediately after class as I have for the other exams.
- The concepts survey (post-test) is being administered
today in Phillips 215. This is the same multiple-choice survey that
you took at the beginning of the course, except that now you should be
able to correctly answer all of the questions! Be sure to
attend either the session this morning from 10:00 – 10:30 or 11:15 – 11:45,
and bring your laptop computer with you so that you can enter your
answers electronically. See page 13 in your lab manual for more
information about this concept survey.
- Web Projects are being published. Check the list and let me know if I should
add or modify your information.
Assignments:
- HW15 is due tonight
- HW16 is due tomorrow night
Chapter 15:
Fluids
Density = Mass/Volume
Pressure = Force/Area
1 Pa = 1 N/m^2
P = Po + Dgh
Gauge pressure is the pressure above atmospheric pressure (i.e. the
measurement you would expect to read from a pressure gauge).
Pascal's principle: Pressure at a given depth is same
everywhere in an enclosed fluid.
A net upward buoyant force results from difference in pressures
acting on the top and bottom surfaces of an object immersed in a fluid.
Archimedes' principle states that the magnitude of the buoyant force
acting on an object is equal to the weight of the fluid displaced by the
object.
An object floats when it displaces an amount of fluid equal to its
weight. This occurs when the overall density of the object is less than
the fluid in which it is immersed.
The equation of continuity for incompressible fluids is A1v1 = A2v2,
which basically means that flow in = flow out.
The equation of continuity for compressible fluids is D1A1v1 =
D2A2v2.
Bernoulli's equation is: P + 0.5Dv^2 + Dgy
= constant
Consequence: Faster fluid flow means lower pressure. (This is how
airplane wings produce lift.)
The Reynolds number is a dimensionless factor that indicates whether
fluid flow will be turbulent (RN > 3000), streamline (RN < 2000), or
somewhere in between.
Viscosity refers to the internal friction of a fluid. Lower
viscosity fluids flow easier than high viscosity fluids.
The rate of flow through a pipe can be found from Poiseuille's
Law. From this equation, we can see that the rate of flow is
proportional to the fourth power of the radius, so a small increase in the
diameter of a tube will result in a very large increase in the flow rate.
Terminal speed of an object falling in a viscous medium is reached
when the magnitude of the frictional resistance equals the weight of the
object.
Surface tension: gamma = F/L
Capillary action is a result of surface tension.
Fick's Law can be used to find the
rate of diffusion, which is proportional to the concentration gradient and the
cross-sectional area between two regions.
Elastic modulus = stress/strain
Stress is proportional to force exerted and has same units as pressure.
Strain is a measure of degree of deformation.
1D - Young's modulus, Y
2D - Shear modulus, S
3D - Bulk modulus, B
Ponderables:
- What determines whether an object floats?
- How much water is displaced by a floating object?
- How much water is displaced by a submerged (sunken)
object?
- Does an object weigh more or less in a vacuum?
- How much would you appear to weigh in a vacuum?
- Why does a can of Coke sink, but Diet Coke floats?
- How can you make a bowling ball float?
- How does the Bernoulli blower work?
- A person in a boat floating in a small pond throws an
anchor overboard. Does the level of the pond rise, fall, or remain
the same? [fall]
- What happens to the water level in a glass as the ice
in it melts? [stays the same]
- What happens to the water level in a glass as the ice
(with small pebbles) in it melts? [falls]
- Should a golf ball have backspin or topspin for maximum
flight distance? [backspin]
- To make a baseball curve to the left, which way should
the pitcher spin the ball? [spin to the left]
- Chicago has been called "the windy
city." How does the Bernoulli effect
contribute to this nickname? [air speeds increases as it passes through
the narrow spaces between the buildings]
- When your instructor lays on a
bed of nails, how much force does each nail contribute to support his
weight? [~150 lbs/1500 nails = 0.1 lb/nail]
- If a person is standing up, should the blood pressure
in a leg be more or less than in an arm?
- If an angioplasty procedure expands an artery too much
so that the diameter is significantly larger than the surrounding region,
would the blood pressure be larger or smaller at that point? How is
this similar to an aneurism?
Problems:
- How much more (or less) would a 70-kg person
"weigh" in a vacuum?
- What is the maximum weight that the suction square
could support (Dia. = 6 in.)? [400 lbs.]
- At what depth in fresh water is the pressure equal to 2
atmospheres? [10 m]
- You decide to weigh your car by applying the physics
you have learned. Each tire is inflated to 35 psi and the area in
contact with the road is about 20 sq. in. How much does the car
weigh? [2800 lbs.]
- What fraction of the total volume of an iceberg is
exposed above water? [10.7%]
- What are the forces on a block of aluminum suspended in
water by a spring scale?
- If an aluminum block is suspended in water by a spring
scale, and the water is held in a container that rests on another scale,
what do the scales read?
- If the outside pressure on the emergency door of an
airplane is 0.3 atm, and the door has an area of 6 s.f., what is the net
force on the door caused by the pressure difference inside and outside the
plane? How many people would it take to hold the door in place if it
were not secured by the door frame?
- Which is more effective for improving the flow rate of
blood through an artery: 1) use of a
blood thinner that reduces the blood's viscosity by 50%, or 2) an
angioplasty procedure that increases the inside diameter of a clogged
artery to twice its original size? [The flow rate will be 8 times
greater for the angioplasty than for the blood thinner.]
Demos:
- Floating bowling ball
- Coke/Diet Coke
- Suction square
- Atmosphere bar
- Bed of nails
- Aluminum block weighed in water
- Lung manometer
- Cartesian diver
- Bernoulli blower, paper
Chapter
16: Temperature and Heat
Temperature
scales:
T(Kelvin) = T(Celsius) + 273.15
T(Fahrenheit) = (9/5)T(Celsius) + 32
Heat is a transfer of energy due to a temperature difference.
A calorie is the amount of heat needed to raise the temperature of 1 g
of water from 14.5o to 15.5o C.
1 Calorie = 1 kcal
The mechanical equivalent of heat is 4.186 J/cal.
The heat (Q) required to change the temperature of a substance by (dT) is Q = mc(dT),
where m is the mass of the substance and c is its specific heat.
Water has one of the highest specific heats of any substance. This means
it can store a lot of thermal energy, and it also takes a lot of heat to change
its temperature compared with other materials.
Heat can be transferred by 3 processes:
conduction
- requires
contact between materials: Q/dt = kA(T2-T1)/L
convection - requires fluid, occurs naturally due to buoyant forces from
density variations
radiation - all objects absorb and emit electromagnetic radiation
(infrared)
The rate of radiation (radiating power) is given by Stefan's law: P =
sAeT^4.
Newton's
law of cooling:
The rate that a substance cools (or warms) is proportional to the
temperature difference between its temperature and the ambient temperature.
Most substances expand when heated, and the rate of expansion depends on the
temperate difference and the linear, area, or volume expansion
coefficient.
Ponderables:
- If you heat up a metal plate with a hole in it, what
will happen to the size of the hole?
- Why does water at 75 degrees Farenheit
feel cold but air at this same temperature feels warm?
- If you get a hot cup of coffee but don't want to drink
it right away, should you add cold dairy creamer right away or just before
you drink it for the hottest cup of coffee? Which method of heat
transfer is most significant for this problem?
- How many Calories does your body expend when you drink
a glass of ice water? Answer
to this nagging question.
- What procedure should be used to make a pitcher of
freshly-brewed iced tea in the shortest time possible?
- How should gloves be constructed to minimize heat loss
from your hands on a cold winter day?
- Why is it difficult to get a sunburn
from sunlight passing through a window?
- If you are in a cool, dark room, would you be warmer
wearing a white or black T-shirt?
- A copper cube is colored black on one side and filled
with water at 52 deg. C. Why does an infrared thermometer show a
reading of 48 deg. C on the black side and 28 deg. C on the copper
sides? (Hint: The thermometer assumes an emissivity of 0.95.)
Problems:
- How does specific heat correspond to thermal
conductivity?
- In terms of energy, how many candy bars does it take to
get to the top of Mt. Mitchell?
- What is the final temperature when 100 mL of water at 10oC is mixed with 100 mL of water at 50oC in a styrofoam cup?
- What is the final temperature when 100 g of ice at -5oC
is mixed with 100 mL of water at 50oC
in a styrofoam cup?
- How long does it take for a cup of water to boil when
placed in a solar cooker?
- How much fluid does a runner loose through
perspiration?
- Estimate the power of a microwave oven if a cup of
water at room temperature boils in 2 min. on high.
- You look "radiant"! How much?
Demos:
- Torch juggling
- Leslie cube
- Infrared thermometer
- Thermal expansion