Download Nats 101 S00 #8

Nats 101 S99 #8 (Mon, 15/2/00)
 Chapter 1, Summary
1. What is science?
2. How is it done these days
 Chapter 2, Summary
1. The origins of modern science
2. Laws of motion, gravity
 Chapter 3, Summary
1. The nature of energy
2. Conservation of energy
 Chapter 4, Summary
1. Heat
 Text 80-91
Review
 A force causes a mass to accelerate.
 F = ma
 W = Fd
 Energy is the ability to do work
 P = W/t
Forms of Energy: Kinetic, Potential
 This will be an interesting chapter. It is a discussion about heat and the
second law of thermodynamics. This law is the one that tells us that we
must grow older, that ultimately, chaos reigns.
1
Nature has direction
 You grow old but you never grow younger. You can peel an orange but
you can never put the peel back. You can pop pop-corn but you can never
un-pop it. What are some other directional things?
 Up till now we have not come across anything that says this must be so.
Conservation of energy tells us that the energy required to unpop corn is
the same as the energy that we used to pop it. But it does not tell us that
in fact we cannot unpop it.
 Have you noticed that a cold drink warms up, and a warm drink cools.
We do not see cold drinks spontaneously cool down even further. The
2nd law of thermodynamics covers these phenomena.
 Other related phenomena include that the smell of a drop of perfume fills
a room. We never find that the smell, after filling the room, collects and
goes back into the bottle. The smell is due to floating small molecules of
the perfume. They evaporate and float around the room, but they do not
collect back into a drop. But if they did, then the energy to collect it
would be the same as it was to dispense it.
 The reason for this entire behavior is due to the vibration of atoms. If a
fast atom is put into a container with all slow moving atoms in it, then the
fast one will collide with the slow ones, and slowly its energy will
dissipate as the slow ones speed up, and the fast one slows down till all
the atoms are moving at the same average speed.
 This is the same effect as you observe when a warm house cools down
once the furnace is turned off.
 So let us examine the nature of heat in some detail, and we will examine
the 2nd law of thermodynamics next lecture.
 First, we need some definitions.
 Define: heat. Heat is a form of kinetic energy related to the vibration of
atoms. It flows from warmer objects to cooler ones. It is a measure of the
quantity of energy stored in the vibrations of the atoms. Its units are often
calories = the amount of heat required to raise 1 gm of room-temperature
water by 1 degree C. (Note, Cal is energy unit associated with food, and
it = 1000 cal.)
 Define: temperature. This is not the same as heat though it may seem so
at first. The temperature of an object is related to how vigorously the
atoms in the object are vibrating. Two objects are at the same
temperature if no heat energy flows between them when they are in
contact.
2
 Therefore, a gallon of boiling water is at the same temperature as a pint
of boiling water. However, the gallon of boiling water contains more
energy than the pint because there is more of it, so it can heat more tings
before it cools.
 The larger the temperature difference between two objects the faster the
heat can flow. This is why cold fronts bring such strong winds, often with
thunderheads at the boundary.
 The actual value associated with temperature can be arbitrary. There are
3 common scales, Fahrenheit, Celsius and Kelvin.
 Fahrenheit is the oldest (~1700), and was defined from 0 to 100 by the
extremes in temperature conditions common to the experience of the
inventor, Fahrenheit, a German.
 Celsius is the scale that has T = 0 at the freezing point of ice and T = 100
at the boiling point. A Swede invented it just after the Fahrenheit scale
was invented (1742). However, you know how cold it can get in Sweden,
he made it so the freezing pt was 100  and the boiling pt was 0 . It used
to be called the centigrade scale but this was officially dropped in 1948.
 Kelvin is the scale that is most used in thermodynamics. It set T = 0 to be
the lowest theoretical temperature that can be obtained. No thermal
energy exists in an object when it is at this temperature. The temperature
interval is set to be the same as for the Celsius scale. You do not same
10 K, but rather 10 K.
 Why are the freezing and boiling points of water used? How do you
calibrate temperature otherwise?
 Define: Specific Heat capacity. This is a measure of the ability of a
material to absorb heat energy. And is defined as the quantity of heat
required to raise the temperature of 1 gm of material by 1C. Water has
the largest heat capacity of any material. This is quite important in a
practical sense. This means it is slow to heat and slow to cool. Think
about heating a pan of water. The pan heats up quickly and it takes a
while for the water to heat. This is also why bodies of water act as a
climate moderator. Diamond conducts heat better than anything else.
3
Transfer of Heat
 You cannot make heat stay in any certain place, i.e. confine it, you can
only hope to slow it down.
 There are 3 ways that heat is transferred.
1. Conduction: the movement of heat by atomic vibrations. If a piece of
metal is heated at one end then the rest of the piece will get hot as the
atoms vibrate with large amplitudes and bump into their neighbors and
thus increase their vibrations. This is the major way that heat loss occurs
in buildings. Air inside the house is warmer than outside. The boundary
is the walls and windows. The molecules of air are moving faster inside
than outside and they collide with the walls and the windows, making
their atoms vibrate quicker. The heat goes by conduction to the outside of
the walls and windows and then transferred to the outside air. There are
several ways to slow down the process, all of them are done by slowing
down the speed of conduction. For windows, make them out of 2 panes
of glass with a vacuum between them, or glass made with lots of air
bubbles. Add insulation to the walls. This is based on that fact that
different materials have different thermal conductivity.
Wood feels warm, or normal, while metal usually feels cold. Actually
both are at the same temperature, its just that the metal conducts the
heat better, thereby taking heat from your hand, making it feel cooler.
2. Convection: This is the process of moving heat around by moving the
molecules that contain the heat. E.g. Boiling water in a pan produces
convection cells. The water at the bottom gets heated and it rises to the
top, forcing the cool water down, where it gets heated. The water at the
top sends its heat into the air so the cycle is continuous. You see this in
the shimmering waves off hot pavement, or above a toaster. This is
typically the major component of weather patterns. Cities are heat sinks
so the temperature is warmer there. Rainfall is larger in a city because it
draws the cool moist air in from the surrounding countryside. Tornadoes,
dust devils and volcanoes are also examples.
3. Radiation: Transfer of heat to electromagnetic radiation. Ever notice the
red glow of a fire. You can hold your hands to it and get its warmth. It is
not doing this by convection nor by conduction but by radiation. The
vibration of the atoms causes the electron density to fluctuate. This in
turn alters its energy, so when the energy drops then radiation is given
off. Much of the heat energy associated with our bodies and homes is low
energy and so it is given off as infrared radiation. This is the only way for
heat to travel through space.
4
 All three types can occur at the same time or various combinations.
 Home insulation: Fiberglass. This is small strands of glass. Because of
the atomic disorder of glass, it is a poor conductor. Furthermore, by using
strands they just touch here and there so the contact is small. Air is a poor
conductor as well. Therefore, a thick layer of fiberglass is a good
insulator. Liquid foam is also used in much the same way, especially on
older homes so you do not have to tear the walls down. Lots of tiny
bubbles of air. Air bubbles can also be introduced to the windows.
 Animal insulation: Birds and mammals are warm-blooded and therefore
need insulation. Feathers are made of hollow fibers with a small number
of contacts, much like fiberglass. They hold the air still. Birds can tense
their muscles to make the feathers stand up and thus hold a thicker
volume of air. Fat on whales etc is a poor conductor of heat. Hair is much
like feathers. Goose bumps are involuntary muscle movements that make
our hair stand up.
Temperature regulation
 Animals need a certain amount of heat in order to stay alive. Some
reptiles can convert radiation directly to heat, that is why lizards sun
themselves.
 Humans have several ways to adjust their body temperatures. If the body
temperature starts to rise, then the blood vessels near the surface of the
skin get bigger in order to move more blood and let the skin radiate the
heat away. That is why you appear flushed after being in the sun. In
addition, you sweat. The water absorbs heat in order to evaporate.
 When the body gets too cold, the blood vessels contract so that less heat
is carried to the surface. This can lead to frostbite where the skin dies
from lack of blood and therefore oxygen. Shivering is an involuntary
response to cold, where the muscles try to generate more heat.
 The shivering associated with the onset of a fever are because the body is
now hotter, and so the contact with the air is colder, making you shiver as
if the temperature outside dropped.
5