Heat
and Temperature Teaching Notes
1)
Heat flows from hot to cold areas
due to a temperature difference only.
example: A small hot block
of a material is placed next to a larger, cooler block. Heat flows from the
small hot block to the larger block till equilibrium is reached.
2) Note the difference between the heat content and temperature. A
lake may be cooler in temperature than a liter flask of water but the lake has
a much greater heat content due to the vast number of particles and their
associated motion.
3) Human perception of heat and temperature is not adequate for
scientific work. So we must investigate tools that provide the accuracy and
repeatability we need.
4) For temperature we will be using thermometers. There are other
devices that allow you to measure temperature.
5) For heat we will be working with simple calorimeters. We will
look at these devices when we look at the transfer of heat.
Calibration
Both
Celsius (Anders Celsius) and Fahrenheit (Gabriel
Fahrenheit) scales are established by using the
boiling
and freezing point of water at 1 atmosphere
of pressure.
step 1 -- establish a mixture of ice and water in equilibrium (0° C)
mark
point of liquid in thermometer at 0° C
establish a mixture of steam and water at equilibrium (both at a
pressure of 1 atm.) steam condensing
and water vaporizing)
label this point of liquid as 100 °C
step
2 -- divide the interval between 0° C and 100° C into 100 equal parts, each
representing a change in temperature of 1° C.
using this scale you can extend your marks below 0 °C and above 100 °C
as far as ,you wish.
The Fahrenheit scale labels the freezing point at 32°
(his
label for the temperature he could achieve with an ice and water mixture and
labeled the boiling point temperature of water at 212 ° which was a
number chosen for convenience apparently creating 180 divisions.
You might ask your self about the amount of heat energy need to
cause a 1 degree change in temperature on a Celsius scale compared to that
needed on a Fahrenheit scale. (more heat needed to cause change of 1 degree on
Celsius scale.)
KELVIN scale
We
know that gases decrease in volume 1/273 of its
original volume for each degree drop in temperature.
Thus
at -273° C the volume of gas would shrink to zero
and the gas would have no molecular
motion. We
know this is impossible (particles have zero-point energy).
To
label these very low temperatures a scale called the
absolute or Kelvin scale is often used. It designates
-273°
C at the zero point, and is called called absolute zero.
Extrapolation
to absolute zero:
A good research project for students or a quick review of graphing can be done
by using the method to calculate absolute zero.
Use a capillary tube with a trapped bubble of air between light machine oil.
Measure the length of the bubble after placing it in different temperature
mixtures. Plot the length versus temperature.
Since it is not easy to obtain very cold temperatures, the linear series of
points that you did obtain should allow you to extrapolate, (extend a curve
beyond the known data points following the apparent pattern of the curve)
until it intersects the temperature axis.
See actual plot!
Transfer of Heat by
Conduction, Convection, Radiation
Conduction is a consequence of the kinetic behavior of matter. Faster
vibrating particles collide with less energetic neighbors and transfer some
of their kinetic energy to the slower moving particle.
Through successive molecular collisions energy travels through a
material without the average position of the particles being changed. There
must be a temperature differential (one end of some object at a higher
temperature than the other) for heat to be conducted.
Gases are poor conductors of heat (compared to liquids and solids)
because the molecules are relatively far apart and collisions are
infrequent.
Metals have the greatest ability to conduct heat (for the same reason
as their high electrical conductivity). This is due to a significant number
of electrons being able to move about freely instead of being bound
permanently to particular atoms.
Thermal conductivity of a material is a measure of its ability
to conduct heat.
Example: wood and metal
Convection involves the actual motion of a hot fluid from one place to
another, displacing a colder fluid in its path and setting up a convection
current. Convection is the chief mechanism of heat transfer in fluids.
Natural convection occurs when the buoyancy of heated fluids leads to
motion. Heated fluids (gas or liquid) expand and becomes less dense than
surrounding cooler fluids. It then rises.
Radiation is defined as the energy that is transmitted by
electromagnetic waves and requires no material medium for passage.
All objects radiate electromagnetic waves with the higher the
temperature of an object the shorter the predominating wavelength of its
radiation.
Example: see glass lined thermos bottle in heat
packet
Thermal Expansion of Water
Ice floats (less dense than the water).
A body of water freezes from the top down.
Ice is such a poor conductor of heat that this
initial ice layer impedes further freezing.
This allows fish and plant life to live through the
winter.
The spaces between molecules in ice are greater than the same spaces
in liquid water.
Ice has what is called an open structure. Each H2O
molecule bonds with 4 other H2O molecules while other solids can
have molecules with as many as a dozen bonds with surrounding molecules
resulting in a compact substance.
The density of water increases from 0° C to 4° C (as the volume
decreases). Large clusters of H2O molecules break into smaller
clusters that occupy less space in the aggregate as the temperature rises to
the 4° C mark. The greatest density of water is at 4° C.
Above
4° C , the normal thermal expansion of materials is seen. Here as the
temperature rises the density decreases.
Transfer of Heat
 |
Conduction
-- place iron rod in fire -- the
end you are holding becomes warm due to conduction |
|
Convection
-- stove heats room by
convection
|
|
Radiation
-- heat the earth receives
from the sun is radiation
|
Natural direction of heat flow is from hot bodies to cold ones.
Conduction -- conduction is a consequence of kinetic behavior
of matter
|
faster vibrating particle
collide with less energetic neighbor and transfer some of their kinetic
energy the slower moving particle
|
|
through successive molecular
collisions energy travels down the iron rod without the average position
of the particles being changed
|
|
for heat to be conducted through a body the ends must be at different
temperatures
|
|
gas are poor conductors of heat (compared to liquids an solids) because
the molecules are relatively far apart and collisions are infrequent
|
|
metals have greatest ability to conduct heat (for the same reason as
their high electrical conductivity) ‑ a significant number of
electrons are able to move about freely instead of being bound
permanently to particular atoms
|
|
thermal conductivity of a material is a measure of its ability to
conduct heat |
example:
place hand on wood and metal sample at same temperature. The metal will seem
cooler because it conducts the heat away much faster than the wood
Convection
-- actual motion of
hot fluid from one place to another, displacing cold fluid in its path
setting up a convection current = chief mechanism of heat transfer in fluids
in most instances
|
natural convection - the
buoyancy of heated fluids leads to motion ‑ heated fluid (gas or
liquid) expands and becomes less dense than surrounding cooler fluids
and rises
|
Radiation --
energy that is transmitted by electromagnetic waves and requires no
material medium for passage
|
all objects radiate
electromagnetic waves but the higher the temperature of an object the
shorter the predominate wavelength of its radiation |
There
are many examples you can use to demonstrate these three ideas but
discussing
a glass lined thermos bottle will allow you discuss them as well as
High
specific heat capacity material demonstrates relatively small change in
temperature for a given change in internal energy content
add 1 calorie of heat to 1 gram
of water, helium, ice, gold
temperature rises: water 1 °C
helium 1.3 °C
ice
2 °C
gold
33 °C
Calorimetry:
1)
Use 2 polystyrene cups (one within the other) -- the polystyrene will
not absorb very much heat.
2) You may find if
difficult for the students to be patient when working with the calorimeters.
Your step-by-step procedure must be very simple and clear. This type lab is
also a very dangerous time for your thermometers.
3) Several labs have been included in the packet involving the use of
the calorimeters and thermometers: a) the heat of reactions and heat of
solutions labs deal with endothermic and exothermic reactions (and you can
incorporate the use of the mole as review) b) the specific heat capacity lab
is an excellent demonstration of the principle of heat exchange as well as
specific heat capacity of different metals and liquids. 1) for better
results with this lab try to use as much metal as possible and as little
fluid as possible and still cover the metal in the cups. Drain the metal
samples quickly after removing them from the boiling water. This is a good
time to have the students watch the boiling process which will be one of the
final topics in this unit. 2) the math may be a bit difficult for you at
first.
Thermal Expansion of water
1) From 0° C to 4°
C the volume of water in a sample decreases (the greatest density is at 4°
C
2) We know that ice
floats (less dense) so that a body of water in winter freezes from the top
down. The ice is a poor conductor of heat so that the initial layer of ice
that freezes impedes further freezing allowing fish and plant life to live
through the winter.
3)
The spaces between molecules in ice are greater than the same spaces
in liquids.
4) Ice has what is
called an Open Structure --> each water molecule can participate
in 4 bonds with other water molecules, while other solid molecules can have
as many as a dozen bonds with surrounding molecules resulting in a more
compact substance.
5) As stated the density of the water increases from 0 °C
to 4 °C. Large
clusters
of water molecules break into smaller clusters that occupy less
space in the aggregate as the temperature rises to 4 °C. Only above
4 °C does the normal thermal expansion show a decreasing density with
increasing temperature.
