CSET Practice Test Subtest II Science

Thermal Conductivity 

In a conductor, electric current can flow freely, in an
insulator it cannot. Metals such as copper typify
conductors, while most non-metallic solids are said
to be good insulators, having extremely high
resistance to the flow of charge through them.
"Conductor" implies that the outer electrons of the
atoms are loosely bound and free to move through
the material. Most atoms hold on to their electrons
tightly and are insulators. In copper, the valence
electrons are essentially free and strongly repel each
other. Any external influence which moves one of
them will cause a repulsion of other electrons which
propagates, "domino fashion" through the conductor. 

Simply stated, most metals are good electrical
conductors, most nonmetals are not. Metals are
also generally good heat conductors while nonmetals
are not.

Heat transfer by conduction involves transfer of
energy within a material without any motion of the
material as a whole. The rate of heat transfer depends
upon the temperature gradient and the thermal
conductivity of the material. Thermal conductivity is 
a reasonably straightforward concept when you are 
discussing heat loss through the walls of your house, 
and you can find tables which characterize the 
building materials and allow you to make reasonable 
calculations. 

More fundamental questions arise when you examine 
the reasons for wide variations in thermal conductivity. 
Gases transfer heat by direct collisions between 
molecules, and as would be expected, their thermal 
conductivity is low compared to most solids since 
they are dilute media. Non-metallic solids transfer 
heat by lattice vibrations so that there is no net 
motion of the media as the energy propagates 
through. Such heat transfer is often described in 
terms of "phonons", quanta of lattice vibrations. 
Metals are much better thermal conductors than 
non-metals because the same mobile electrons 
which participate in electrical conduction also take 
part in the transfer of heat. 

Conceptually, the thermal conductivity can be 
thought of as the container for the medium-dependent 
properties which relate the rate of heat loss per unit 
area to the rate of change of temperature. 

For an ideal gas the heat transfer rate is proportional 
to the average molecular velocity, the mean free path, 
and the molar heat capacity of the gas. 

For non-metallic solids, the heat transfer is view as 
being transferred via lattice vibrations, as atoms 
vibrating more energetically at one part of a solid 
transfer that energy to less energetic neighboring 
atoms. This can be enhanced by cooperative motion 
in the form of propagating lattice waves, which in the 
quantum limit are quantized as phonons. Practically, 
there is so much variability for non-metallic solids that 
we normally just characterize the substance with a 
measured thermal conductivity when doing ordinary 
calculations.

For metals, the thermal conductivity is quite high, 
and those metals which are the best electrical 
conductors are also the best thermal conductors. 
At a given temperature, the thermal and electrical 
conductivities of metals are proportional, but raising 
the temperature increases the thermal conductivity 
while decreasing the electrical conductivity. 

Qualitatively, this relationship is based upon the fact 
that the heat and electrical transport both involve the 
free electrons in the metal. The thermal conductivity 
increases with the average particle velocity since that 
increases the forward transport of energy. However, the 
electrical conductivity decreases with particle velocity 
increases because the collisions divert the electrons 
from forward transport of charge. This means that the 
ratio of thermal to electrical conductivity depends upon 
the average velocity squared, which is proportional to 
the kinetic temperature. 

Is there a relationship between electrical conductivity 
and thermal conductivity? 

There is a relationship for metals and it is known as 
the Wiedemann-Franz law. Metals are good electrical 
conductors because there are lots of free charges in 
them. The free charges are usually negative electrons, 
but in some metals, e.g., tungsten, they are positive 
'holes.' For purposes of discussion, let's assume we have 
free electron charges. 

When a voltage difference exists between two points 
in a metal, it creates an electric field which causes the 
electrons to move, i.e., it causes a current. Of course, 
the electrons bump into some of the stationary atoms 
(actually, 'ion cores') of the metal and this frictional 
'resistance' tends to slow them down. The resistance 
depends on the specific type of metal we're dealing 
with. E.g., the friction in silver is much less than it is in 
iron. The greater the distance an electron can travel 
without bumping into an ion core, the smaller is the 
resistance, i.e., the greater is the electrical conductivity. 
The average distance an electron can travel without 
colliding is called the 'mean free path.' But there's 
another factor at work too. The electrons which are 
free to respond to the electric field have a thermal 
speed a sizable percentage of the speed of light, but 
since they travel randomly with this high speed, they 
go nowhere on average, i.e., this thermal speed itself 
doesn't create any current. 

The thermal conductivity of this metal is, like electrical 
conductivity, determined largely by the free electrons. 
Suppose now that the metal has different temperatures 
at its ends. The electrons are moving slightly faster at 
the hot end and slower at the cool end. The faster 
electrons transmit energy to the cooler, slower ones by 
colliding with them, and just as for electrical conductivity, 
the longer the mean free path, the faster the energy 
can be transmitted, i.e., the greater the thermal 
conductivity. But the rate is also determined by the 
very high thermal speed-the higher the speed, the 
more rapidly does heat energy flow(i.e., the more 
rapidly collisions occur). In fact, the thermal conductivity 
is directly proportional to the product of the mean free 
path and thermal speed. 

Both thermal and electrical conductivity depend in the 
same way on not just the mean free path, but also on 
other properties such as electron mass and even the 
number of free electrons per unit volume. But as we 
have seen, they depend differently on the thermal speed 
of the electrons-electrical conductivity is inversely 
proportional to it and thermal conductivity is directly 
proportional to it. The upshot is that the ratio of thermal 
to electrical conductivity depends primarily on the square 
of the thermal speed. But this square is proportional to 
the temperature, with the result that the ratio depends 
on temperature, T, and two physical constants: 
Boltzmann's constant, k, and the electron charge, e. 
Boltzmann's constant is, in this context, a measure of 
how much kinetic energy an electron has per degree of 
temperature. 

Putting it all together, the ratio of thermal to electrical 
conductivity is: 

( 2 / 3 ) * ( (k/e)2 ) * T 

the value of the constant multiplying T being: 2.45x10-8 
W-ohm-K-squared.

Plants and Gravity

Humans have always been mesmerized by the thought 
of space travel and inhabiting other worlds. But if we 
are going to be successful colonists, we need to be 
self-sufficient and that includes knowing how to grow 
crops in space. Here's why: Future astronauts will 
need food, oxygen and water to sustain themselves, 
and plants can provide these basic needs. Plants are 
a source of food, turn carbon dioxide into oxygen 
and they can absorb and hold water. But to make the 
leap into space gardening, NASA has had to struggle 
with some fascinating problems. To date, they've 
come a long way in their discoveries. Here's what 
they've learned so far: 

Garden Troubles in Space
On Earth, plants need gravity, water, carbon dioxide, 
soil, light and pollination to survive. The lack of these 
elements in space causes several problems. 

1. Gravity and Seeds: In an orbiting ship, plants don't 
have the constant downward pull of gravity, and that 
often confuses their orientation. For example, the stalk 
will sometimes grow the same way as their roots. Also, 
plant cell development and function is slower than 
normal, and most cells do not grow to maturity.

2. Gravity and Water: Here on earth, gravity forces 
water down to the roots, where the plant absorbs it. 
But in space, watering becomes difficult because zero 
gravity causes the water to spread out evenly in a 
horizontal layer through the soil-like material, making 
it difficult to reach the roots. Or worse, the water 
may form a ball and not move towards the roots at 
all. 

3. Natural Atmosphere: In space, there is less natural 
air circulating, so plants could literally suffocate on 
their own "exhaled" oxygen. Scientists have to provide 
special containers with fans or air pumps for 
experimentation to keep the air moving. Future crops 
will have to live within a controlled environment, much 
like a greenhouse.

4. Soil: Soil on other planets will differ from that on 
Earth's, so researchers carefully try to mimic soils on 
other planets and still make sure plants get the 
nutrients they need.

5. Light: In an orbiting station, plants in a window 
would get a pattern of 45 minutes of light, and 45 
minutes of darkness, so artificial light is needed to 
simulate normal Earth conditions. Otherwise, without 
their normal daylight cycle, plants can't manufacture 
enough food to survive, because photosynthesis is 
thwarted.

6. Pollination: Plants rely heavily on bees and other 
insects to spread the pollen from flower to flower so 
they can repopulate. But in space, there are no 
insects to rely upon. Furthermore, insects studied 
aboard spacecrafts have had very difficult times 
adapting to the zero gravity environment. Researchers 
are forced to devise alternative methods of pollination 
or rely on self-pollinating or asexual plants.


Source:
Excerpt from an article entitled "2001: A Gardening 
Odyssey For future space travelers, gardening will be
 a matter of survival" By Tiffany McKinnon. Special 
thanks to Homestore.com, visit them on the web for 
all your home gardening needs.