Worlds of David Darling > Children's
Encyclopedia of Science > Between Fire and Ice > Chapter 4
BETWEEN FIRE AND ICE:
The Science of Heat
a book in the eXperiment! series by David Darling
4. Passing the Heat Along
Leave a metal spoon in a cup of coffee and quite soon the handle of the
spoon will become hot. This is an example of heat traveling by CONDUCTION.
We can understand why the end of the spoon gets hot by once again thinking
The fast-moving molecules of water in the coffee bump into the part of the
spoon that is next to the hot liquid. This makes the metal molecules of
the spoon vibrate harder so that the metal heats up. Quickly the vibrations
are passed on, from molecule to molecule, up the handle of the spoon.
A Test of Conductors
You will need:
- Copper and steel wire of the same thickness
- Wire made from other metals of the same thickness
- A stopwatch
- Three short, fat candles
- Vaseline or melted candle wax
- Paper clips
- Two thumbtacks
- A ruler
- Two wooden blocks
What to do:
Twist the ends of the two wires together, making sure that the twisted
part of each wire is the same length. Support the wires between the
two wooden blocks and fasten their free ends to one of the blocks
with thumbtacks. Attach paper clips one inch apart along each wire,
using Vaseline or drops of melted wax from a candle. Ask an adult
to help you with this. Space the three candles evenly under the middle
of the twisted wires and light them. Observe what happens. Time how
long it takes for each paper clip along each wire to fall off. Plot
a graph of time versus distance. From your results, decide which metal
is the better conductor.
Taking it further:
Continue the experiment with wire made from other metals. Are the
best conductors also those metals that expanded most in the experiment
You may also wish to try comparing two wires of the same metal but
of different thickness. Does heat travel faster along a thinner wire
of copper than a thicker one? If so, how much faster? For instance,
if one wire is half as thick as another, does heat travel twice as
fast along it?
Try making up you own questions and then devising an experiment to
answer them. You may find, as scientists often do, that while one
experiment may help solve a particular problem, it leads to other
interesting problems and further ideas for experiments.
Conductors and Insulators
Substances through which heat can travel easily are said to be good conductors.
Metals are the best conductors. That is why we use them for making such
things as saucepans and radiators. It also explains why metals usually feel
cool to the touch. If a metal object is at a lower temperature than your
body, it will quickly carry heat away from your skin when you touch it.
The opposite is true if the metal is warmer than you are. Then the metal
will quickly give up its heat to your skin so that it fells hot to the touch.
A substance through which heat cannot travel well is called an INSULATOR.
Air, for example, is a very good insulator.
Can the Heat Get Through
You will need:
- A rubber hot-water bottle
- Pieces of wood, plastic, paper, cardboard, cork, and woolen
and cotton materials Stack each of these so that they are all
of roughly the same thickness.
What to do:
Fill the hot-water bottle with hot (but not boiling) water and lay
it on a table. Place the piece of wood on the bottle and leave it
there for 5 minutes. Put your hand on top of the wood. Record whether
it feels "very warm," "quite warm," "warm," or "cool." Repeat this
with the plastic, paper, and other substances. From your results,
make a list of the materials, with the best conductors at the top
and the best insulators at the bottom. You may occasionally need to
replace the hot water so that the hot-water bottle is at more or less
the same temperature throughout the experiment.
Protecting the Space Shuttle
As the orbiter of the space shuttle returns to Earth, it plunges through
the upper atmosphere at very high speed. Without proper protection this
would cause the metal hull of the craft to melt. Scientists have therefore
developed a special insulating material, containing silicon, which can be
made into thin, lightweight tiles. These tiles are used to cover the orbiterís
underside, nose, and wing tips.
A space shuttle tile is such a good insulator that one side can be held
in bare hands while the opposite side is red-hot. It can withstand temperatures
of up to 2,300°F (1,260°C).
You will need:
- A jar with a metal lid
- Two thermometers
- Various materials with which to wrap the jar, such as aluminum
foil, cloth, cotton, and newspaper
- A plastic jug
- A stopwatch
What to do:
Ask an adult to make a hole in the middle of the metal lid of the
jar, just big enough for one of the thermometers to fit through. Push
the thermometer through the hole and seal any gap with some Play-Doh.
Wrap the sides and base of the jar with aluminum
foil, leaving enough foil free at the top to cover the lid. Fill the
jug with hot water from a hot-water faucet (or ask an adult to heat
up some water for you). Measure the temperature of this water with
the second thermometer. Immediately fill the jar with hot water from
the jug. Put on the lid, cover the lid with foil, and start the stopwatch.
After 10 minutes, take off the foil wrapping and record the thermometer
reading. Throw out the water and wrap the jar with a different material.
Fill the jug again with fresh hot water and make sure that this is
at the same temperature as before. Repeat the experiment as with the
foil. Do the same with the other wrapping materials that you have.
Which provided the best insulation?
Try different combinations of wrappings to see if these insulate the
jar batter. For example, you could put some cotton between two layers
In designing an experiment it is always important to make it as fair
as possible. In other words, apart from those things you want to study,
all other conditions should remain unchanged. For example, in this
experiment, the thicknesses of the wrapping materials used should
be the same. Otherwise it would be impossible to say whether one wrapper
kept in more heat than another because it was made of better insulating
material or simply because it was thicker.
For the experiment to be fair, you must also make sure that all the
wrappings completely cover the jar. Even a small opening will allow
heat to escape without first having to pass through the material surrounding
the glass. This kind of care is important in all experiments, since
without it you bring in other factors that can have an unknown effect
upon your results.
Try to think of ways to improve this experiment or extend it. For
instance, in a class project, you might try to design an experiment
to test the insulating properties of materials used in the building.
Pockets of Air
Air is one of the best of all insulators. This means that a good insulator
can be made by trapping little pockets of air within a substance. The insulation
that is often found around hot-water tanks in in attics of houses works
in this way. It holds still air between the fibers of a fluffy material.
|Well-insulated clothing keeps us warm in
the coldest weather
You can use this idea to help stay warm in winter. By dressing in several
layers of thinner clothing instead of wearing one or two thick items, you
trap more air between the layers and so prevent your body heat from escaping.
Boots and coats designed for use in low temperatures are themselves made
from many layers for this purpose.