Elenco Solar Energy - Manual (Page 10)

9
To concentrate heat, your solar furnace must face the sun.
It is constructed in such a way that the parabolic reflector and its
polystyrene support can be tilted so that you can point the reflector
directly at the sun.
You will achieve the best results in your experiments with the solar
furnace if you try them when the sun is directly overhead. This is
due to the fact that the sun’s rays are strongest when they strike
the earth at an angle of 90
o
.
When the sun is directly overhead, its rays travel the shortest possible distance before
they reach you. Rays that strike your reflector at a greater angle (when the sun is not
directly overhead) are weaker since they travel a longer distance through the earth’s
atmosphere. As the rays travel from the sun, the atmosphere of the earth absorbs some
of their heat. If the rays travel a longer distance, they lose more heat. In addition to
that, when the sun’s rays strike the ground at an angle, they cover a greater area and
less heat is available than when the sun is overhead.
A breeze or wind will cool the items while you are trying to heat them. Needless to say,
on a cloudy day you will be unable to perform some experiments.
Experiment no. 10 Determining the focal point
The focal point (F.P.) of our parabolic reflector is located 6 cm above its deepest place.
Place your solar furnace in sunlight. Take a white sheet of paper and slowly move it
towards the reflectors. As the sheet approaches the focal point of the reflector you will
begin to see a circular bright spot. The closer you get, the smaller the spot becomes.
It is quite small at the focal point. At this point, our solar furnace will be the most
effective.
The focal point is the point in mid-air where all the light hitting our parabolic reflector
is reflected upon. What happens as you move the paper closer to the reflector than its
focal point?
Experiment no. 11 Burning a feather
For this experiment, you will need a small dark birds feather, which can be found out
in the garden.
Hold this feather 2 – 3 inches above the reflector with the
reflector pointing towards the sun. Move the feather a little
up and down, right and left (horizontally and vertically),
until you reach a point where the feather begins to smoke.
This is the focal point. At this point, the concentrated
sunlight is enough to singe the feather.
It is easy to burn a dark feather, while a white one will
hardly be affected.
While performing this experiment, and the previous and
following ones, take care not to block the sunlight with your
hand, from reaching the reflector.
USING THE SOLAR FURNACE
USE SUNGLASSES WHILE PERFORMING THIS EXPERIMENT!!
NEVER LOOK DIRECLY AT THE SUN!!