Elenco Solar Energy - Manual (Page 13)

12
Experiment no. 22 Charring paper
Place your lens in bright sunlight with a white piece of paper at its focal point. Can you
char the paper? If the sun is bright enough, you may be able to char it. It will char
best right at the edge of the paper.
The white paper reflects most of the sunlight and solar heat. Even though the solar
heat is concentrated at a small point the paper barely, if at all, reaches its kindling point.
Experiment no. 23 A lenses’ shadow
Try to locate a small bottle cap the same size
as your lens. Fill the cap with water.
Place it in direct sunlight for 10 minutes.
Measure and record the water temperature at
the beginning and end of the time period.
Now start again. Place an equal amount of
water in your bottle cap. Make sure the water
is at the same starting temperature as before.
This time place the water in sunlight with the
lens in front of the cap and the water at the F.P.
of the lens. Compare the temperature of the
water after 10 minutes with the results of your
previous try – they should be very similar.
When the lens is placed in front of the water in the bottle cap the concentrated rays
converge on a small area of the water. The surrounding area will be in the shadow of
the lens, so it does not receive the energy that is being gathered by the lens. The total
amount of heat received remains the same. If the area of the body of water is smaller
than that of the lens, then the water will receive more energy and the lens can be
thought of as a collector.
Experiment no. 24 The solar trap
For this and the next two experiments, you need fairly large pieces of card of stiff
drawing paper. You also need some glue and aluminum foil.
Find a piece of thin card or stiff drawing paper about 50 cm x 30cm. Cover a table
with a piece of old newspaper to protect it. Place the card on the newspaper. With a
pain brush, or cotton ball, spread the glue evenly over the card. Take a piece of
aluminum foil the same size as the card. Stick it down smoothly and firmly.
You have made a crude cardboard mirror! Let the glue dry for at least half an hour.
In the meantime take it outdoors and try, by
means of this mirror, to reflect sunlight onto
the wall of a house. See what happens when
you bend the cardboard mirror slightly. After
alf an hour, roll the card into a cone (funnel)
as illustrated. The side with the aluminum
foil must be inside. The top opening should
be approximately 20cm across, the bottom
about 2.5 cm. Fix together with tape, glue or
staples.
You solar trap is ready.