September 11, 2015
By: Ted Beyer
Summertime—sun and fun! For most of us (in the northern hemisphere at least) that means hot weather. Heat does interesting things to the world around us, and to us as well. On a hot day you tend to perspire. Your body does this for a good reason: as the moisture evaporates, it cools your skin, and thus helps to regulate your body temperature.
In contrast, dogs don’t perspire—they don’t have sweat glands! So on a hot day you will see dogs panting—lots of rapid, shallow breaths with their tongues looking bigger than usual hanging out of their mouths. That’s the doggie way of cooling off. They are moving air over a wet surface—again using evaporation to lower their body temperature.
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July 9, 2010
by: Martin Sagendorf
On a Bright Day:
A great deal of energy falls on the Earth’s surface – roughly 1 kW per square meter. This is about 0.6 Watt per square inch. This doesn’t sound like much energy, but suppose we collect and concentrate 63 square inches of this sunlight? These 63 square inches would collect about 38 Watts of energy. This doesn’t sound like much, but…
Suppose We Could Then:
Concentrate these 38 Watts into an area of only 1/8 of a square inch? This is exactly what we can do with an inexpensive plastic Fresnel lens. We’ll focus the sunlight into an area 3/8” in diameter – this is the equivalent of 300 Watts per square inch! With this energy level, we can easily ignite a piece of wood, boil some water, and even melt a penny.
A Suitable Device:
Is described in the book, Physics Demonstration Apparatus and in the blog The Sun’s Energy.
Now we’re going to describe how to build a much simpler version that works just as well – one that uses a very inexpensive Fresnel lens and is very easy to construct. Read the rest of this entry »